RECOMMENDED RESIDENTIAL CONSTRUCTION FOR COASTAL AREAS
Building on Strong and Safe Foundations
FEMA P-550, Second Edition / December 2009
About the Cover
On August 29, 2005, Hurricane Katrina struck the Gulf Coast with
recordbreaking
storm surge that destroyed foundations and devastated homes from Louisiana
east
to Alabama. Katrina was so destructive that engineers assessing the carnage
no
longer looked for “success stories” (i.e., homes that were only moderately
damaged), but rather searched for “survivor” homes that, while extensively
dam-
aged, still bore a slight resemblance to a residential building. Hurricane
Katrina proved that, without strong foundations, homes on the coast can and
will
be destroyed.
Preface
Since the publication of the First Edition of FEMA 550 in July 2006, several
advances have been made in nationally adopted codes and standards. Two
editions
of the International Residential Code® (the 2006 IRC® and the 2009 IRC) and
the
International Building Code® (the 2006 IBC® and the 2009 IBC) have been
published and the long awaited International Code Council (ICC) 600 Standard
for
Residential Construction in High Wind Areas (a successor to the legacy
standard
SSTD-10) has been issued.
To keep pace with developing codes and standards and to improve its guidance,
FEMA is issuing this Second Edition of FEMA 550. In addition to being renamed
to
more accurately reflect its applicability, the Second Edition of FEMA 550
contains a new foundation style Case H, which incorporates an elevated
concrete
beam for improved structural efficiency. The Second Edition of FEMA 550 has
also
been updated for consistency with the 2006 and 2009 editions of the IRC and
IBC,
and the 2005 Edition of ASCE 7 Minimum Design Loads for Buildings and Other
Structures.
RECOMMENDED RESIDENTIAL CONSTRUCTION FOR COASTAL AREAS
Building on Strong and Safe Foundations
FEMA P-550, Second Edition / December 2009
Acknowledgments
FEMA would like to thank the following individuals who provided information,
data, review, and guidance in developing the Second Edition of this
publication.
FEMA
John Ingargiola
FEMA Headquarters
Consultants
Dave Conrad
PBSJ
Deb Daly
Greenhorne & O'Mara, Inc.
Julie Liptak
Greenhorne & O'Mara, Inc.
David K. Low, PE
DK Low & Associates, LLC
Kelly Park
Greenhorne & O'Mara, Inc.
Scott Sundberg
Category X Coastal Consulting
Scott Tezak
URS Corporation
Jimmy Yeung, PhD, PE
Greenhorne & O’Mara, Inc.
In addition, FEMA would like to acknowledge the members of the project team
for
the First Edition of the publication. (Note: all affiliations were current as
of
July 2006.)
Principal Authors
Bill Coulbourne, PE
URS Corporation
Matt Haupt, PE
URS Corporation
Scott Sundberg, PE
URS Corporation
David K. Low, PE
DK Low & Associates, LLC
Jimmy Yeung, PhD, PE
Greenhorne & O’Mara, Inc.
John Squerciati, PE
Dewberry & Davis, LLC
Contributors and Reviewers
John Ingargiola
FEMA Headquarters
Shabbar Saifee
FEMA, Region IV
Dan Powell
FEMA, Region IV
Alan Springett
FEMA, Region IV
Keith Turi
FEMA Headquarters
Christopher Hudson
FEMA Headquarters
Christopher P. Jones, PE
Dan Deegan, PE, CFM
PBSJ
Ken Ford
National Association of Homebuilders (NAHB)
Mike Hornbeck
Gulf Construction Company, Inc.
David Kriebel, PhD, PE
U.S. Naval Academy
Jim Puglisi
Dewberry & Davis, LLC
John Ruble
Bayou Plantation Homes
Bob Speight, PE
URS Corporation
Deb Daly
Greenhorne & O’Mara, Inc.
Julie Liptak
Greenhorne & O’Mara, Inc.
Wanda Rizer
Design4Impact
Naomi Chang Zajic
Greenhorne & O’Mara, Inc.
RECOMMENDED RESIDENTIAL CONSTRUCTION FOR COASTAL AREAS
Building on Strong and Safe Foundations
FEMA P-550, Second Edition / December 2009
Table of Contents
Preface iii
Acknowledgments v
Introduction xv
Chapter 1. Types of Hazards 1-1
1.1 High Winds 1-1
1.2 Storm Surge 1-5
1.3 Flood Effects 1-5
1.3.1 Hydrostatic Forces 1-7
1.3.2 Hydrodynamic Forces 1-7
1.3.3 Waves 1-8
1.3.4 Floodborne Debris 1-10
1.3.5 Erosion and Scour 1-10
Chapter 2. Foundations 2-1
2.1 Foundation Design Criteria 2-1
2.2 Foundation Design in Coastal Areas 2-2
2.3 Foundation Styles in Coastal Areas 2-4
2.3.1 Open Foundations 2-5
2.3.1.1 Piles 2-5
2.3.1.2 Piers 2-7
2.3.2 Closed Foundations 2-8
2.3.2.1 Perimeter Walls 2-8
2.3.2.2 Slab-on-Grade 2-10
2.4 Introduction to Foundation Design and Construction 2-11
2.4.1 Site Characterization 2-11
2.4.2 Types of Foundation Construction 2-12
2.4.2.1 Piles 2-12
2.4.2.2 Diagonal Bracing of Piles 2-13
2.4.2.3 Knee Bracing of Piles 2-14
2.4.2.4 Wood-Pile-to-Wood-Girder Connections 2-15
2.4.2.5 Grade Beams in Pile/Column Foundations 2-15
Chapter 3. Foundation Design Loads 3-1
3.1 Wind Loads 3-2
3.2 Flood Loads 3-2
3.2.1 Design Flood and DFE 3-2
3.2.2 Design Stillwater Flood Depth (d subscript s) 3-4
3.2.3 Design Wave Height (H subscript b) 3-5
3.2.4 Design Flood Velocity (V) 3-5
3.3 Hydrostatic Loads 3-6
3.4 Wave Loads 3-7
3.4.1 Breaking Wave Loads on Vertical Piles 3-8
3.4.2 Breaking Wave Loads on Vertical Walls 3-8
3.5 Hydrodynamic Loads 3-9
3.6 Debris Impact Loads 3-12
3.7 Erosion and Localized Scour 3-14
3.7.1 Localized Scour Around Vertical Piles 3-15
3.7.2 Localized Scour Around Vertical Walls and Enclosures 3-19
3.8 Flood Load Combinations 3-19
Chapter 4. Overview of Recommended Foundation Types and Construction for
Coastal
Areas 4-1
4.1 Critical Factors Affecting Foundation Design 4-2
4.1.1 Wind Speed 4-2
4.1.2 Elevation 4-3
4.1.3 Construction Materials 4-4
4.1.3.1 Masonry Foundation Construction 4-4
4.1.3.2 Concrete Foundation Construction 4-4
4.1.3.3 Field Preservative Treatment for Wood Members 4-5
4.1.4 Foundation Design Loads 4-5
4.1.5 Foundation Design Loads and Analyses 4-8
4.2 Recommended Foundation Types for Coastal Areas 4-14
4.2.1 Open/Deep Foundation: Timber Pile (Case A) 4-15
4.2.2 Open/Deep Foundation: Steel Pipe Pile with Concrete Column and Grade
Beam
(Case B) 4-17
4.2.3 Open/Deep Foundation: Timber Pile with Concrete Column and Grade Beam
(Case C) 4-17
4.2.4 Open/Deep Foundation: Timber Pile with Concrete Grade and Elevated
Beams
and Concrete Columns (Case H) 4-19
4.2.5 Open/Shallow Foundation: Concrete Column and Grade Beam with Slabs
(Cases
D and G) 4-21
4.2.6 Closed/Shallow Foundation: Reinforced Masonry – Crawlspace (Case E) 4-
21
4.2.7 Closed/Shallow Foundation: Reinforced Masonry – Stem Wall (Case F) 4-23
Chapter 5. Foundation Selection 5-1
5.1 Foundation Design Types 5-1
5.2 Foundation Design Considerations 5-2
5.3 Cost Estimating 5-4
5.4 How to Use This Manual 5-4
5.5 Design Examples 5-7
Appendices
Appendix A Foundation Designs
Appendix B Pattern Book Design
Appendix C Assumptions Used in Design
Appendix D Foundation Analysis and Design Examples
Appendix E Cost Estimating
Appendix F Pertinent Coastal Construction Information
Appendix G FEMA Publications and Additional References
Appendix H Glossary
Appendix I Abbreviations and Acronyms
Tables
Chapter 2
Table 2-1. Foundation Type Dependent on Coastal Area 2-5
Chapter 3
Table 3-1. Building Category and Corresponding Dynamic Pressure Coefficient
(Cp)
3-9
Table 3-2. Drag Coefficient Based on Width to Depth Ratio 3-11
Table 3-3.Example Foundation Adequacy Calculations for a Two-Story Home
Supported on Square Timber Piles. 3-17
Table 3-4. Local Scour Depth as a Function of Soil Type 3-19
Table 3-5. Selection of Flood Load Combinations for Design 3-21
Chapter 4
Table 4-1a. Design Perimeter Wall Reactions (lb/lf) for One-Story Elevated
Homes
4-7
Table 4-1b. Design Perimeter Wall Reactions (lb/lf) for Two-Story Elevated
Homes
4-7
Table 4-2. Flood Forces (in pounds) on an 18-Inch Square Column 4-7
Table 4-3. Wind Reactions Used to Develop Case H Foundations 4-8
Table 4-4. Design Moments (K-ft), Axial Loads (in kips), and Shears (in kips)
for 10-Foot Tall 3-Bay Foundations 4-10
Table 4-5. Design Moments (K-ft), Axial Loads (in kips), and Shears (in kips)
for 15-Foot Tall 3-Bay Foundations 4-11
Table 4-6. Design Moments (K-ft), Axial Loads (in kips), and Shears (in kips)
for 10-Foot Tall 6-Bay Foundations 4-12
Table 4-7.Design Moments (K-ft), Axial Loads (in kips), and Shears (in kips)
for
15-Foot Tall 6-Bay Foundations 4-12
Table 4-8. Design Moments (K-ft), Axial Loads (in kips), and Shears (in kips)
for 10-Foot Tall 9-Bay Foundations 4-13
Table 4-9. Design Moments (K-ft), Axial Loads (in kips), and Shears (in kips)
for 15-Foot Tall 9-Bay Foundations 4-14
Table 4-10. Recommended Foundation Types Based on Zone 4-15
Chapter 5
Table 5-1a. Foundation Design Cases for One-Story Homes Based on Height of
Elevation and Wind Velocity 5-10
Table 5-1b. Foundation Design Cases for Two-Story Homes Based on Height of
Elevation and Wind Velocity 5-11
Figures
Introduction
Figure 1. Damage to residential properties as a result of Hurricane Katrina's
winds and storm surge. Note the building that was knocked off its foundation.
xvi
Figure 2. Schematic range of home dimensions and roof pitches used as the
basis
for the foundation designs presented in this manual. viii
Chapter 1
Figure 1-1. Wind damage to roof structure and gable end wall from Hurricane
Katrina (2005) (Pass Christian, Mississippi). 1-2
Figure 1-2. Saffir-Simpson Scale. -3
Figure 1-3. Wind speeds (in mph) for the entire U.S. 1-4
Figure 1-4. Graphical depiction of a hurricane moving ashore. In this
example, a
15-foot surge added to the normal 2-foot tide creates a total storm tide of
17
feet. 1-5
Figure 1-5. Storm tide and waves from Hurricane Dennis on July 10, 2005, near
Panacea, Florida. 1-6
Figure 1-6. Comparison of storm surge levels along the shorelines of the Gulf
Coast for Category 1, 3, and 5 storms. 1-6
Figure 1-7. Building floated off of its foundation (Plaquemines Parish,
Louisiana). 1-7
Figure 1-8. Aerial view of damage to one of the levees caused by Hurricane
Katrina. 1-8
Figure 1-9. During Hurricane Opal (1995), this house was in an area of
channeled
flow between large buildings. As a result, the house was undermined and
washed
into the bay behind a barrier island. 1-8
Figure 1-10. Storm waves breaking against a seawall in front of a coastal
residence at Stinson Beach, California. 1-9
Figure 1-11.Storm surge and waves overtopping a coastal barrier island in
Alabama (Hurricane Frederic, 1979). 1-9
Figure 1-12. Pier piles were carried over 2 miles by the storm surge and
waves
of Hurricane Opal (1995) before coming to rest in Pensacola Beach, Florida.
1-10
Figure 1-13. Extreme case of localized scour undermining a slab-on-grade
house
in Topsail Island, North Carolina, after Hurricane Fran (1996). 1-11
Chapter 2
Figure 2-1. Recommended open foundation practice for buildings in A zones,
Coastal A zones, and V zones. 2-3
Figure 2-2 Slab-on-grade foundation failure due to erosion and scour
undermining from Hurricane Dennis, 2005 (Navarre Beach, Florida). 2-3
Figure 2-3. Compression strut at base of a wood pile. Struts provide some
lateral support for the pile, but very little resistance to rotation. 2-6
Figure 2-4. Near collapse due to insufficient pile embedment (Dauphin Island,
Alabama). 2-6
Figure 2-5. Successful pile foundation following Hurricane Katrina (Dauphin
Island, Alabama). 2-6
Figure 2-6. Column connection failure (Belle Fontaine Point, Jackson County,
Mississippi). 2-7
Figure 2-7. Performance comparison of pier foundations. Piers on discrete
footings failed while piers on more substantial footings survived (Pass
Christian, Mississippi). 2-8
Figure 2-8. Isometric view of an open foundation with grade beam. 2-9
Figure 2-9. Isometric view of a closed foundation with crawlspace. 2-10
Figure 2-10. Pile installation methods. 2-12
Figure 2-11. Diagonal bracing schematic. 2-14
Chapter 3
Figure 3-1. Wind speeds (in mph) for the entire U.S. 3-3
Figure 3-2. Parameters that determine or are affected by flood depth. 3-4
Figure 3-3. Normally incident breaking wave pressures against a vertical wall
(space behind vertical wall is dry). 3-10
Figure 3-4. Normally incident breaking wave pressures against a vertical wall
(stillwater level equal on both sides of wall). 3-10
Figure 3-5. Distinguishing between coastal erosion and scour. A building may
be
subject to either or both, depending on the building location, soil
characteristics, and flood conditions. 3-14
Figure 3-6. Scour at vertical foundation member stopped by underlying scour-
resistant stratum. 3-16
Chapter 4
Figure 4-1. The BFE, freeboard, erosion, and ground elevation determine the
foundation height required. 4-3
Figure 4-2. Design loads acting on a column. 4-6
Figure 4-3. Shear panel reactions for the 3- and 6-bay models. Reactions for
the
9-bay model were similar to those of the 6-bay. 4-10
Figure 4-4.Profile of Case A foundation type. 4-16
Figure 4-5. Profile of Case B foundation type. 4-18
Figure 4-6. Profile of Case C foundation type. 4-19
Figure 4-7. Profile of Case H foundation type. 4-20
Figure 4-8. Profile of Case G foundation type. 4-22
Figure 4-9. Profile of Case D foundation type. 4-23
Figure 4-10. Profile of Case F foundation type. 4-24
Figure 4-11. Profile of Case E foundation type.4-24
Chapter 5
Figure 5-1. Schematic of a basic module and two footprints. 5-3
Figure 5-2. Foundation selection decision tree 5-8
Figure 5-3. “T” shaped modular design 5-12
Figure 5-4. “L” shaped modular design 5-12
Figure 5-5. “Z” shaped modular design 5-13
Introduction
The purpose of this design manual is to provide recommended foundation
designs
and guidance for rebuilding homes destroyed by hurricanes in coastal areas.
In
addition, the manual is intended to provide guidance in designing and
building
safer and less vulnerable homes to reduce the risk to life and property.
Past storms such as Hurricanes Andrew, Hugo, Charley, Katrina, and Rita, and
recent events such as Hurricane Ike continue to show the vulnerability of our
“built environment” (Figure 1). While good design and construction cannot
totally eliminate risk, every storm has shown that sound design and
construction
can significantly reduce the risk to life and damage to property. With that
in
mind, the Federal Emergency Management Agency (FEMA) has developed this
manual
to help the community of homebuilders, contractors, and local engineering
professionals in rebuilding homes destroyed by hurricanes, and designing and
building safer and less vulnerable new homes.
[Begin figure]
Figure 1 is a photo showing damage to residential properties as a result of
Hurricane Katrina's winds and storm surge. Note the building that was knocked
off its foundation (circled).
Source: Hurricane Katrina MATphoto
[End figure]
Intent of the Manual
The intent of the manual is to provide homebuilders, contractors, and
engineering professionals with a series of recommended foundation designs
that
will help create safer and stronger buildings in coastal areas. The designs
are
intended to help support rebuilding efforts after coastal areas have been
damaged by floods, high winds, or other natural hazards.
The foundations may differ somewhat from traditional construction techniques;
however, they represent what are considered to be some of the better
approaches
to constructing strong and safe foundations in hazardous coastal areas. The
objectives used to guide the development of this manual are:
- To provide residential foundation designs that will require minimal
engineering oversight
- To provide foundation designs that are flexible enough to accommodate many
of
the homes identified in A Pattern Book for Gulf Coast Neighborhoods prepared
for
the Mississippi Governor’s Rebuilding Commission on Recovery, Rebuilding, and
Renewal (see Appendix B)
- To utilize "model" layouts so that many homes can be constructed without
significant additional engineering efforts
The focus of this document is on the foundations of residential buildings.
The
assumption is that those who are designing and building new homes will be
responsible for ensuring that the building itself is designed according to
the
latest building code (International Building Code® [IBC®], International
Residential Code® [IRC®], and FEMA guidance) and any local requirements. The
user of this manual is directed to other publications that also address
disaster-resistant construction (see Appendix G).
Although the foundation designs are geared to the coastal environment subject
to
storm surge, waves, floating debris, and high winds, several are suitable for
supporting homes on sites protected by levees and floodwalls or in riverine
areas subjected to high-velocity flows. Design professionals can be contacted
to
ensure the foundation designs provided in this manual are suitable for
specific
sites.
[Begin text box]
CAUTION: Although sites inside levees are not exposed to wave loads, sites
immediately adjacent to floodwalls and levees can be exposed to extremely
high
flood velocities and scour if a breach occurs. Design professionals should be
consulted before using these foundation designs on sites close to floodwalls
or
levees to determine if they are appropriate.
[End text box[
This manual contains closed foundation designs for elevating homes up to 8
feet
above ground level and open foundation designs for elevating homes up to 15
feet
above ground level. These upper limits are a function of constructability
limitations and overturning and stability issues for more elevated
foundations.
Eight-foot tall foundations are a practical upper limit for 8-inch thick
reinforced concrete masonry unit (CMU) walls exposed to flood forces
anticipated
in non-coastal A zones. The upper limit of 15 feet for open foundations was
established by estimating the amount a home needs to be elevated to achieve
the
2005 Advisory Base Flood Elevations (ABFEs) as determined in response to
Hurricanes Katrina and Rita. The ABFEs published in the Hurricane Recovery
Maps
were compared to local topographic maps for the Gulf Coast. The comparison
revealed that providing foundation designs up to 15 feet tall would allow
over
80 percent of the homes damaged by Hurricane Katrina to be protected from
flood
events reflected in the ABFEs and in the Hurricane Recovery Maps. Many homes
can
be elevated to the BFE on foundations that are 4 feet tall or less.
This updated edition of FEMA 550 introduces the Case H foundation, which is
an
open/deep foundation developed for use in coastal high hazard areas (V
zones).
It is also appropriate to use the Case H foundation in Coastal A and non-
coastal
A zones. Case H foundations incorporate elevated reinforced concrete beams
that
provide three important benefits. One, the elevated beams work in conjunction
with the reinforced concrete columns and grade beams to produce a structural
frame that is more efficient at resisting lateral loads than the grade beams
and
cantilevered columns used in other FEMA 550 open foundations. The increased
efficiency allows foundations to be constructed with smaller columns that are
less exposed to flood forces.
The second benefit is that the elevated reinforced concrete beams provide a
continuous foundation that can support many homes constructed to prescriptive
designs from codes and standards such as the IRC, the American Forest and
Paper
Association’s (AF&PA’s) Wood Frame Construction Manual for One- and Two-
Family
Dwellings (WFCM), and the International Code Council’s (ICC’s) Standard for
Residential Construction in High Wind Regions (ICC-600).
The third benefit that Case H foundations provide is the ability to support
relatively narrow (14-foot wide) homes. It is anticipated that Case H
foundations can be used for several styles of modular homes.
Using the Manual
The following information is needed to use this manual:
- Design wind speed and the Design Flood Elevation (DFE) at the site
- The flood zone(s) at the site
- Building layout
- Topographic elevation of existing building site
- Soil conditions for the site. Soil condition assumptions used in the load
calculations are intentionally conservative. Users are encouraged to
determine
soil conditions at the site to potentially improve the cost-effectiveness of
the
design.
Most of the information can be obtained from the local building official or
floodplain manager.
This document is not intended to supplant involvement from local design
professionals. While the designs included can be used without modification
(provided that the home to be elevated falls within the design criteria),
consulting with local engineers should be considered. Local engineers may
assist
with the following:
- Incorporating local site conditions into the design
- Addressing and supporting unique features of the home
- Confirming the suitability of the designs for a specific home on a specific
site
- Allowing use of value engineering to produce a more efficient design
These "prescriptive" designs have been developed to support homes with a
range
of dimensions, weights, and roof pitches. Figure 2 schematically shows the
diverse range of dimensions and roof pitches. Appendix C contains a complete
list of criteria and assumptions used in these designs.
[Begin figure]
Figure 2 is an illustration of a schematic range of home dimensions and roof
pitches used as the basis for the foundation designs presented in this
manual.
[End figure]
This manual concentrates on foundations that resist the extreme hurricane
wind
and flooding conditions found in many coastal areas. For successful, natural
hazard-resistant installations, both the foundation and the home it supports
must be properly designed and constructed to take all loads on the structure
into the ground through the foundation. Designing and constructing the home
to
meet all the requirements of the IBC or the IRC is the minimum action
necessary
to producing hazard-resistant homes. However, any model code must contain
minimum requirements. FEMA supports the use of best practice approaches for
improved resistance to natural hazards.
The foundations presented in this manual have been designed to resist the
flood,
wind, and gravity loads specified in Appendix C and load combinations
specified
in the American Society of Civil Engineers’ (ASCE’s) Minimum Design Loads for
Buildings and Other Structures (ASCE 7-05). Although the designs provide
significant resistance to gravity, lateral, and uplift loads, they have not
been
specifically designed for seismic events. In coastal areas where seismic
risks
exist, design professionals should confirm that the foundation designs
presented
herein are adequate to resist seismic loads on site.
To gain the benefits of a “best practices” approach, readers are directed to
publications such as FEMA 499, Home Builder’s Guide to Coastal Construction
Technical Fact Sheet Series, and FEMA 55, Coastal Construction Manual. A more
complete list of available publications is contained in Appendix G.
Organization of the Manual
There are five chapters and nine appendices in this manual. The intent is to
cover the essential information in the chapters and provide all the details
in
the appendices. Chapter 1 provides a description of the different types of
hazards that must be considered in the design of a residential building
foundation in a coastal area. The primary issues related to designing
foundations for residential buildings are described in Chapter 2. Chapter 3
provides guidance on how to determine the magnitude of the loads placed on a
building by a particular natural hazard event or a combination of events. The
different foundation types and methods of construction foundation for a
residential building are discussed in Chapter 4. Chapter 5 and Appendix A
present foundation designs to assist the homebuilders, contractors, and local
engineering professionals in developing safe and strong foundations.
In addition to Chapters 1 through 5 and Appendix A, the following appendices
are
presented herein:
- Appendix B presents examples of how the foundation designs in this manual
can
be used with some of the homes in the publication A Pattern Book for Gulf
Coast
Neighborhoods.
- Appendix C provides a list of assumptions used in developing the foundation
design presented in this manual.
- Appendix D presents detailed calculations on how to design the foundation
of
residential buildings. Two examples, one for open foundations and the other
for
closed foundations, are included.
- Appendix E provides cost information that the homebuilders can use to
estimate
the cost of installing the foundation systems proposed in this manual.
- Appendix F includes fact sheets contained in FEMA 499 (Home Builder’s Guide
to
Coastal Construction Technical Fact Sheet Series) and a recovery advisory
contained in FEMA P-757 (Hurricane Ike in Texas and Louisiana: Mitigation
Assessment Team Report, Building Performance Observations, Recommendations,
and
Technical Guidance) that are pertinent to construction in coastal areas.
- Appendix G presents a list of references and other FEMA publications that
can
be of assistance to the users of this manual.
- Appendix H contains a glossary of terms used in this manual.
- Appendix I defines abbreviations and acronyms used in this manual.
[Begin text box]
Limitations of the Manual
This manual has been provided to assist in reconstruction efforts after
coastal
areas have been damaged by floods, high winds, or other natural hazards.
Builders, architects, or engineers using this manual assume responsibility
for
the resulting designs and their performance during a natural hazard event.
The foundation designs and analyses presented in this manual were based on
ASCE7-02 and the 2003 version of the IRC. While FEMA 550 was being developed,
released its 2005 edition of 7 (7-05) and the ICC issued their 2006 editions
of
the IBC and The 7 revisions did not affect the load calculations controlling
the
designs and there were no substantive flood provision changes to the IRC that
affect foundation designs in coastal areas.
This Second Edition of FEMA550 is consistent with the 2009 editions of the
IBC
and IRC.
[End text box]
Chapter 1.Types of Hazards
This chapter discusses the following types of hazards that must be considered
in
the design of a residential building foundation for coastal areas: high
winds,
storm surge, and associated flood effects, including hydrostatic forces,
hydrodynamic forces, waves, floodborne debris, and erosion and scour.
1.1 High Winds
Hurricanes and typhoons are the basis for design wind speeds for many
portions
of the U.S. and its territories. High winds during a hurricane can create
extreme positive and negative forces on a building; the net result is that
wind
forces simultaneously try to push over the building and lift it off its
foundation. If the foundation is not strong enough to resist these forces,
the
home may slide, overturn, collapse, or incur substantial damage (Figure 1-1).
[Begin figure]
Figure 1-1 is a photo showing wind damage to roof structure and gable end
wall
from Hurricane Katrina (2005) (Pass Christian, Mississippi).
Source: Hurricane Katrina MAT photo
[End figure]
The most current design wind speeds are provided by the American Society of
Civil Engineers (ASCE) document Minimum Design Loads for Buildings and Other
Structures (ASCE 7). ASCE 7 is typically updated every 3 years. The 2002
edition
(ASCE 7-02) is referenced by the following model building codes: the 2009
editions of the International Building Code® (IBC®) and the International
Residential Code® (IRC®).
[Begin text box]
NOTE: Hurricanes are classified into five categories according to the Saffir-
Simpson Scale, which uses wind speed and central pressure as the principal
parameters to categorize storm damage potential. Hurricanes can range from
Category 1 to the devastating Category 5 (Figure 1-2).
Hurricanes can produce storm surge that is higher or lower than what the wind
speed at landfall would predict. While Hurricane Katrina’s storm surge was
roughly that of a Category 5, its winds at landfall were only a Category 3. A
hurricane that is a Category 3 or above is generally considered a major
hurricane.
[End text box]
Design wind speeds given by ASCE 7 are 3-second gust speeds, not the
sustained
wind speeds associated with the Saffir-Simpson hurricane classification scale
(Figure 1-2). Figure 1-3 shows the design wind speeds for portions of the
Gulf
Coast region based on 3-second gusts (measured at 33 feet above the ground in
Exposure C).
[Begin figures]
Figure 1-2. Saffir-Simpson Scale.
Source: Hurricane Katrina in the GULF COAST (FEMA 549)
Category 1 Hurricane – Winds 74 to 95 mph, sustained (91 to 116 mph, 3-second
gust)
No real damage to buildings. Damage to unanchored mobile homes. Some damage
to
poorly constructed signs. Also, some coastal flooding and minor pier damage.
Examples: Hurricanes Irene (1999) and Allison (1995).
Category 2 Hurricane – Winds 96 to 110 mph, sustained (117 to 140 mph, 3-
second
gust)
Some damage to building roofs, doors, and windows. Considerable damage to
mobile
homes. Flooding damages piers, and small crafts in unprotected moorings may
break. Some trees blown down. Examples: Hurricanes Bonnie (1998), Georges (FL
and LA, 1998), and Gloria (1985).
Category 3 Hurricane – Winds 111 to 130 mph, sustained (141 to 165 mph, 3-
second
gust)
Some structural damage to small residences and utility buildings. Large trees
blown down. Mobile homes and poorly built signs destroyed. Flooding near the
coast destroys smaller structures with larger structures damaged by floating
debris. Terrain may be flooded far inland. Examples: Hurricanes Keith (2000),
Fran (1996), Opal (1995), Alicia (1983), Betsy (1965), and Katrina at
landfall
(2005).
Category 4 Hurricane – Winds 131 to 155 mph, sustained (166 to 195 mph, 3-
second
gust)
More extensive curtainwall failures with some complete roof structure failure
on
small residences. Major erosion of beach areas. Terrain may be flooded far
inland. Examples: Hurricanes Hugo (1989), Donna (1960), and Charley (2004).
Category 5 Hurricane – Winds greater than 155 mph, sustained (195 mph and
greater, 3-second gust)
Complete roof failure on many residences and industrial buildings. Some
complete
building failures with small utility buildings blown over or away. Flooding
causes major damage to lower floors of all structures near the shoreline.
Massive evacuation of residential areas may be required. Examples: Hurricanes
Andrew (1992), Camille (1969), and the unnamed Labor Day storm (1935).
Figure 1-3 is a map showing wind speeds (in mph) for the entire U.S.
Source: ASCE 7-05
[End figures]
1.2 Storm Surge
Storm surge is water that is pushed toward the shore by the combined force of
the lower barometric pressure and the wind-driven waves advancing to the
shoreline. This advancing surge combines with the normal tides to create the
hurricane storm tide, which in many areas can increase the sea level by as
much
as 20 to 30 feet. Figure 1-4 is a graphical depiction of how wind-driven
waves
are superimposed on the storm tide. This rise in water level can cause severe
flooding in coastal areas, particularly when the storm tide coincides with
high
tides (Figure 1-5). Because much of the United States’ densely populated
coastlines lie less than 20 feet above sea level, the danger from storm surge
is
great. This is particularly true along the Gulf of Mexico where the shape and
bathymetry of the Gulf contribute to storm surge levels that can exceed most
other areas in the U.S. (Figure 1-6).
[Begin figures]
Figure 1-4 is a graphical depiction of a hurricane moving ashore. In this
example, a 15-foot surge added to the normal 2-foot tide creates a total
storm
tide of 17 feet.
Figure 1-5 is a photo of storm tide and waves from Hurricane Dennis on July
10,
2005, near Panacea, Florida.
Source: U.S. Geological Survey (USGS) Sound Waves Monthly newsletter.
Photograph
courtesy of The Forgotten Coastline (Copyright 2005)
Figure 1-6 is a graph showing the comparison of storm surge levels along the
shorelines of the Gulf Coast for Category 1, 3, and 5 storms.
Source: Hurricane Katrina in the Gulf Coast (FEMA 549)
[End figures]
1.3 Flood Effects
Although coastal flooding can originate from a number of sources, hurricanes
and
weaker tropical storms not categorized as hurricanes are the primary cause of
flooding (Figure 1-2). The flooding can lead to a variety of impacts on
coastal
buildings and their foundations: hydrostatic forces, hydrodynamic forces,
waves,
floodborne debris forces, and erosion and scour.
1.3.1 Hydrostatic Forces
Horizontal hydrostatic forces against a structure are created when the level
of
standing or slowly moving floodwaters on opposite sides of the structure are
not
equal. Flooding can also cause vertical hydrostatic forces, resulting in
flotation. Rapidly rising floodwaters can also cause structures to float off
of
their foundations (Figure 1-7). If floodwaters rise slowly enough, water can
seep into a structure to reduce buoyancy forces. While slowly rising
floodwaters
reduce the adverse effects of buoyancy, any flooding that inundates a home
can
cause extreme damage.
[Begin figure]
Figure 1-7 is a photo showing a building floated off of its foundation
(Plaquemines Parish, Louisiana).
Source: Hurricane Katrina in the gulf coast (FEMA 549)
[End figure]
1.3.2 Hydrodynamic Forces
Moving floodwaters create hydrodynamic forces on submerged foundations and
buildings. These hydrodynamic forces can destroy solid walls and dislodge
buildings with inadequate connections or load paths. Moving floodwaters can
also
move large quantities of sediment and debris that can cause additional
damage.
In coastal areas, moving floodwaters are usually associated with one or more
of
the following:
- Storm surge and wave runup flowing landward through breaks in sand dunes,
levees, or across low-lying areas (Figure 1-8)
[Begin figure]
Figure 1-8 is an aerial view of damage to one of the levees caused by
Hurricane
Katrina (photo taken on August 30, 2005, the day after the storm hit, New
Orleans, Louisiana).
Source: FEMA NEWS PHOTO/ Jocelyn Augustino
[End figure]
- Outflow (flow in the seaward direction) of floodwaters driven into bay or
upland areas by a storm
- Strong currents along the shoreline driven by storm waves moving in an
angular
direction to the shore
High-velocity flows can be created or exacerbated by the presence of manmade
or
natural obstructions along the shoreline and by “weak points” formed by
shore-
normal (i.e., perpendicular to the shoreline) roads and access paths that
cross
dunes, bridges, or shore-normal canals, channels, or drainage features. For
example, evidence after Hurricane Opal (1995) struck Navarre Beach, Florida,
suggests that flow was channeled in between large, engineered buildings. The
resulting constricted flow accelerated the storm surge and caused deep scour
channels across the island. These channels eventually undermined pile-
supported
houses between large buildings while also washing out roads and houses
farther
landward (Figure 1-9).
[Begin figure]
Figure 1-9 is a photo taken during Hurricane Opal (1995). This house was in
an
area of channeled flow between large buildings. As a result, the house was
undermined and washed into the bay behind a barrier island.
Source: COastal Construction manual(FEMA 55)
[End figure]
1.3.3 Waves
Waves can affect coastal buildings in a number of ways. The most severe
damage
is caused by breaking waves (Figures 1-10 and 1-11). The height of these
waves
can vary by flood zone: V zone wave heights can exceed 3 feet, while Coastal
A
zone wave heights are between 1.5 and 3 feet. The force created by waves
breaking against a vertical surface is often ten or more times higher than
the
force created by high winds during a storm event. Waves are particularly
damaging due to their cyclic nature and resulting repetitive loading. Because
typical wave periods during hurricanes range from about 6 to 12 seconds, a
structure can be exposed to 300 to 600 waves per hour, resulting in possibly
several thousand load cycles over the duration of the storm.
[Begin figures]
Figure 1-10i s a photo of storm waves breaking against a seawall in front of
a
coastal residence at Stinson Beach, California.
Source: COastal COnStruction manual (FEMA 55)
Figure 1-11 is a photo of storm surge and waves overtopping a coastal barrier
island in Alabama (Hurricane Frederic, 1979).
Source: COastal COnStruction manual (FEMA 55)
[End figures]
Wave runup occurs as waves break and run up beaches, sloping surfaces, and
vertical surfaces. Wave runup can drive large volumes of water against or
around
coastal buildings, creating hydrodynamic forces (although smaller than
breaking
wave forces), drag forces from the current, and localized erosion and scour.
Wave runup under a vertical surface (such as a wall) will create an upward
force
by the wave action due to the sudden termination of its flow. This upward
force
is much greater than the force generated as a wave moves along a sloping
surface. In some instances, the force is large enough to destroy overhanging
elements such as carports, decks, porches, or awnings. Another negative
effect
of waves is reflection or deflection occurring when a wave is suddenly
redirected as it impacts a building or structure.
1.3.4 Floodborne Debris
Floodborne debris produced by coastal flood events and storms typically
includes
carports, decks, porches, awnings, steps, ramps, breakaway wall panels,
portions
of or entire houses, fuel tanks, vehicles, boats, piles, fences, destroyed
erosion control structures, and a variety of smaller objects (Figure 1-12).
In
some cases, larger pieces of floodborne debris can strike buildings (e.g.,
shipping containers and barges), but the designs contained herein are not
intended to withstand the loads from these larger debris elements. Floodborne
debris is capable of destroying unreinforced masonry walls, light wood frame
construction, and small-diameter posts and piles (and the components of
structures they support). Debris trapped by cross-bracing, closely spaced
piles,
grade beams, or other components is also capable of transferring flood and
wave
loads to the foundation of an elevated structure.
[Begin figure]
Figure 1-12 is a photo of pier piles carried over 2 miles by the storm surge
and
waves of Hurricane Opal (1995) before coming to rest near this elevated house
in
Pensacola Beach, Florida.
Source: Coastal Construction Manual (FEMA 55)
[End figure]
1.3.5 Erosion and Scour
Erosion refers to the wearing and washing away of coastal lands, including
sand
and soil. It is part of the larger process of shoreline changes. Erosion
occurs
when more sediment leaves a shoreline area than enters from either manmade
objects or natural forces. Because of the dynamic nature of erosion, it is
one
of the most complex hazards to understand and difficult to accurately predict
at
any given site along coastal areas.
Short-term erosion changes can occur from storms and periods of high wave
activity, lasting over periods ranging from a few days to a few years.
Because
of the variability in direction and magnitude, short-term erosion (storm-
induced) effects can be orders of magnitude greater than long-term erosion.
Long-term shoreline changes occur over a period of decades or longer and tend
to
average out the short-term erosion. Both short-term and long-term changes
should
be considered in the siting and design of coastal residential construction.
Refer to Chapter 7 of FEMA 55, Coastal Construction Manual for additional
guidance on assessing short- and long-term erosion.
Scour can occur when water flows at high velocities past an object embedded
in
or sitting on soil that can be eroded. Scour occurs around the object itself,
such as a pile or foundation element, and contributes to the loss of support
provided by the soil. In addition to any storm or flood-induced erosion that
occurs in the general area, scour is generally limited to small, cone-shaped
depressions. Localized scour is capable of undermining slabs, piles, and
grade
beam structures, and, in severe cases, can lead to structural failure (Figure
1-
13). This document considers these effects on the foundation size and depth
of
embedment requirements.
[Begin figure]
Figure 1-13 is a photo showing an extreme case of localized scour undermining
a
slab-on-grade house in Topsail Island, North Carolina, after Hurricane Fran
(1996). Prior to the storm, the lot was several hundred feet from the
shoreline
and mapped as an A zone on the FIRM. This case illustrates the need for open
foundations in Coastal A zones.
Source: COastal COnStruction manual (FEMA 55)
[End figure]
FEMA 55 contains guidance on predicting scour, much of which is based on
conditions observed after numerous coastal storms. FEMA 55 suggests that
scour
depths around individual plies be estimated at two times the pile diameter
for
circular piles and two times the diagonal dimension for square or rectangular
piles.
In some storms (e.g., Hurricane Ike, which struck the Texas coast in October
2008), observed scour depths exceed those suggested in FEMA 55. Because
erosion
and scour reduce the resistance of the pile (by reducing its embedment) and
increases stresses within the pile (by increasing the bending moment within
the
pile that the lateral forces create), they can readily cause failure of a
coastal foundation. To help prevent failure, erosion and scour depths should
be
approximated conservatively. After Hurricane Ike, FEMA developed eight
Hurricane
Ike Recovery Advisories (RAs). One of the RAs, Erosion, Scour, and Foundation
Design, discusses erosion and scour and their effects on coastal homes with
pile
foundations and is presented in Appendix F. All of the Hurricane Ike RAs are
available at http://www.fema.gov/library/viewRecord.do?id=3539.
Chapter 2. Foundations
This chapter discusses the primary issues related to designing foundations
for
residential buildings in coastal areas: foundation design criteria, National
Flood Insurance Program (NFIP) requirements on coastal construction in A and
V
zones, the performance of various foundation types, and foundation
construction.
2.1 Foundation Design Criteria
Foundations in coastal areas should be designed in accordance with the 2006
or
2009 edition of the IBC or IRC; both contain up-to-date wind provisions and
are
consistent with NFIP flood provisions. In addition, any locally adopted
building
ordinances must be addressed. Foundations should be designed and constructed
to:
- Properly support the elevated home and resist all loads expected to be
imposed
on the home and its foundation during a design event
- Prevent flotation, collapse, or lateral movement of the building
- Function after being exposed to the anticipated levels of erosion and scour
that may occur over the life of the building.
In addition, the foundation should be constructed with flood-resistant
materials
below the Base Flood Elevation (BFE).
2.2 Foundation Design in Coastal Areas
Building in a coastal environment is different from building in an inland
area
because:
- Storm surge, wave action, and erosion in coastal areas make coastal
flooding
more damaging than inland flooding.
- Design wind speeds are higher in coastal areas and thus require buildings
and
their foundations to be able to resist higher wind loads.
Foundations in coastal areas must be constructed such that the top of the
lowest
floor (in A zones) or the bottom of the lowest horizontal structural members
(in
V zones) of the buildings are elevated above the BFE, while withstanding
flood
forces, high winds, erosion and scour, and floodborne debris. Deeply embedded
pile or other open foundations are required for V zones because they allow
waves
and floodwaters to pass beneath elevated buildings. Because of the increased
flood, wave, floodborne debris, and erosion hazards in V zones, NFIP design
and
construction requirements are more stringent in V zones than in A zones.
[Begin text box]
NFIP Minimum Elevation Requirements for New Construction*
A zone: Elevate top of lowest floor to or above BFE
V zone: Elevate bottom of lowest horizontal structural member supporting the
lowest floor to or above BFE
In both V and A zones, many property owners have decided to elevate one full
story above grade, even if not required, to allow below-building parking.
Fact
Sheet No. 2 of FEMA 499 contains information about NFIP requirements and
recommended best practices in A and V zones (see Appendix F).
[End text box]
Some coastal areas mapped as A zones may also be subject to damaging waves
and
erosion (referred to as “Coastal A zones”). A Coastal A zone is also known as
the Limit of Moderate Wave Action (LiMWA), which is the landward extent of
coastal areas designated Zone AE where waves higher than 1.5 feet can exist
during a design flood. Buildings in these areas that are constructed to
minimum
NFIP A zone requirements may sustain major damage or be destroyed during the
base flood. It is strongly recommended that buildings in A zones subject to
breaking waves and erosion be designed and constructed with V zone type
foundations (Figure 2-1). Open foundations are often recommended instead of
solid wall, crawlspace, slab, or shallow foundations, which can restrict
floodwaters and be undermined easily. Figure 2-2 shows examples of building
failures due to erosion and scour under a slab-on-grade foundation.
[Begin figures]
Figure 2-1 is an illustration of recommended open foundation practice for
buildings in A zones, Coastal A zones, and V zones.
Source: Coastal Construction Manual (FEMA 55)
Figure 2-2 is a photo of a slab-on-grade foundation failure due to erosion
and
scour undermining and closeup of the foundation failure from Hurricane
Dennis,
2005 (Navarre Beach, Florida).
Source: HURRICANE DENNIS MAT PHOTO
[End figures]
2.3 Foundation Styles in Coastal Areas
Several styles of foundations can be used to elevate homes. In discussing
foundation styles, it is beneficial to categorize them as open, closed,
shallow,
or deep.
As the name implies, open foundations generally consist of piles, piers, or
columns and present minimal obstructions to moving floodwaters. With open
foundations, moving floodwaters, breaking waves, and smaller pieces of
floodborne debris should meet relatively few obstructions and hopefully be
able
to pass under the home without imparting large flood loads on the foundation.
Open foundations have the added benefit of disrupting flood flows less than
larger obstructions. This can help to reduce scour around foundation
elements.
On the other hand, closed foundations typically consist of continuous
foundation
walls (constructed of masonry, concrete, or treated wood) that can enclose
crawlspaces or, as in the case of stem walls, areas of retained soils. Closed
foundation walls create large obstructions to moving floodwaters and large
flood
forces can be imparted on them by breaking waves, floodborne debris, and the
hydrodynamic loads associated with moving water. Closed foundations are also
more vulnerable to scour than open foundations.
The terms shallow and deep signify the relative depth of the soils on which
the
homes are founded. Shallow foundations are set on soils that are relatively
close to the surface of the surrounding grade, generally within 3 feet of the
finished grade. In cold climates, shallow foundations may need to be extended
4
feet or more below grade to set the foundation beneath the design frost
depth.
Shallow foundations can consist of discrete concrete pad footings, strip
footings, or a matrix of strip footings placed to create a mat foundation.
Mat
foundations have the added benefit of better resisting uplift and overturning
forces than foundations consisting of discrete pad footings.
Deep foundations are designed to be supported on much deeper soils or rock.
These foundations frequently are used where soils near the surface have
relatively weak bearing capacities (typically 700 pounds per square foot
[psf]
or less), when soils near the surface contain expansive clays (also called
shrink/swell soils because they shrink when dry and swell when wet) or where
surface soils are vulnerable to being removed by erosion or scour.
Although typical foundation styles vary geographically, deep foundations for
residential construction in coastal areas generally consist of driven treated
timber piles or treated square piles. Driven concrete piles are common in
other
areas.
Only open foundations with base members or elements (piles or beams) located
below expected erosion and scour are allowed in V zones; as a “best
practices”
approach, open foundations are recommended, but are not NFIP required, in
Coastal A zones. Table 2-1 shows the recommended type of foundation depending
on
the coastal area. Additional information concerning foundation performance in
coastal areas can be found in FEMA 499, Fact Sheet No. 11 (see Appendix F).
[Begin table]
Table 2-1. Foundation Type Dependent on Coastal Area
Open Foundation Type:
V Zone: Acceptable
Coastal A Zone: Acceptable
A Zone: Acceptable
Closed Foundation Type:
V Zone: not permitted
Coastal A Zone: not recommended
A Zone: Acceptable
[End table]
2.3.1 Open Foundations
Open foundations are required in V zones and recommended in Coastal A zones.
As
previously mentioned, this type of foundation allows water to pass beneath an
elevated building through the foundation and reduces lateral flood loads on
the
structure. Open foundations also have the added benefit of being less
susceptible to damage from floodborne debris because debris is less likely to
be
trapped.
2.3.1.1 Piles
Pile foundations consist of deeply placed vertical piles installed under the
elevated structure. The piles support the elevated structure by remaining
solidly placed in the soil. Because pile foundations are set deeply, they are
inherently more tolerant to erosion and scour. Piles rely primarily on the
friction forces that develop between the pile and the surrounding soils (to
resist gravity and uplift forces) and the compressive strength of the soils
(to
resist lateral movement). The soils at the ends of the piles also contribute
to
resist gravity loads.
Piles are typically treated wood timbers, steel pipes, or pre-cast concrete.
Other materials like fiber reinforced polyester (FRP) are available, but are
rarely used in residential construction. Piles can be used with or without
grade
beams. When used without grade beams, piles extend to the lowest floor of the
elevated structure. Improved performance is achieved when the piles extend
beyond the lowest floor to the roof (or an upper floor level) above. Doing so
provides resistance to rotation (also called “fixity”) in the top of the pile
and improves stiffness of the pile foundation. Occasionally, wood framing
members are installed at the base of a wood pile (Figure 2-3). These members
are
not true grade beams but rather are compression struts. They provide lateral
support for portions of the pile near grade and reduce the potential for
column
buckling; however, due to the difficulties of constructing moment connections
with wood, the compression struts provide very little resistance to rotation.
[Begin figure]
Figure 2-3 is a photo of a compression strut at base of a wood pile. Struts
provide some lateral support for the pile, but very little resistance to
rotation.
Source: COastal COnStruction manual(FEMA 55)
[End figure]
Critical aspects of a pile foundation include the pile size, installation
method, and embedment depth, bracing, and their connections to the elevated
structure (see FEMA 499, Fact Sheet Nos. 12 and 13 in Appendix F). Pile
foundations with inadequate embedment will not have the structural capacity
to
resist sliding and overturning (Figure 2-4). Inadequate embedment and
improperly
sized piles greatly increase the probability for structural collapse.
However,
when properly sized, installed, and braced with adequate embedment into the
soil
(with consideration for erosion and scour effects), a building’s pile
foundation
performance will allow the building to remain standing and intact following a
design flood event (Figure 2-5).
[Begin figures]
Figure 2-4 is a photo of a near collapse of a home due to insufficient pile
embedment (Dauphin Island, Alabama).
Source: Hurricane Katrina in the Gulf Coast (FEMA 549)
Figure 2-5 is a photo of a successful pile foundation following Hurricane
Katrina (Dauphin Island, Alabama).
Source: Hurricane Katrina in the Gulf Coast (FEMA 549)
[End figures]
When used with grade beams, the piles and grade beams work in conjunction to
elevate the structure, provide vertical and lateral support for the elevated
home, and transfer loads imposed on the elevated home and foundation to the
ground below.
Pile foundations with grade beams must be constructed with adequate strength
to
resist all lateral and vertical loads. Failures experienced during Hurricane
Katrina often resulted from inadequate connections between the columns and
footings or grade beams below (Figure 2-6). Pile and grade beam foundations
should be designed and constructed so that the grade beams act only to
provide
fixity to the foundation system and not to support the lowest elevated floor.
If
grade beams support the lowest elevated floor of the home, they become the
lowest horizontal structural member and significantly higher flood insurance
premiums would result. Also, if the grade beams support the structure, the
structure would become vulnerable to erosion and scour. Grade beams must also
be
designed to span between adjacent piles and the piles must be capable of
resisting both the weight of the grade beams when undermined by erosion and
scour, and the loads imposed on them by forces acting on the structure.
[Begin figure]
Figure 2-6 is a photo of a column connection failure (Belle Fontaine Point,
Jackson County, Mississippi).
Source: Hurricane Katrina in the Gulf Coast (FEMA 549)
[End figure]
2.3.1.2 Piers
Piers are generally placed on footings to support the elevated structure.
Without footings, piers function as short piles and rarely have sufficient
capacity to resist uplift and gravity loads. The type of footing used in pier
foundations greatly affects the foundation’s performance (Figure 2-7). When
exposed to lateral loads, discrete footings can rotate so piers placed on
discrete footings are only suitable when wind and flood loads are relatively
low. Piers placed on continuous concrete grade beams or concrete strip
footings
provide much greater resistance to lateral loads because the grade
beams/footings act as an integral unit and are less prone to rotation.
Footings
and grade beams must be reinforced to resist the moment forces that develop
at
the base of the piers due to the lateral loads on the foundation and the
elevated home (Figure 2-8).
[Begin figures]
Figure 2-7 is a photo of a performance comparison of pier foundations. Piers
on
discrete footings (foreground) failed by rotating and overturning while piers
on
more substantial footings (in this case a concrete mat) survived (Pass
Christian, Mississippi).
Source: HURRICANE KATRINA MAT PHOTO
Figure 2-8 is an illustration of an isometric view of an open foundation with
grade beam.
[End figures]
Since pier foundation footings or grade beams are limited in depth of
placement,
they are appropriate only where there is limited potential for erosion or
scour.
The maximum estimated depth for long- and short-term erosion and localized
scour
should not extend below the bottom of the footing or grade beam.
2.3.2 Closed Foundations
A closed foundation is typically constructed using foundation walls, a
crawlspace foundation, or a stem wall foundation (usually filled with
compacted
soil). A closed foundation does not allow water to pass easily through the
foundation elements below the elevated building. Thus, these types of
foundations are said to obstruct the flow. These foundations also present a
large surface area upon which waves and flood forces act; therefore, they are
prohibited in V zones and not recommended for Coastal A zones. If foundation
or
crawlspace walls enclose space below the Base Flood Elevation (BFE), they
must
be equipped with openings that allow floodwaters to flow in and out of the
area
enclosed by the walls (Figure 2-9 presents an isometric view). The entry and
exit of floodwaters will equalize the water pressure on both sides of the
wall
and reduce the likelihood of the wall collapsing (see FEMA 499, Fact Sheet
No.
15 in Appendix F). Two types of closed foundations are discussed in this
manual,
perimeter walls and slab-on-grade.
[Begin figure]
Figure 2-9 is an illustration of an isometric view of a closed foundation
with
crawlspace.
[End figure]
2.3.2.1 Perimeter Walls
Perimeter walls are conventional walls (typically masonry or wood frame) that
extend from the ground up to the elevated building. They typically bear on
shallow footings. Crawlspaces and stem walls are two types of foundations
with
perimeter walls.
Crawlspaces. Crawlspace foundations are typically low masonry perimeter
walls,
some requiring interior piers supporting a floor system if the structure is
wide. These foundations are usually supported by shallow footings and are
prone
to failure caused by erosion and scour.
This type of foundation is characterized by a solid perimeter foundation wall
around a structure with a continuous spread footing with reinforced masonry
or
concrete piers. All crawlspace foundation walls in the Special Flood Hazard
Area
(SFHA) must be equipped with flood openings. These openings are required to
equalize the pressure on either side of the wall (see FEMA 499, Fact Sheet
Nos.
15 and 26 in Appendix F). However, even with flood vents, hydrodynamic and
wave
forces in Coastal A zones can damage or destroy these foundations.
Stem Walls. Stem walls (i.e., a solid perimeter foundation wall on a
continuous
spread footing backfilled to the underside of the floor slab) are similar to
crawlspace foundations, but the interior that would otherwise form the
crawlspace is filled with soil or gravel that supports a floor slab. Stem
wall
foundations have been observed to perform better than crawlspace foundations
in
Coastal A zones (but only where erosion and scour effects are minor). Flood
openings are not required in filled stem wall foundations.
2.3.2.2 Slab-on-Grade
A slab-on-grade foundation is concrete placed directly on-grade (to form the
slab) with generally thickened, reinforced sections around the edges and
under
loadbearing walls. The slab itself is typically 4 inches thick where not
exposed
to concentrated loads and 8 to 12 inches thick under loadbearing walls. The
thickened portions of slab-on-grade foundations are typically reinforced with
deformed steel bars to provide structural support; the areas not thickened
are
typically reinforced with welded wire fabrics (WWFs) for shrinkage control.
While commonly used in residential structures in A zones, slab-on-grade
foundations are prone to erosion, prohibited in V zones, and not recommended
for
Coastal A zones.
Slab-on-grade foundations can be used with structural fill to elevate
buildings.
Fill is usually placed in layers called “lifts” with each lift compacted at
the
site. Because fill is susceptible to erosion, it is prohibited for providing
structural support in V zones. Structural fill is not recommended for Coastal
A
zones, but may be appropriate for non-Coastal A zones.
2.4 Introduction to Foundation Design and Construction
This section introduces two main issues related to foundation design and
construction: site characterization and types of foundation construction.
Construction materials and methods are addressed in Chapter 4.
2.4.1 Site Characterization
The foundation design chosen should be based on the characteristics that
exist
at the building site. A site characteristic study should include the
following:
- The type of foundations that have been installed in the area in the past. A
review of the latest FIRM is recommended to ensure that construction
characteristics have not been changed.
- The proposed site history, which would indicate whether there are any
buried
materials or if the site has been regraded.
- How the site may have been used in the past, from a search of land records
for
past ownership.
- A soil investigation report, which should include:
- Soil borings sampled from the site or taken from test pits
- A review of soil borings from the immediate area adjacent to the site
- Information from the local office of the Natural Resource Conservation
Service
(NRCS) (formerly the Soil Conservation Service [SCS]) and soil surveys
published
for each county
One of the parameters derived from a soil investigation report is the bearing
capacity, which measures the ability of soils to support gravity loads
without
soil shear failure or excessive settlement. Measured in psf, soil bearing
capacity typically ranges from 1,000 psf for relatively weak soils to over
10,000 psf for bedrock.
Frequently, designs are initially prepared on a presumed bearing capacity. It
is
then the homebuilder’s responsibility to verify actual site conditions. The
actual soil bearing capacity should be determined. If soils are found to have
higher bearing capacity, the foundation can be constructed as designed or the
foundation can be revised to take advantage of the better soils.
Allowable load bearing values of soils given in Section 1806 of the 2009 IBC
can
be used when other data are not available. However, soils can vary
significantly
in bearing capacity from one site to the next. A geotechnical engineer should
be
consulted when any unusual or unknown soil condition is encountered.
2.4.2 Types of Foundation Construction
2.4.2.1 Piles
A common type of pile foundation is the elevated wood pile foundation, where
the
piles extend from deep in the ground to an elevation at or above the Design
Flood Elevation (DFE). Horizontal framing members are used to connect the
piles
in both directions. This grid forms a platform on which the house is built
(see
FEMA 499, Fact Sheet No. 12 in Appendix F).
The method of installation is a major consideration in the structural
integrity
of pile foundations. The ideal method is to use a pile driver. In this
method,
the pile is held in place with leads while a single-acting, double-acting
diesel, or air-powered hammer drives the pile into the ground (Figure 2-10).
[Begin figure]
Figure 2-10 is an illustration of pile installation methods. Driven: hammer
(or
vibratory hammer) drives pile. Augered: hammer to set in place. Jetted: water
injected to create hole for pile, then hammer to set in place.
Source: COastal COnstruction manual(FEMA 55)
[End figure]
If steel piles are used, only the hammer driving method mentioned above
should
be used. For any pile driving, the authority having jurisdiction or the
engineer-of-record may require that a driving log is kept for each pile. The
log
will tabulate the number of blows per foot as the driving progresses. This
log
is a key factor used in determining the pile capacity.
Another method for driving piles is the drop hammer method. It is a lower
cost
alternative to the pile driver. A drop hammer consists of a heavy weight
raised
by a cable attached to a power-driven winch and then dropped onto the pile.
Holes for piles may be excavated by an auger if the soil has sufficient clay
or
silt content. Augering can be used by itself or in conjunction with pile
driving. If the hole is full-sized, the pile is dropped in and the void
backfilled. Alternatively, an undersized hole can be drilled and a pile
driven
into it. When the soil conditions are appropriate, the hole will stay open
long
enough to drop or drive in a pile. In general, this method may not have as
much
capacity as those methods previously mentioned. Like jetted piles, augered
piles
are not appropriate for the designs provided in this manual unless the method
for compressing the soil is approved by a geotechnical engineer.
A less desirable but frequently used method of inserting piles into sandy
soil
is “jetting,” which involves forcing a high-pressure stream of water through
a
pipe that advances with the pile. The water creates a hole in the sand as the
pile is driven until the required depth is reached. Unfortunately, jetting
loosens the soil both around the pile and the tip. This results in a lower
load
capacity due to less frictional resistance. Jetted piles are not appropriate
for
the designs provided in this manual unless capacity is verified by a
geotechnical engineer.
2.4.2.2 Diagonal Bracing of Piles
The foundation design may include diagonal bracing to stiffen the pile
foundation in one or more directions. When installed properly, bracing lowers
the point where lateral loads are applied to the piles. The lowering of load
application points reduces the bending forces that piles must resist (so
piles
in a braced pile foundation do not need to be as strong as piles in an
unbraced
pile foundation) and also reduces lateral movement in the building. Outside
piles are sufficiently designed to withstand external forces, because bracing
will not assist in countering these forces. A drawback to bracing, however,
is
that the braces themselves can become obstructions to moving floodwaters and
increase a foundation’s exposure to wave and debris impact.
Because braces tend to be slender, they are vulnerable to compression
buckling.
Therefore, most bracing is considered tension-only bracing. Because wind and,
to
a lesser extent, flood loads can act in opposite directions, tension-only
bracing must be installed in pairs. One set of braces resists loads from one
direction while the second set resists loads from the opposite direction.
Figure
2-11 shows how tension only bracing pairs resist lateral loads on a home.
[Begin figure]
Figure 2-11 us a diagonal bracing schematic.
[End figure]
The braced pile design can only function when all of the following conditions
are met:
- The home must be constructed with a stiff horizontal diaphragm such as a
floor
system that transfers loads to laterally braced piles.
- Solid connections, usually achieved with bolts, must be provided to
transmit
forces from the brace to the pile or floor system.
The placement of the lower bolted connection of the diagonal to the pile
requires some judgment. If the connection is placed too high above grade, the
pile length below the connection is not braced and the overall bracing is
less
strong and stiff. If the connection is placed too close to grade, the bolt
hole
is more likely to be flooded or infested with termites. Because the bolt hole
passes through the untreated part of the pile, flooding and subsequent decay
or
termite infestation will weaken the pile at a vulnerable location. Therefore,
the bolt hole should be treated with preservative after drilling and prior to
bolt placement.
The braced wood pile designs developed for this manual use steel rods for
bracing. Steel rods were used because:
- Steel has greater tensile strength than even wide dimensional lumber.
- There are fewer obstructions to waves and floodborne debris.
- The rod bracing can easily be tensioned with turnbuckles and can be
adjusted
throughout the life of the home.
- A balanced double shear connection is two to three times stronger than a
wood
to wood connection made with 2-inch thick dimensional lumber.
Alternative bracing should only be installed when designed by a licensed
engineer.
2.4.2.3 Knee Bracing of Piles
Knee braces involve installing short diagonal braces between the upper
portions
of the piles and the floor system of the elevated structure. The braces
increase
the stiffness of an elevated pile foundation and can be effective at
resisting
the lateral forces on a home. Although knee braces do not stiffen a
foundation
as much as diagonal bracing, they do offer some advantages over diagonal
braces.
For example, knee braces present less obstruction to waves and debris, are
shorter than diagonal braces, and are usually designed for both tension and
compression loads. Unlike diagonal braces, knee braces do not reduce bending
moments in the piles (they can actually increase bending moments) and will
not
reduce the diameter of the piles required to resist lateral loads.
The entire load path into and through the knee brace must be designed with
sufficient capacity. The connections at each end of each knee brace must have
sufficient capacity to handle both tension and compression and to resist
axial
loads in the brace. The brace itself must have sufficient cross-sectional
area
to resist compression and tensile loads. Due to the complexity of knee
bracing,
they have not been used in the foundation designs included in Appendix A
herein.
2.4.2.4 Wood-Pile-to-Wood-Girder Connections
Wood piles are often notched to provide a bearing surface for a girder.
However,
a notch should not reduce more than 50 percent of the pile cross-section
(such
information is typically provided by a designer on contract documents). For
proper load transfer, the girder should bear on the surface of the pile
notch.
Although connections play an integral role in the design of structures, they
are
typically regarded as the weakest link. The connection between a wood pile
and
the elevated structure should be designed by a licensed engineer (see FEMA
499,
Fact Sheet No. 13 in Appendix F).
2.4.2.5 Grade Beams in Pile/Column Foundations
Grade beams are sometimes used in conjunction with pile and column
foundations
to generate more stiffness. They generate stiffness by forcing the piles to
move
as a group rather than individually and by providing fixity (i.e., resistance
to
rotation) at the ends of the piles. Typically, they extend in both directions
and are usually made of reinforced concrete. The mix design, the amount and
placement of reinforcement, the cover, and the curing process are important
parameters in optimizing durability. To reduce the effect of erosion and
scour
on foundations, grade beams must be designed to be self-supporting foundation
elements. The supporting piers should be designed to carry the weight of the
grade beams and resist all loads transferred to the piers.
In V zones, grade beams must be used only for lateral support of the piles.
If,
during construction, the floor is made monolithic with the grade beams, the
bottom of the beams become the lowest horizontal structural member. This
elevation must be at or above the BFE.
If grade beams are used with wood piles, the possibility of rot occurring
must
be considered when designing the connection between the grade beam and the
pile.
The connection must not encourage water retention. The maximum bending moment
in
the piles occurs at the grade beams, and decay caused by water retention at
critical points in the piles could induce failure under high-wind or flood
forces.
Chapter 3. Foundation Design Loads
This chapter provides guidance on how to determine the magnitude of the loads
placed on a building by a particular natural hazard event or a combination of
events. The methods presented are intended to serve as the basis of a
methodology for applying the calculated loads to the building during the
design
process.
The process for determining site-specific loads from natural hazards begins
with
identifying the building codes or engineering standards in place for the
selected site (e.g., the International Building Code 2009 (IBC 2009) or ASCE
7-
05, Minimum Design Loads for Buildings and Other Structures), if model
building
codes and other building standards do not provide load determination and
design
guidance for each of the hazards identified. In these instances, supplemental
guidance such as FEMA 55 should be sought, the loads imposed by each of the
identified hazards should be calculated, and the load combinations
appropriate
for the building site should be determined. The load combinations used in
this
manual are those specified by ASCE 7-05, the standard referenced by the IBC
2009. Either allowable stress design (ASD) or strength design methods can be
used to design a building. For this manual, all of the calculations,
analyses,
and load combinations presented are based on ASD. The use of strength design
methods will require the designer to modify the design values to accommodate
strength design concepts. Assumptions utilized in this manual can be found in
Appendix C.
3.1 Wind Loads
Wind loads on a building structure are calculated using the methodology
presented in ASCE 7-05. This document is the wind standard referenced by the
2003 editions of the IBC and IRC. Equations used to calculate wind loads are
presented in Appendix D.
The most important variable in calculating wind load is the design wind
speed.
Design wind speed can be obtained from the local building official or the
ASCE
7-05 wind speed map (Figure 3-1). The speeds shown in this figure are 3-
second
gust speeds for Exposure Category C at a 33-foot (10-meter) height. ASCE 7-05
includes scaling factors for other exposures and heights.
[Begin figure]
Figure 3-1 shows wind speeds (in mph) for the entire U.S.
Source: ASCE 7-05
[End figure]
ASCE 7-05 specifies wind loads for structural components known as a main wind
force resisting system (MWFRS). The foundation designs developed for this
manual
are based on MWFRS pressures calculated for Exposure Category C, the category
with the highest anticipated wind loads for land-based structures.
ASCE 7-05 also specifies wind loads for components and cladding (C&C).
Components and cladding are considered part of the building envelope, and
ASCE
7-05 requires C&C to be designed to resist higher wind pressures than a
MWFRS.
3.2 Flood Loads
This manual develops in more detail flood load calculations and incorporates
the
methodology presented in ASCE 7-05. Although wind loads can directly affect a
structure and dictate the actual foundation design, the foundation is more
affected by flood loads. ASCE 24-05 discusses floodproof construction. Loads
developed in ASCE 24-05 come directly from ASCE 7-05, which is what the
designs
presented herein are based upon.
The effects of flood loads on buildings can be exacerbated by storm-induced
erosion and localized scour, and by long-term erosion. Erosion and scour
lower
the ground surface around foundation members and can cause the loss of load-
bearing capacity and resistance to lateral and uplift loads. Erosion and
scour
also increase flood depths and, therefore, increase depth dependent flood
loads.
3.2.1 Design Flood and DFE
The design flood is defined by ASCE 7-05 as the greater of the following two
flood events:
1. Base flood, affecting those areas identified as SFHAs on the community’s
FIRM, or
2. The flood corresponding to the area designated as a flood hazard area on a
community’s flood hazard map or otherwise legally designated.
The DFE is defined as the elevation of the design flood, including wave
height
and freeboard, relative to the datum specified on a community’s flood hazard
map. Figure 3-2 shows the parameters that determine or are affected by flood
depth.
[Begin figure]
Figure 3-2 is an illustration showing parameters that determine or are
affected
by flood depth.
Source: Coastal Construction Manual (FEMA 55)
[End figure]
3.2.2 Design Stillwater Flood Depth (d sub s)
Design stillwater flood depth (d sub s) is the vertical distance between the
eroded ground elevation and the stillwater flood elevation associated with
the
design flood. Determining the maximum design stillwater flood depth over the
life of a building is the single most important flood load calculation that
will
be made; nearly all other coastal flood load parameters or calculations
(e.g.,
hydrostatic load, design flood velocity, hydrodynamic load, design wave
height,
DFE, debris impact load, local scour depth) depend directly or indirectly on
the
design stillwater flood depth. The design stillwater flood depth (d sub s) is
defined as:
d sub s = E sub sw – GS
Where
D sub s = Design stillwater flood depth (ft)
E sub sw = Design stillwater flood elevation (ft) above the datum (e.g.,
National Geodetic Vertical Datum [NGVD], North American Vertical Datum
[NAVD]),
including wave setup effects
GS = Lowest eroded ground elevation above datum (ft), adjacent to building,
including the effects of localized sour around piles
GS is not the lowest existing pre-flood ground surface; it is the lowest
ground
surface that will result from long-term erosion and the amount of erosion
expected to occur during a design flood, excluding local scour effects. The
process for determining GS is described in Chapter 7 of FEMA 55.
Values for E sub sw are not shown on a FIRM, but they are given in the Flood
Insurance Study (FIS) report, which is produced in conjunction with the FIRM
for
a community. FIS reports are usually available from community officials, from
NFIP State Coordinating Agencies, and on the web at the FEMA Map Service
Center
(http://store.msc.fema.gov). Some States have FIS reports available on their
individual web sites.
3.2.3 Design Wave Height (H sub b)
The design wave height at a coastal building site will be one of the most
important design parameters. Therefore, unless detailed analysis shows that
natural or manmade obstructions will protect the site during a design event,
wave heights at a site will be calculated from Equation 5-2 of ASCE 7-05 as
the
heights of depth-limited breaking waves (H sub b), which are equivalent to
0.78
times the design stillwater flood depth:
H sub b = 0.78d sub s
Note: 70 percent of the breaking wave height (0.7Hb) lies above the
stillwater
flood level.
3.2.4 Design Flood Velocity (V)
Estimating design flood velocities in coastal flood hazard areas is subject
to
considerable uncertainty. Little reliable historical information exists
concerning the velocity of floodwaters during coastal flood events. The
direction and velocity of floodwaters can vary significantly throughout a
coastal flood event, approaching a site from one direction during the
beginning
of the flood event before shifting to another (or several directions).
Floodwaters can inundate some low-lying coastal sites from both the front
(e.g.,
ocean) and the back (e.g., bay, sound, river). In a similar manner, flow
velocities can vary from close to zero to high velocities during a single
flood
event. For these reasons, flood velocities should be estimated conservatively
by
assuming that floodwaters can approach from the most critical direction and
that
flow velocities can be high.
For design purposes, the Commentary of ASCE 7-05 suggested a range of flood
velocities from:
V = d sub s ÷ t (expected lower bound)
to
V = (gd sub s) 0.5 (expected upper bound)
Where
V = Average velocity of water in ft/s
d sub s = Design stillwater flood depth
t = Time (1 second)
g = Gravitational constant (32.2 ft/sec squared)
Factors that should be considered before selecting the upper- or lower-bound
flood velocity for design include:
- Flood zone
- Topography and slope
- Distance from the source of flooding
- Proximity to other buildings or obstructions
The upper bound should be taken as the design flood velocity if the building
site is near the flood source, in a V zone, in an AO zone adjacent to a V
zone,
in an A zone subject to velocity flow and wave action, steeply sloping, or
adjacent to other buildings or obstructions that will confine floodwaters and
accelerate flood velocities. The lower bound is a more appropriate design
flood
velocity if the site is distant from the flood source, in an A zone, flat or
gently sloping, or unaffected by other buildings or obstructions.
3.3 Hydrostatic Loads
Hydrostatic loads occur when standing or slowly moving water comes into
contact
with a building or building component. These loads can act laterally
(pressure)
or vertically (buoyancy).
Lateral hydrostatic forces are generally not sufficient to cause deflection
or
displacement of a building or building component unless there is a
substantial
difference in water elevation on opposite sides of the building or component;
therefore, the NFIP requires that floodwater openings be provided in vertical
walls that form an enclosed space below the BFE for a building in an A zone.
Lateral hydrostatic force is calculated by the following:
F sub stat = ½ gamma d sub s squared
Where
F sub stat = Hydrostatic force per unit width (lb/ft) resulting from flooding
against vertical element
gamma = Specific weight of water (62.4 lb/ft cubed for freshwater and 64
lb/ft
cubed for saltwater)
Vertical hydrostatic forces during design flood conditions are not generally
a
concern for properly constructed and elevated coastal buildings. Buoyant or
flotation forces on a building can be of concern if the actual stillwater
flood
depth exceeds the design stillwater flood depth.
Vertical (buoyancy) hydrostatic force is calculated by the following:
F sub Buoy = gamma (Vol)
Where
F sub Buoy = vertical hydrostatic force (lb) resulting from the displacement
of
a given volume of floodwater
Vol = volume of floodwater displaced by a submerged object (ft cubed) =
displaced area x depth of flooding
Buoyant force acting on an object must be resisted by the weight of the
object
and any other opposing force (e.g., anchorage forces) resisting flotation. In
the case of a building, the live load on floors should not be counted on to
resist buoyant forces.
3.4 Wave Loads
Calculating wave loads requires information about expected wave heights. For
the
purposes of this manual, the calculations will be limited by water depths at
the
site of interest. Wave forces can be separated into four categories:
- Non-breaking waves (can usually be computed as hydrostatic forces against
walls and hydrodynamic forces against piles)
- Breaking waves (short duration but large magnitude forces against walls and
piles)
- Broken waves (similar to hydrodynamic forces caused by flowing or surging
water)
- Uplift (often caused by wave runup, deflection, or peaking against the
underside of horizontal surfaces)
Of these four categories, the forces from breaking waves are the largest and
produce the most severe loads. Therefore, it is strongly recommended that the
breaking wave load be used as the design wave load.
Two breaking wave loading conditions are of interest in residential
construction: waves breaking on small-diameter vertical elements below the
DFE
(e.g., piles, columns in the foundation of a building in a V zone) and waves
breaking against vertical walls below the DFE (e.g., solid foundation walls
in A
zones, breakaway walls in V zones).
3.4.1 Breaking Wave Loads on Vertical Piles
The breaking wave load (F sub brkp) on a pile can be assumed to act at the
stillwater flood level and is calculated by Equation 5-4 from ASCE 7-05:
F sub brkp = (1/2)CD gamm DH sub b squared
Where
F sub brkp = Net wave force (lb)
CD = Coefficient of drag for breaking waves = 1.75 for round piles or column,
and 2.25 for square piles or columns
gamma = Specific weight of water (lb/ft cubed)
D = Pile or column diameter (ft) for circular section. For a square pile or
column, 1.4 times the width of the pile or column (ft).
H sub b = Breaking wave height (ft)
3.4.2 Breaking Wave Loads on Vertical Walls
The net force resulting from a normally incident breaking wave (depth limited
in
size, with H sub b = 0.78d sub s) acting on a rigid vertical wall, can be
calculated by Equation 5-6 from ASCE 7-05:
F sub b sub rkw = 1.1C sub p gamma d sub s squared + 2.4 gamma d sub s
squared
Where
F sub brkw = net breaking wave force per unit length of structure (lb/ft)
acting
near the stillwater flood elevation
C sub p = Dynamic pressure coefficient (1.6 < C sub p < 3.5) (see Table 3-1)
[Begin table]
Table 3-1. Building Category and Corresponding Dynamic Pressure Coefficient
(C
sub p)
Building Category I – Buildings and other structures that represent a low
hazard
to human life in the event of a failure
Cp: 1.6
Building Category II – Buildings not in Category I, III, or IV
Cp: 2.8
Building Category III – Buildings and other structures that represent a
substantial hazard to human life in the event of a failure
Cp: 3.2
Building Category IV – Buildings and other structures designated as essential
facilities
Cp: 3.5
Source: ASCE 7-02
[End table]
gamma = Specific weight of water (lb/ft cubed)
d sub s = Design stillwater flood depth (ft) at base of building where the
wave
breaks
This formula assumes the following:
- The vertical wall causes a reflected or standing wave against the seaward
side
of the wall with the crest of the wave, reaching a height of 1.2d sub s above
the design stillwater flood elevation, and
- The space behind the vertical wall is dry, with no fluid balancing the
static
component of the wave force on the outside of the wall (Figure 3-3).
[Begin figure]
Figure 3-3 is an illustration of normally incident breaking wave pressures
against a vertical wall (space behind vertical wall is dry).
Source: ASCE 7-05
[End figure]
If free-standing water exists behind the wall (Figure 3-4), a portion of the
hydrostatic component of the wave pressure and force disappears and the net
force can be computed using Equation 5-7 from ASCE 7-05:
[Begin figure]
Figure 3-4 is an illustration of normally incident breaking wave pressures
against a vertical wall (stillwater level equal on both sides of wall).
Source: ASCE 7-05
[End figure]
F sub brkw = 1.1C sub p gamma d sub s squared + 1.9 gamma d sub s squared
Post-storm damage inspections show that breaking wave loads have virtually
destroyed all wood frame or unreinforced masonry walls below the wave crest
elevation; only highly engineered, massive structural elements are capable of
withstanding breaking wave loads. Damaging wave pressures and loads can be
generated by waves much lower than the 3-foot wave currently used by FEMA to
distinguish between A and V zones.
3.5 Hydrodynamic Loads
Water flowing around a building (or a structural element or other object)
imposes additional loads on the building. The loads (which are a function of
flow velocity and structural geometry) include frontal impact on the upstream
face, drag along the sides, and suction on the downstream side. This manual
assumes that the velocity of the floodwaters is constant (i.e., steady state
flow).
One of the most difficult steps in quantifying loads imposed by moving water
is
determining the expected flood velocity. Refer to Section 3.2.4 for guidance
concerning design flood velocities.
The following equation from FEMA 55 can be used to calculate the hydrodynamic
load from flows with velocity greater than 10 ft/sec:
F sub dyn = ½C sub d rho V squared A
Where
F sub dyn = Hydrodynamic force (lb) acting at the stillwater mid-depth
(halfway
between the stillwater elevation and the eroded ground surface)
C sub d = Drag coefficient (recommended values are 2.0 for square or
rectangular
piles and 1.2 for round piles)
rho = Mass density of fluid (1.94 slugs/ft cubed for freshwater and 1.99
slugs/ft cubed for saltwater)
V = Velocity of water (ft/sec)
A = Surface area of obstruction normal to flow (ft squared)
Note that the use of this formula will provide the total force against a
building of a given impacted surface area (A). Dividing the total force by
either length or width would yield a force per unit length; dividing by “A”
would yield a force per unit area.
The drag coefficient used in the previously stated equations is a function of
the shape of the object around which flow is directed. If the object is
something other than a round, square, or rectangular pile, the drag
coefficient
can be determined using Table 3-2.
[Begin table]
Table 3-2. Drag Coefficient Based on Width to Depth Ratio
Width to Depth Ratio (w/d sub s or w/h)
Drag Coefficient (C sub d)
Width to Depth Ratio: 1 to 12
Drag Coefficient: 1.25
Width to Depth Ratio: 13 to 20
Drag Coefficient: 1.30
Width to Depth Ratio: 21 to 32
Drag Coefficient: 1.40
Width to Depth Ratio: 33 to 40
Drag Coefficient: 1.50
Width to Depth Ratio: 41 to 80
Drag Coefficient: 1.75
Width to Depth Ratio: 81 to 120
Drag Coefficient: 1.80
Width to Depth Ratio: >120
Drag Coefficient: 2.00
Note: “h” refers to the height of an object completely immersed in water.
Source: COASTAL CONSTRUCTION MANUAL (FEMA 55)
[End table]
Flow around a building or building component will also create flow-
perpendicular
forces (lift forces). If the building component is rigid, lift forces can be
assumed to be small. But if the building component is not rigid, lift forces
can
be greater than drag forces. The formula for lift force is similar to the
formula for hydrodynamic force except that the drag coefficient (C sub d) is
replaced with the lift coefficient (C sub l). For the purposes of this
manual,
the foundations of coastal residential buildings can be considered rigid, and
hydrodynamic lift forces can therefore be ignored.
3.6 Debris Impact Loads
Debris or impact loads are imposed on a building by objects carried by moving
water. The magnitude of these loads is very difficult to predict, yet some
reasonable allowance must be made for them. The loads are influenced by where
the building is located in the potential debris stream:
- Immediately adjacent to or downstream from another building
- Downstream from large floatable objects (e.g., exposed or minimally covered
storage tanks)
- Among closely spaced buildings
The following equation to calculate the magnitude of impact load is provided
in
the Commentary of ASCE 7-05:
F sub i = (pi WV sub b C sub I C sub O C sub D C sub B R sub max) ÷ (2g Delta
t)
Where
F sub i = Impact force acting at the stillwater level (lb)
pi = 3.14
W = Weight of debris (lb), suggest using 1,000 if no site-specific
information
is available
V sub b = Velocity of object (assume equal to velocity of water) (ft/sec)
C sub I = Importance coefficient (see Table C5-1 of ASCE 7-05)
C sub O = Orientation coefficient = 0.8
C sub D = Depth coefficient (see Table C5-2 and Figure C5-1 of ASCE 7-05)
C sub B = Blockage coefficient (see Table C5-3 and Figure C5-2 of ASCE 7-05)
R sub max= Maximum response ratio for impulsive load (see Table C5-4 of ASCE
7-
05)
G = Gravitational constant (32.2 ft/sec2)
Delta t = Duration of impact (sec)
When the C coefficients and R sub max are set to 1.0, the above equation
reduces
to
F sub i = (pi WV) ÷ (2g Delta t)
This equation is very similar to the equation provided in ASCE 7-98 and FEMA
55.
The only difference is the pi/2 term, which results from the half-sine form
of
the impulse load.
The following uncertainties must be quantified before the impact of debris
loading on the building can be determined using the above equation:
- Size, shape, and weight (W) of the waterborne object
- Flood velocity (V)
- Velocity of the object compared to the flood velocity
- Portion of the building that will be struck and most vulnerable to
collapsing
- Duration of the impact (t)
Once floodborne debris impact loads have been quantified, decisions must be
made
on how to apply them to the foundation and how to design foundation elements
to
resist them. For open foundations, the Coastal Construction Manual (FEMA 55)
advises applying impact loading to a corner or critical column or pile
concurrently with other flood loads (see FEMA 55, Table 11-6). For closed
foundations (which are not recommended in Coastal A zones and are not allowed
in
V zones), FEMA 55 advises that the designer assume that one corner of the
foundation will be destroyed by debris and recommends the foundation and the
structure above be designed to contain redundancy to allow load
redistribution
to prevent collapse or localized failure. The following should be considered
in
determining debris impact loads:
Size, shape, and weight of the debris. It is recommended that, in the absence
of
information about the nature of the potential debris, a weight of 1,000
pounds
be used for the debris weight (W). Objects of this weight could include
portions
of damaged buildings, utility poles, portions of previously embedded piles,
and
empty storage tanks.
Debris velocity. Flood velocity can be approximated by one of the equations
discussed in Section 3.2.4. For the calculation of debris loads, the velocity
of
the waterborne object is assumed to be the same as the flood velocity. Note
that, although this assumption may be accurate for small objects, it will
overstate debris velocities for large objects (e.g., trees, logs, pier
piles).
The Commentary of ASCE 7-05 provides guidance on estimating debris velocities
for large debris.
Portion of building to be struck. The object is assumed to be at or near the
water surface level when it strikes the building. Therefore, the object is
assumed to strike the building at the stillwater flood level.
Duration of impact. Uncertainty about the duration of impact (Delta t) (the
time
from initial impact, through the maximum deflection caused by the impact, to
the
time the object leaves) is the most likely cause of error in the calculation
of
debris impact loads. ASCE 7-05 showed that measured impact duration (from
initial impact to time of maximum force) from laboratory tests varied from
0.01
to 0.05 second. The ASCE 7-05 recommended value for Delta t is 0.03 second.
3.7 Erosion and Localized Scour
[Begin text box]
NOTE: The method for determining debris impact loads in ASCE 7-05 was
developed
for riverine impact loads and has not been evaluated for coastal debris that
may
impact a building over several wave cycles. Although these impact loads are
very
large but of short duration, a structural engineer should be consulted to
determine the structural response to the short load duration (0.03 second
recommended).
[End text box]
Erosion is defined by Section 1-2 of ASCE 24-05 as the "wearing away of the
land
surface by detachment and movement of soil and rock fragments, during a flood
or
storm or over a period of years, through the action of wind, water, or other
geological processes." Section 7.5 of FEMA 55 describes erosion as “the
wearing
or washing away of coastal lands.” Since the exact configuration of the soil
loss is important for foundation design purposes, a more specific definition
is
used in this document (see the text box above and Figure 3-5).
[Begin text box]
Erosion refers to a general lowering of the ground surface over a wide area.
Scour refers to a localized loss of soil, often around a foundation element.
[End text box]
[Begin figure]
Figure 3-5 is an illustration of distinguishing between coastal erosion and
scour. A building may be subject to either or both, depending on the building
location, soil characteristics, and flood conditions.
[End figure]
Waves and currents during coastal flood conditions are capable of creating
turbulence around foundation elements and causing localized scour, and the
moving floodwaters can cause generalized erosion. Determining potential for
localized scour and generalized erosion is critical in designing coastal
foundations to ensure that failure during and after flooding does not occur
as a
result of the loss in either bearing capacity or anchoring resistance around
the
posts, piles, piers, columns, footings, or walls. Localized scour and
generalized erosion determinations will require knowledge of the flood depth,
flow conditions, soil characteristics, and foundation type.
In some locations, soil at or below the ground surface can be resistant to
localized scour, and scour depths calculated below will be excessive. In
instances where the designer believes the soil at a site will be scour-
resistant, a geotechnical engineer should be consulted before calculated
scour
depths are reduced.
3.7.1 Localized Scour Around Vertical Piles
The methods for calculating localized scour (S sub max) in coastal areas have
been largely based on empirical evidence gathered after storms. Much of the
evidence gathered suggests that localized scour depths around piles and other
thin vertical members are approximately equal to 1.0 to 1.5 times the pile
diameter. Figure 3-6 illustrates localized scour at a pile, with and without
a
scour-resistant terminating stratum. Currently, there is no design guidance
in
ASCE 7-05 on how to calculate scour. FEMA 55 suggests that localized scour
around a foundation element be calculated by the following equation:
S sub max = 2.0a
Where
S sub max = Maximum localized scour depth (ft)
a = Diameter of a round foundation element or the maximum diagonal cross-
section
dimension for a rectangular element (ft)
[Begin figure]
Figure 3-6 is an illustration of scour at vertical foundation member stopped
by
underlying scour-resistant stratum.
Source: COastal COnstruction manual(FEMA 55)
[end figure]
[Begin text box]
NOTE: Resisting higher bending moments brought about by erosion and scour may
necessitate a larger cross-section or decreased pile spacing (i.e., more
piles)
or, in some cases, use of a different pile material (e.g., concrete or steel
instead of wood). Resisting increased lateral flood loads brought about by
erosion (and possibly by linear scour) would necessitate a similar approach.
However, designers should remember that increasing the number of piles or
increasing the pile diameter will, in turn, also increase lateral flood loads
on
the foundation.
Resisting increased unbraced lengths brought about by erosion and scour will
require deeper embedment of the foundation into the ground.
[End text box]
However, recent storms (e.g., Hurricane Ike, which struck the Texas coast in
October 2008) have produced localized scour that exceeded the suggested
depths.
Because scour, coupled with erosion, can cause foundation systems to fail, a
more conservative approach should be considered. Foundation systems should be
analyzed for their ability to resist scour depths of 3 to 4 times pile
diameters
in addition to anticipated erosion levels. This guidance is more conservative
than what has been recommended in FEMA 55, FEMA 499, and other publications.
Erosion and scour can have several adverse impacts on coastal foundations:
- Erosion and scour can reduce the embedment of the foundation into the soil,
causing shallow foundations to collapse and making buildings on deep
foundations
more susceptible to settlement, lateral movement, or overturning from lateral
loads.
- Erosion and scour can increase the unbraced length of pile foundations,
increase the bending moment to which they are subjected, and overstress
piles.
- Erosion over a large area between a foundation and a flood source can
expose
the foundation to increased lateral flood loads (i.e., greater stillwater
depths, possible higher wave heights, and higher flow velocities).
- Local scour around individual piles will not generally expose foundations
to
greater flood loads, but scour across a building site may do so.
To illustrate these points, calculations were made to examine the effects of
erosion and scour on foundation design for a simple case – a 32-foot x 32-
foot
two-story home (10-foot story height), situated away from the shoreline and
elevated 8 feet above grade on 25 square timber piles (spaced 8 feet apart),
on
medium dense sand. The home was subjected to a design wind event with a 130-
mph
(3-second gust speed) wind speed and a 4-foot stillwater depth above the
uneroded grade, with storm surge and broken waves passing under the elevated
building. Lateral wind and flood loads were calculated in accordance with
ASCE.
For simplicity, the piles were analyzed under lateral wind and flood loads
only;
dead, live, and wind uplift loads were neglected. If dead, live, and wind
uplift
loads were included in the analysis, deeper pile embedment and possibly
larger
piles may be needed.
Three different timber pile sizes (8-, 10-, and 12-inch square) were
evaluated
using pre-storm embedment depths of 10-, 15-, and 20-feet, and five different
erosion and scour conditions (erosion = 0 or 1 foot; scour ranges from 2.0
times
the pile diameter to 4.0 times the pile diameter). The results of the
analysis
are shown in Table 3-3. A shaded cell indicates the combination of pile size,
pre-storm embedment, and erosion and scour would not provide the bending
resistance and/or embedment required to resist the lateral loads imposed on
them. The reason(s) for a foundation failure is indicated in each shaded
cell,
using “P” for pile failure due to bending and overstress within the pile and
“E”
for an embedment failure from the pile/soil interaction. An unshaded cell
with
“OK” indicates bending and foundation embedment criteria would both be
satisfied
by the particular pile size/pile embedment/erosion and scour combination.
[Begin table]
Table 3-3. Example Foundation Adequacy Calculations for a Two-Story Home
Supported on Square Timber Piles (and situated away from the shoreline, with
storm surge passing under the home, a 130-mph wind zone, and soil is medium
dense sand)
It shows Pile Embedment Before Erosion, and Scour for 10, 15, and 20 feet
Erosion and Scour Conditions
Pile Diameter, a for 8 inch,10 inch, and 12 inch piles
Where:
P = pile failure due to bending and overstress within the pile
E = embedment failure from the pile/soil interaction
OK = bending and foundation embedment criteria both satisfied by the
particular
pile size/pile embedment/erosion and scour combination
[End table]
A review of Table 3-3 shows several key points:
- Increasing pile embedment will not offset foundation inadequacy (bending
failure) resulting from too small a pile cross-section or too weak a pile
material.
- Increasing cross-section (or material strength) will not compensate for
inadequate pile embedment.
- Given the building and foundation configuration used in the example, the 8-
inch square pile is not strong enough to resist the lateral loads resulting
from
the 130-mph design wind speed under any of the erosion and scour conditions
evaluated, even if there is no erosion or scour. Homes supported by 8-inch
square timber piles, with embedment depths of 10 feet or less, will likely
fail
in large numbers when subjected to design or near design loads and
conditions.
Homes supported by deeper 8-inch piles may still be lost during a design
event
due to pile (bending failures).
- The 10-inch square pile is strong enough to resist bending under all but
the
most severe erosion and scour conditions analyzed.
- The 12-inch pile is the only pile size evaluated that satisfies bending
requirements under all erosion and scour conditions analyzed. This pile works
with 10 feet of embedment under the no erosion and scour condition. However,
introducing as little as 1 foot of erosion and scour equal to twice the pile
diameter was enough to render the foundation too shallow.
- Fifteen feet of pile embedment is adequate for both 10- and 12-inch piles
subject to 1 foot of erosion and scour up to three times the pile diameter.
However, when the scour is increased to four times the pile diameter
(frequently
observed following Hurricane Ike), 15 feet of embedment is inadequate for
both
piles. In general terms, approximately 11 feet of embedment is required in
this
example home to resist the loads and conditions after erosion and scour are
imposed.
- The 12-inch pile with 20 feet of embedment was the only foundation that
worked
under all erosion and scour conditions analyzed. This pile design may be
justified for the example home analyzed when expected erosion and scour
conditions are unknown or uncertain.
[Begin text box]
CAUTION: The results in Table 3-3 should not be used in lieu of building- and
site-specific engineering analyses and foundation design. The table is
intended
for illustrative purposes only and is based upon certain assumptions and
simplifications, and for the combinations of building characteristics, soil
conditions, and wind and flood conditions described above. Registered design
professionals should be consulted for foundation designs.
[End text box]
These analyses were based on only 1 foot of erosion, which historically is a
relatively small amount. Many storms like Hurricanes Isabel, Ivan, and Ike
caused much more extensive erosion. In some areas, these storms stripped away
several feet of soil.
A foot of erosion is more damaging than a foot of scour. While scour reduces
pile embedment and increases stresses within the pile, erosion reduces
embedment, increases stresses, and, since it increases stillwater depths, it
also increases the flood loads that the foundation must resist.
Table 3-3 suggests that increasing embedment beyond 15 feet is not necessary
for
10- and 12-inch piles. This is only the case for relatively small amounts of
erosion (like the 1 foot of erosion in the example). If erosion depths are
greater, pile embedment must be increased.
3.7.2 Localized Scour Around Vertical Walls and Enclosures
Localized scour around vertical walls and enclosed areas (e.g., typical A
zone
construction) can be greater than that around vertical piles, and should be
estimated using Table 3-4.
[Begin table]
Table 3-4. Local Scour Depth as a Function of Soil Type
Expected Depth (% of d sub s)
Loose sand: 80
Dense sand: 50
Soft silt: 50
Stiff silt: 25
Soft clay: 25
Stiff clay: 10
Source: COASTAL CONSTRUCTION MANUAL (FEMA 55)
[End table]
3.8 Flood Load Combinations
Load combinations (including those for flood loads) are given in ASCE 7-05,
Sections 2.3.2 and 2.3.3 for strength design and Sections 2.4.1 and 2.4.2 for
allowable stress design.
The basic load combinations are:
Allowable Stress Design
(1) D + F
(2) D + H + F + L + T
(3) D + H + F + (L sub r or S or R)
(4) D + H + F + 0.75(L + T) + 0.75(L sub r or S or R)
(5) D + H + F + (W or 0.7E)
(6) D + H + F + 0.75(W or 0.7E) + 0.75L + 1.5F sub a + 0.75(L sub r or S or
R)
(7) 0.6D + W + H
(8) 0.6D + 0.7 E + H
Strength Design
(1) 1.4 (D + F)
(2) 1.2 (D + F + T) + 1.6(L + H) + 0.5(L sub r or S or R)
(3) 1.2D + 1.6(L sub r or S or R) + (L or 0.8W)
(4) 1.2D + 1.6W + L + 0.5(L sub r or S or R)
(5) 1.2D + 1.0E + L + 0.2S
(6) 0.9D + 1.6W + 1.6H
(7) 0.9D + 1.0E + 1.6H
For structures located in V or Coastal A zones:
Allowable Stress Design
Load combinations 5, 6, and 7 shall be replaced with the following:
(5) D + H + F + 1.5F sub a + W
(6) D + H + F + 0.75W + 0.75L + 1.5F sub a + 0.75(L sub r or S or R)
(7) 0.6D + W + H + 1.5F sub a
Strength Design
Load combinations 4 and 6 given in ASCE 7-05 Section 2.3.1 shall be replaced
with the following:
(4) 1.2D + 1.6W + 2.0F sub a + L + 0.5(L sub r or S or R)
(6) 0.9D + 1.6W + 2.0 F sub a + 1.6H
Where
D = dead load
W = wind load
E = earthquake load
F sub a = flood load
F = load due to fluids with well defined pressures and maximum heights
L = live load
L sub r = roof live load
S = snow load
R = rain load
H = lateral earth pressure
Flood loads were included in the load combinations to account for the strong
correlation between flood and winds in hurricane-prone regions that run along
the Gulf of Mexico and the Atlantic Coast.
In non-Coastal A zones, for ASD, replace the 1.5F sub a with 0.75F sub a in
load
combinations 5, 6, and 7 given above. For strength design, replace
coefficients
W and F sub a in equations 4 and 6 above with 0.8 and 1.0, respectively.
Designers should be aware that not all of the flood loads will act at certain
locations or against certain building types. Table 3-5 provides guidance to
designers for the calculation of appropriate flood loads in V zones and
Coastal
A zones (non-Coastal A zone flood load combinations are shown for
comparison).
The floodplain management regulations enacted by communities that participate
in
the NFIP prohibit the construction of solid perimeter wall foundations in V
zones, but allow such foundations in A zones. Therefore, the designer should
assume that breaking waves will impact piles in V zones and walls in A zones.
It
is generally unrealistic to assume that impact loads will occur on all piles
at
the same time as breaking wave loads; therefore, this manual recommends that
impact loads be evaluated for strategic locations such as a building corner.
[Begin table]
Table 3-5. Selection of Flood Load Combinations for Design
Case 1 Pile or Open Foundation in V Zone (Required)
F sub brkp (on all piles) + F sub i (on one corner or critical pile only)
or
F sub brkp (on front row of piles only) + F sub dyn (on all piles but front
row)
+ F sub i (on one corner or critical pile only)
Case 2 Pile or Open Foundation in Coastal A Zone (Recommended)
sub Fbrkp (on all piles) + F sub i (on one corner or critical pile only)
or F sub brkp (on front row of piles only) + F sub dyn (on all piles but
front
row) +F sub i (on one corner or critical pile only)
Case 3 Solid (Wall) Foundation in Coastal A Zone (NOT Recommended)
F sub brkp (on walls facing shoreline, including hydrostatic component) + F
sub
dyn; assume one corner is destroyed by debris, and design in redundancy
Case 4 Solid (Wall) Foundation in Non-Coastal A Zone (Shown for Comparison)
F sub sta + F sub dyn
Source: Coastal Construction Manual (FEMA 55)
[End table]
Chapter 4. Overview of Recommended Foundation Types and Construction for
Coastal
Areas
Chapters 1 through 3 discussed foundation design loads and calculations and
how
these issues can be influenced by coastal natural hazards. This chapter will
tie
all of these issues together with a discussion of foundation types and
methods
of constructing a foundation for a residential structure.
4.1 Critical Factors Affecting Foundation Design
Foundation construction types are dependent upon the following critical
factors:
- Design wind speed
- Elevation height required by the BFE and local ordinances
- Flood zone
- Soil parameters
Soil parameters, like bearing capacities, shear coefficients, and subgrade
moduli, are important in designing efficient and effective foundations. But,
for
the purpose of creating the standardized foundation concepts for use in a
variety of sites, some soil parameters have been assumed (as in the case of
bearing capacity for shallow foundations) and others have been stipulated (as
those required to produce specific performance – as in the case for deep
driven
piles). Assumptions used in developing the foundations are listed in Appendix
C,
where stipulations on pile capacity are also listed in the individual
drawings.
4.1.1 Wind Speed
The basic wind speed determines the wind velocity used in establishing wind
loads for a building. It can also have a significant influence on the size
and
strength of foundations that support homes. Contemporary codes and standards
like the IRC, IBC, and ASCE 7 specify basic wind speeds as 3-second gust wind
speeds. Earlier versions of codes and standards specified wind speeds with
different averaging periods. One example is the fastest mile wind speed that
was
specified in the 1988 (and earlier) versions of ASCE 7 and in pre-2000
versions
of most model building codes.
The wind speed map shown in Figure 3-1 illustrates that the basic (3-second
gust) wind speeds along most of the Gulf of Mexico, the Atlantic coast, and
coastal Alaska range between 120 and 150 mph. The basic wind speed for most
of
the Pacific coast is 85 mph. Several areas in the Pacific Northwest are
designated as special wind regions and wind speeds are dictated locally. The
design wind speeds for many of the U.S. territories and protectorates are
tabulated in ASCE 7.
To determine forces on the building and foundation, the wind speed is
critical.
Wind speed creates wind pressures that act upon the building. These pressures
are proportional to the square of the wind speed, so a doubling of the wind
speed increases the wind pressure by a factor of four. The pressure applied
to
an area of the building will develop forces that must be resisted. To
transfer
these forces from the building to the foundation, properly designed load
paths
are required. For the foundation to be properly designed, all forces
including
uplift, compression, and lateral must be taken into account.
Although wind loads are important in the design of a building, in coastal
areas
flood loads often have a much greater effect on the design of the foundation
itself.
4.1.2 Elevation
The required height of the foundation depends on three factors: the DFE, the
site elevation, and the flood zone. The flood zone dictates whether the
lowest
habitable finished floor must be placed at the DFE or, in the case of homes
in
the V zone, the bottom of the lowest horizontal member must be placed at the
DFE. Figure 4-1 illustrates how the BFE, freeboard, erosion, and the ground
elevation determine the foundation height required. While not required by the
NFIP, V zone criteria are recommended for Coastal A zones. Stated
mathematically:
H = DFE – G + Erosion
or
H = BFE – G + Erosion + Freeboard
Where
H = Required foundation height (in ft)
DFE = Design Flood Elevation
BFE = Base Flood Elevation
G = Non-eroded ground elevation
Erosion = Short-term plus long-term erosion
Freeboard = 2009 IRC required in SFHAs, locally adopted or owner desired
freeboard
[Begin figure]
Figure 4-1 is illustrates that the BFE, freeboard, erosion, and ground
elevation
determine the foundation height required.
[End figure]
The height to which a home should be elevated is one of the key factors in
determining which pre-engineered foundation to use. Elevation height is
dependent upon several factors, including the BFE, local ordinances requiring
freeboard, and the desire of the homeowner to elevate the lowest horizontal
structural member above the BFE (see also Chapter 2). This manual provides
designs for closed foundations up to 8 feet above ground level and open
foundations up to 15 feet above ground level. Custom designs can be developed
for open and closed foundations to position the homes above those elevation
levels. Foundations for homes that need to be elevated higher than 15 feet
should be designed by a licensed professional engineer.
4.1.3 Construction Materials
The use of flood-resistant materials below the BFE is also covered in FEMA
NFIP
Technical Bulletin 2, Flood Damage-Resistant Materials Requirements for
Buildings Located in Special Flood Hazard Areas in accordance with the
National
Flood Insurance Program and FEMA 499, Fact Sheet No. 8 (see Appendix F). This
manual will cover the materials used in masonry and concrete foundation
construction, and field preservative treatment for wood.
4.1.3.1 Masonry Foundation Construction
The combination of high winds, moisture, and salt-laden air creates a
damaging
recipe for masonry construction. All three can penetrate the tiniest cracks
or
openings in the masonry joints. This can corrode reinforcement, weaken the
bond
between the mortar and the brick, and create fissures in the mortar. Moisture
resistance is highly influenced by the quality of the materials and the
workmanship.
4.1.3.2 Concrete Foundation Construction
Cast-in-place concrete elements in coastal environments should be constructed
with 3 inches or more of concrete cover over the reinforcing bars. The
concrete
cover physically protects the reinforcing bars from corrosion. However, if
salt
water penetrates the concrete cover and reaches the reinforcing steel, the
concrete alkalinity is reduced by the salt chloride, thereby corroding the
steel. As the corrosion forms, it expands and cracks the concrete, allowing
the
additional entry of water and further corrosion. Eventually, this process
weakens the concrete structural element and its load carrying capacity.
Alternatively, epoxy-coated reinforcing steel can be used if properly
handled,
stored, and placed. Epoxy-coated steel, however, requires more sophisticated
construction techniques and more highly trained contractors than are usually
involved with residential construction.
Concrete mix used in coastal areas must be designed for durability. The first
step in this process is to start with the mix design. The American Concrete
Institute (ACI) 318 manual recommends that a maximum water-cement ratio by
weight of 0.40 and a minimum compressive strength of 4,000 pounds per square
inch (psi) be used for concrete used in coastal environments. Since the
amount
of water in a concrete mix largely determines the amount that concrete will
shrink and promote unwanted cracks, the water-cement ratio of the concrete
mix
is a critical parameter in promoting concrete durability. Adding more water
to
the mix to improve the workability increases the potential for cracking in
the
concrete and can severely affect its durability.
Another way to improve the durability of a concrete mix is with ideal mix
proportions. Concrete mixes typically consist of a mixture of sand,
aggregate,
and cement. How these elements are proportioned is as critical as the water-
cement ratio. The sand should be clean and free of contaminants. The
aggregate
should be washed and graded. The type of aggregate is also very important.
Recent research has shown that certain types of gravel do not promote a tight
bond with the paste. The builder or contractor should consult expert advice
prior to specifying the concrete mix.
Addition of admixtures such as pozzolans (fly ash) is recommended for
concrete
construction along the coast. Fly ash when introduced in concrete mix has
benefits such as better workability and increased resistance to sulfates and
chlorates, thus reducing corrosion from attacking the steel reinforcing.
4.1.3.3 Field Preservative Treatment for Wood Members
In order to properly connect the pile foundation to the floor framing system,
making field cuts, notches, and boring holes are some of the activities
associated with construction. Since pressure-preservative-treated piles,
timbers, and lumber are used for many purposes in coastal construction, the
interior, untreated parts of the wood are exposed to possible decay and
infestation. Although treatments applied in the field are much less effective
than factory treatments, the potential for decay can be minimized. The
American
Wood Preservers’ Association (AWPA) AWPA M4-08 Standard for the Care of
Preservative-Treated Wood Products (AWPA 2008) describes field treatment
procedures and field cutting restrictions for poles, piles, and sawn lumber.
Field application of preservatives should always be done in accordance with
instructions on the label. When detailed instructions are not provided, dip
soaking for at least 3 minutes can be considered effective for field
applications. When this is impractical, treatment may be done by thoroughly
brushing or spraying the exposed area. It should be noted that the material
is
more absorptive at the end of a member, or end grains, than it is for the
sides
or side grains. To safeguard against decay in bored holes, the holes should
be
poured full of preservative. If the hole passes through a check (such as a
shrinkage crack caused by drying), it will be necessary to brush the hole;
otherwise, the preservative would run into the check instead of saturating
the
hole.
Waterborne arsenicals, pentachlorophenol, and creosote are unacceptable for
field applications. Copper napthenate is the most widely used field
treatment.
Its deep green color may be objectionable, but the wood can be painted with
alkyd paints in dark colors after extended drying. Zinc napthenate is a clear
alternative to copper napthenate. However, it is not quite as effective in
preventing insect infestation, and it should not be painted with latex
paints.
Tributyltin oxide (TBTO) is available, but should not be used in or near
marine
environments, because the leachates are toxic to aquatic organisms. Sodium
borate is also available, but it does not readily penetrate dry wood and it
rapidly leaches out when water is present. Therefore, sodium borate is not
recommended.
4.1.4 Foundation Design Loads
To provide flexibility in the home designs, tension connections have been
specified between the tops of all wood piles and the grade beams. Depending
on
the location of shear walls, shear wall openings, and the orientation of
floor
and roof framing, some wood piles may not experience tension forces. Design
professionals can analyze the elevated structure to identify compression only
piles to reduce construction costs. For foundation design and example
calculations, see Appendix D.
Figure 4-2 illustrates design loads acting on a column. The reactions at the
base of the elevated structure used in most of the foundation designs are
presented in Tables 4-1a (one-story) and 4-1b (two-story). These reactions
are
the controlling forces for the range of building weights and dimensions
listed
in Appendix A and shown in Figure 2 of the Introduction. Design reactions
have
been included for the various design wind speeds and various building
elevations
above exterior grade. ASCE 7-05 load combination 4 (D + 0.75L + 0.75Lr)
controls
for gravity loading and load combination 7 controls for uplift and lateral
loads. Load combination 7 is 0.6D + W + 0.75Fa in non-Coastal A zones and
0.6D +
W + 1.5Fa in Coastal A and V zones. Refer to Section 3.8 for the list of
flood
load combinations.
[Begin figure]
Figure 4-2 illustrates design loads acting on a column.
[End figure]
Loads on the foundation elements themselves are more difficult to tabulate
because they depend on the foundation style (open or enclosed), foundation
dimensions, and foundation height. Table 4-2 provides reactions for the 18-
inch
square columns used in most of the open foundation designs.
[Begin tables]
Table 4-1a. Design Perimeter Wall Reactions (lb/lf) for One-Story Elevated
Homes
(Note: Reactions are taken at the base of the elevated home/top of the
foundation element.)
This table shows the break down for wind speeds of 120, 130 ,140, 150 mph,
and
(All V); the height of the foundation above grade (both horizontal and
vertical)
for 5, 6, 7, 8, 10, 12, 14, and 15 feet.
Table 4-1b. Design Perimeter Wall Reactions (lb/lf) for Two-Story Elevated
Homes
(Note: Reactions are taken at the base of the elevated home/top of the
foundation element.)
This table shows the break down for wind speeds of 120, 130 ,140, 150 mph,
and
(All V); the height of the foundation above grade (both horizontal and
vertical)
for 5, 6, 7, 8, 10, 12, 14, and 15 feet.
Table 4-2. Flood Forces (in pounds) on an 18-Inch Square Column
Flood Depth: 5 ft
Hydrodynamic: 1,000
Breaking Wave: 684
Impact: 3,165
Buoyancy: 465
Flood Depth: 6 ft
Hydrodynamic: 1,440
Breaking Wave: 985
Impact: 3,476
Buoyancy: 577
Flood Depth: 7 ft
Hydrodynamic: 1,960
Breaking Wave: 1,340
Impact: 3,745
Buoyancy: 650
Flood Depth: 8 ft
Hydrodynamic: 2,560
Breaking Wave: 1,750
Impact: 4,004
Buoyancy: 743
Flood Depth: 10 ft
Hydrodynamic: 4,001
Breaking Wave: 2,735
Impact: 4,476
Buoyancy: 939
Flood Depth: 12 ft
Hydrodynamic: 5,761
Breaking Wave: 3,938
Impact: 4,903
Buoyancy: 1,115
Flood Depth: 14 ft
Hydrodynamic: 7,841
Breaking Wave: 5,360
Impact: 5,296
Buoyancy: 1,300
Flood Depth: 15 ft
Hydrodynamic: 9,002
Breaking Wave: 6,155
Impact: 5,482
Buoyancy: 1,394
[End tables]
4.1.5 Foundation Design Loads and Analyses
Load analyses used to develop Case H foundations are similar to the analyses
completed for the original FEMA 550 designs. Live loads used were those
specified by the IRC and the original and augmented foundations were
developed
to support a range of dead loads. Wind and flood loads were calculated per
ASCE
7, Minimum Design Loads for Buildings and Other Structures (the Case H design
loads were calculated using ASCE 7-05; loads used in the original designs
were
calculated using ASCE 7-02, which are consistent with the 2005 edition).
Design
assumptions are listed in Appendix C.
Some noteworthy differences exist. Wind loads used in the original FEMA 550
designs were the worst case loads for a home that varied in width from 24
feet
to 42 feet and in roof slope from 3:12 to 12:12. The foundation reactions for
the original designs are listed in Table 4-1b. In the Case H designs,
separate
wind loads were determined based on the number of stories (one or two) and
the
building width (14 feet for the 3-bay designs, 28 feet for the 6-bay designs,
and 42 feet for the 9-bay designs). The more precise matching of wind loads
to
building widths and heights provide greater design efficiencies.
Wind loads used to develop the Case H foundations are listed in Table 4-3.
[Begin table]
Table 4-3. Wind Reactions Used to Develop Case H Foundations
This table shows 3-Bay, 6-Bay, and 9-Bay construction for vertical forces on
windward edge of foundation, vertical forces on leeward edge of foundation,
and
horizontal forces on windward and leeward edges of foundation at 120, 130,
140,
and 150m mph.
Note
1. (+) loads act upward; (-) pressures act downward.
2. Lateral loads are applied to both windward and leeward foundation
elements.
[End table]
To account for shear panel reactions from segmented shear walls, the analyses
of
foundations supporting one-story homes included 6.72 kip quarter span point
loads for the 3-bay design (point loads were applied at mid-span for the 6-
and
9-bay models, Figure 4-3). The loads correspond to 10-foot tall wood framed
shear panels constructed with 7/16-inch blocked wood structural panels
fastened
with 8d common nails 6 inches on center (o.c.). Foundations supporting two-
story
homes were analyzed with 13.44 kip shear panel reactions or twice that of the
one-story home. The foundations will also support homes constructed with
perforated shear walls.
[Begin figure]
Figure 4-3 illustrates shear panel reactions for the 3- and 6-bay models.
Reactions for the 9-bay model were similar to those of the 6-bay.
[End figure]
Another difference in design methodology was required due to the nature of
structural frames. In the original designs, the concrete columns were
considered
statically determinant and analyzed as such. The structural frames created by
the concrete grade beams, concrete columns, and elevated beams, however, are
not
statically determinant and computer modeling was warranted. To analyze the
frame
action developed by those structural elements, computer models using RISA©
structural software were created. Design loads were applied to the frames and
critical shears and moments were tabulated for the grade beams, columns, and
elevated beams. Critical axial forces were also tabulated for the columns.
Tables 4-4 through 4-9 summarize the critical shears, moments, and axial
loads
of the computer models used to develop the Case H foundations.
[Begin table]Table 4-4 shows the Design Moments (K-ft), Axial Loads (in
kips),
and Shears (in kips) for 10-Foot Tall 3-Bay Foundations with winds at 120,
130,
140, and 150 mph.
10-Foot Foundations consist of:
Column Moment +, Column Moment –, Column Shear Bottom, Column Shear Top,
Axial
Maximum, Axial Minimum, Elevated Beam Moment +, Elevated Beam Moment,
Elevated
Beam Shear at Column, Elevated Beam Shear at Mid-Span, Grade Beam Moment +,
Grade Beam Moment -, Grade Beam Shear at Column, Grade Beam Shear at Mid-
Span=
Note:
1. (+) loads act upward; (-) pressures act downward.
[End table]
[Begin table]
Table 4-5. Design Moments (K-ft), Axial Loads (in kips), and Shears (in kips)
for 15-Foot Tall 3-Bay Foundations with winds at 120, 130, 140, and 150 mph.
15-Foot Foundations consist of:
Column Moment +, Column Moment –, Column Shear Bottom, Column Shear Top,
Axial
Maximum, Axial Minimum, Elevated Beam Moment +, Elevated Beam Moment,
Elevated
Beam Shear at Column, Elevated Beam Shear at Mid-Span, Grade Beam Moment +,
Grade Beam Moment -, Grade Beam Shear at Column, Grade Beam Shear at Mid-
Span=
Note:
1. (+) loads act upward; (-) pressures act downward.
[End table]
[Begin table]
Table 4-6. Design Moments (K-ft), Axial Loads (in kips), and Shears (in kips)
for 10-Foot Tall 6-Bay Foundations with winds at 120, 130, 140, and 150 mph.
10-Foot Foundations consist of:
Column Moment +, Column Moment –, Column Shear Bottom, Column Shear Top,
Axial
Maximum, Axial Minimum, Elevated Beam Moment +, Elevated Beam Moment,
Elevated
Beam Shear at Column, Elevated Beam Shear at Mid-Span, Grade Beam Moment +,
Grade Beam Moment -, Grade Beam Shear at Column, Grade Beam Shear at Mid-
Span=
Note:
1. (+) loads act upward; (-) pressures act downward.
[End table]
[Begin table]
Table 4-7. Design Moments (K-ft), Axial Loads (in kips), and Shears (in kips)
for 15-Foot Tall 6-Bay Foundations with winds at 120, 130, 140, and 150 mph.
15-Foot Foundations consist of:
Column Moment +, Column Moment –, Column Shear Bottom, Column Shear Top,
Axial
Maximum, Axial Minimum, Elevated Beam Moment +, Elevated Beam Moment,
Elevated
Beam Shear at Column, Elevated Beam Shear at Mid-Span, Grade Beam Moment +,
Grade Beam Moment -, Grade Beam Shear at Column, Grade Beam Shear at Mid-
Span=
Note:
1. (+) loads act upward; (-) pressures act downward.
[End table]
[Begin table]
Table 4-8. Design Moments (K-ft), Axial Loads (in kips), and Shears (in kips)
for 10-Foot Tall 9-Bay Foundations with winds at 120, 130, 140, and 150 mph.
10-Foot Foundations consist of:
Column Moment +, Column Moment –, Column Shear Bottom, Column Shear Top,
Axial
Maximum, Axial Minimum, Elevated Beam Moment +, Elevated Beam Moment,
Elevated
Beam Shear at Column, Elevated Beam Shear at Mid-Span, Grade Beam Moment +,
Grade Beam Moment -, Grade Beam Shear at Column, Grade Beam Shear at Mid-
Span=
Note:
1. (+) loads act upward; (-) pressures act downward.
[End table]
[Begin table]
Table 4-9. Design Moments (K-ft), Axial Loads (in kips), and Shears (in kips)
for 15-Foot Tall 9-Bay Foundations with winds at 120, 130, 140, and 150 mph.
15-Foot Foundations consist of:
Column Moment +, Column Moment –, Column Shear Bottom, Column Shear Top,
Axial
Maximum, Axial Minimum, Elevated Beam Moment +, Elevated Beam Moment,
Elevated
Beam Shear at Column, Elevated Beam Shear at Mid-Span, Grade Beam Moment +,
Grade Beam Moment -, Grade Beam Shear at Column, Grade Beam Shear at Mid-
Span=
Note:
1. (+) loads act upward; (-) pressures act downward.
[End table]
4.2 Recommended Foundation Types for Coastal Areas
Table 4-10 provides six open (deep and shallow) foundation types and two
closed
foundations discussed in this manual. Appendix A provides the foundation
design
drawings for the cases specified.
[Begin table]Table 4-10. Recommended Foundation Types Based on Zone
Open Foundation (deep)
Braced timber pile, Case A, acceptable in V Zones, Coastal A Zone, and A Zone
Steel pipe pile with concrete column and grade beam, Case B, acceptable in V
Zones, Coastal A Zone, and A Zone
Timber pile with concrete column and grade beam, Case C, acceptable in V
Zones,
Coastal A Zone, and A Zone
Timber pile with concrete grade and elevated beams and concrete columns, Case
H,
acceptable in V Zones, Coastal A Zone, and A Zone
Open Foundation (shallow)
Concrete column and grade beam, Case D, not recommended in V zones,
acceptable
Coastal A Zone and A Zone
Concrete column and grade beam with integral slab, Case G, not recommended in
V
zones, acceptable Coastal A Zone and A Zone
Closed Foundation (shallow)
Reinforced masonry – crawlspace, Case E, not permitted in V zones, not
recommended in Coastal A zones, acceptable in A zones
Reinforced masonry – stem wall, Case F, not permitted in V zones, not
recommended in Coastal A zones, acceptable in A zones
[End table]
The foundation designs contained in this manual are based on soils having a
bearing capacity of 1,500 pounds per square foot (psf). The 1,500-psf bearing
capacity value corresponds to the presumptive value contained in Section 1806
of
the 2009 IBC. The presumptive bearing capacity is for clay, sandy clay, silty
clay, clayey silt, and sandy silt (CL, ML, MH, and CH soils).
The size of the perimeter footings and grade beams are generally not
controlled
by bearing capacity (uplift and lateral loads typically control footing size
and
grade beam dimensions). Refining the designs for soils with greater bearing
capacities may not significantly reduce construction costs. However, the size
of
the interior pad footings for the crawlspace foundation (Table 4-10, Case E)
depends greatly on the soil’s bearing capacity. Design refinements can reduce
footing sizes in areas where soils have greater bearing capacities. The
following discussion of the foundation designs listed in Table 4-10 is also
presented in Appendix A. Figures 4-4 through 4-10 are based on Appendix A.
4.2.1 `Open/Deep Foundation: Timber Pile (Case A)
This pre-engineered, timber pile foundation uses conventional, tapered,
treated
piles and steel rod bracing to support the elevated structure. No concrete,
masonry, or reinforcing steel is needed (see Figure 4-4). Often called a
“stilt”
foundation, the driven timber pile system is suitable for moderate elevations
if
the homebuilder prefers to minimize the number of different construction
trades
used. Once the piles are driven, the wood guides and floor system are
attached
to the piles; the remainder of the home is constructed off the floor
platform.
[Begin figure]
Figure 4-4 is a profile of Case A foundation type (see Appendix A for
additional
drawings).
[End figure]
The recommended design for Case A that is presented in this manual
accommodates
home elevations up to 10 feet above grade. With customized designs and longer
piles, the designs can be modified to achieve higher elevations. However,
elevations greater than 10 feet will likely be prevented by pile
availability,
the pile strength required to resist lateral forces, and the pile embedment
required to resist erosion and scour. A construction approach that can
improve
performance is to extend the piles above the first floor diaphragm to the
second
floor or roof diaphragm. Doing so allows the foundation and the elevated home
to
function more like a single, integrated structural frame. Extending the piles
stiffens the structure, reduces stresses in the piles, and reduces lateral
deflections. Post disaster assessments of pile supported homes indicate that
extending piles in this fashion improves survivability. Licensed professional
engineers should be consulted to analyze the pile foundations and design the
appropriate connections.
One drawback of the timber pile system is the exposure of the piles to
floodborne debris. During a hurricane event, individual piles can be damaged
or
destroyed by large, floating debris. With the home in place, damaged piles
are
difficult to replace. Two separate ways of addressing this potential problem
is
to use piles with a diameter larger than is called for in the foundation
design
or to use a greater number of piles to increase structural redundancy.
4.2.2 Open/Deep Foundation: Steel Pipe Pile with Concrete Column and Grade
Beam
(Case B)
This foundation incorporates open-ended steel pipe piles; this style is
somewhat
unique to the Gulf Coast region where the prevalence of steel pipe piles used
to
support oil platforms has created local sources for these piles. Like treated
wood piles, steel pipe piles are driven but have the advantage of greater
bending strength and load carrying capacity (see Figure 4-5). The open steel
pipe pile foundation is resistant to the effects of erosion and scour. The
grade
beam can be undermined by scour without compromising the entire foundation
system.
[Begin figure]
Figure 4-5 is a Profile of Case B foundation type (see Appendix A for
additional
drawings).
[End figure]
The number of piles required depends on local soil conditions. Like other
soil
dependent foundation designs, consideration should be given to performing
soil
tests on the site so the foundation design can be optimized. With guidance
from
engineers, the open-ended steel pipe pile foundation can be designed for
higher
elevations. Additional piles can be driven for increased resistance to
lateral
forces, and columns can be made larger and stronger to resist the increased
bending moments that occur where the columns join the grade beam. Because
only a
certain amount of steel can be installed to a given cross-section of concrete
before the column sizes and the flood loads become unmanageable, a maximum
elevation of 15 feet exists for the use of this type of foundation.
4.2.3 Open/Deep Foundation: Timber Pile with Concrete Column and Grade Beam
(Case C)
This foundation is similar to the steel pipe pile with concrete column and
grade
beam foundation (Case B). Elevations as high as 15 feet can be achieved for
wind
speeds up to 150 mph for both one- and two-story structures. However, because
wood piles have a lower strength to resist the loads than steel piles,
approximately twice as many timber piles are needed to resist loads imposed
on
the home and the exposed portions of the foundation (Figure 4-6).
[Begin figure]
Figure 4-6 is a profile of Case C foundation type (see Appendix A for
additional
drawings).
[End figure]
While treated to resist rot and damage from insects, wood piles may become
vulnerable to damage from wood destroying organisms in areas where they are
not
constantly submerged by groundwater. If constantly submerged, there is not
enough oxygen to sustain fungal growth and insect colonies; if only
periodically
submerged, the piles can have moisture levels and oxygen levels sufficient to
sustain wood destroying organisms. Consultation with local design
professionals
in the area familiar with the use and performance of driven treated wood
piles
will help quantify this potential risk. Grade beams can be constructed at
greater depths or alternative pile materials can be selected if wood
destroying
organism damage is a major concern.
4.2.4 Open/Deep Foundation: Timber Pile with Concrete Grade and Elevated
Beams
and Concrete Columns (Case H)
Case H foundation designs augment designs contained in the first edition of
FEMA
550. They incorporate elevated reinforced beams into the V zone timber pile
foundation design. The elevated beams provide two important benefits:
The elevated beams, columns, and grade beams function as structural frames
that
resist lateral loads. The frame action allows smaller concrete columns to be
used. Smaller columns reduce the flood loads imposed on the foundation and
provide more efficient designs.
The elevated beams provide attachment points for homes constructed per
prescriptive codes and standards like ANSI/AF&PA Wood Frame Construction
Manual,
American Iron and Steel Institute (AISI) Standard for Cold-Formed Steel
Framing
– Prescriptive Method for One and Two Family Dwellings, SSTD10-99 Standard
for
Hurricane Resistant Residential Construction, and ICC-600 Standard for
Residential Construction in High Wind Regions.
Case H designs include a 3-bay design suitable for homes as narrow as 14
feet.
Designs for foundation heights of 10 and 15 feet are provided.
As previously stated, the Case H designs are more precise and foundation
strengths more closely match design loads. In addition, the structural frame
action provided by the grade beams, columns, and elevated beams allow smaller
columns to be constructed. One drawback of the design, however, is that
constructing elevated concrete beams is more complicated than constructing
grade
beams and reinforced columns; therefore, more knowledgeable and experienced
contractors would be needed (Figure 4-7).
[Begin figure]
Figure 4-7 is a profile of Case H foundation type (see Appendix A for
additional
drawings).
[End figure]
Sections were designed with an emphasis on strength, ductility, and
constructability. To simplify detailing and construction, axial and shear
reinforcement were made consistent through each section. Section properties
throughout each member were selected to resist the maximum forces (positive
and
negative moments, axial loads, and shears) that exist within the elements
(grade
beams, columns, or elevated beams). To simplify forming, the dimensions of
the
elevated beams matched the columns into which they frame. The designs were
based
on a belief that the potential increase in concrete costs would be more than
offset by the savings in labor costs in constructing simple forms. Design
professionals using the guidance contained in this manual may find it
beneficial
to vary from this approach.
The approach used to design and detail the Case H foundation system was to
develop easily scalable column and beam systems. The size allows for the use
of
variable amounts of steel while keeping rebar congestion to a minimum. A 16-
inch
wide system to allow for the economical use of structural form panels with
minimal waste was selected. The consistent column and elevated beam sizes
also
allow for reuse of the concrete forms between columns and beams. The member
size
and aspect ratio (member shape factor) allow for high lateral capacities.
The continuous bars in the beams provide a tie around the entire structure,
imparting redundancy should an element fail, as well as the ability for the
system to bridge over failed elements below. This redundancy improves the
foundation performance, especially with impact from floodborne debris that
exceeds design loads.
4.2.5 Open/Shallow Foundation: Concrete Column and Grade Beam with Slabs
(Cases
D and G)
These open foundation types make use of a rigid mat to resist lateral forces
and
overturning moments. Frictional resistance between the grade beams and the
supporting soils resist lateral loads while the weight of the grade beam and
the
above grade columns resist uplift. Case G (foundation with slab) contains
additional reinforcement to tie the on-grade slab to the grade beams to
provide
additional weight to resist uplift (Figure 4-8). With the integral slab,
elevations up to 15 feet above grade are achievable. Without the slab (as for
Case D), the designs as detailed are limited to 10-foot elevations (Figure 4-
9).
[Begin figures]
Figure 4-8 is a profile of Case G foundation type (see Appendix A for
additional
drawings).
Figure 4-9 is a profile of Case D foundation type (see Appendix A for
additional
drawings).
[End figures]
Unlike the deep driven pile foundations, both shallow grade beam foundation
styles can be undermined by erosion and scour if exposed to waves and high
flow
velocities. Neither style of foundation should be used where anticipated
erosion
or scour would expose the grade beam.
4.2.6 Closed/Shallow Foundation: Reinforced Masonry Crawlspace (Case E)
The reinforced masonry with crawlspace type of foundation utilizes
conventional
construction similar to foundations used outside of SFHAs. Footings are cast-
in-
place reinforced concrete; walls are constructed with reinforced masonry
(Figure
4-10). The foundation designs presented in Appendix A permit elevated homes
to
be raised to 8 feet. Higher elevations are achievable with larger or more
closely spaced reinforcing steel or with walls constructed with thicker
masonry.
[Begin figure]
Figure 4-10 is a profile of Case F foundation type (see Appendix A for
additional drawings).
[End figure]
The required strength of a masonry wall is determined by breaking wave loads
for
wall heights 3 feet or less, by non-breaking waves and hydrodynamic loads for
taller walls, and by uplift for all walls. Perimeter footing sizes are
controlled by uplift and must be relatively large for short foundation walls.
The weight of taller walls contributes to uplift resistance and allows for
smaller perimeter footings. Solid grouting of perimeter walls is recommended
for
additional weight and improved resistance to water infiltration.
Interior footing sizes are controlled by gravity loads and by the bearing
capacity of the supporting soils. Since the foundation designs are based on
relatively low bearing capacities, obtaining soils tests for the building
site
may allow the interior footing sizes to be reduced.
The crawlspace foundation walls incorporate NFIP required flood vents, which
must allow floodwaters to flow into the crawlspace. In doing so, hydrostatic,
hydrodynamic, and breaking wave loads are reduced. Crawlspace foundations are
vulnerable to scour and flood forces and should not be used in Coastal A
zones;
the NFIP prohibits their use in V zones.
4.2.7 Closed/Shallow Foundation: Reinforced Masonry Stem Wall (Case F)
The reinforced masonry stem walls (commonly referred to as chain walls in
portions of the Gulf Coast) type of foundation also utilizes conventional
construction to contain fill that supports the floor slab. They are
constructed
with hollow masonry block with grouted and reinforced cells (Figure 4-11).
Full
grouting is recommended to provide increased weight, resist uplift, and
improve
longevity of the foundation.
[Begin figure]
Figure 4-11 is a profile of Case E foundation type (see Appendix A for
additional drawings).
[End figure]
The amount and size of the reinforcement are controlled primarily by the
lateral
forces created by the retained soils and by surcharge loading from the floor
slab and imposed live loads. Because the retained soils can be exposed to
long
duration flooding, loads from saturated soils should be considered in the
analyses. The lateral forces on stem walls can be relatively high and even
short
cantilevered stem walls (those not laterally supported by the floor slab)
need
to be heavily reinforced. Tying the top of the stem walls into the floor slab
provides lateral support for the walls and significantly reduces
reinforcement
requirements. Because backfill needs to be placed before the slab is poured,
walls that will be tied to the floor slab need to be temporarily braced when
the
foundation is backfilled until the slab is poured and cured.
[Begin text box]
NOTE: Stem wall foundations are vulnerable to scour and should not be used in
Coastal A zones without a deep footing. The NFIP prohibits the use of this
foundation type in V zones.
[End text box]
Chapter 5. Foundation Selection
This chapter provides foundation designs, along with the use of the drawings
in
Appendix A, to assist the homebuilder, contractor, and local engineering
professional in developing a safe and strong foundation. Foundation design
types, foundation design considerations, cost estimating, and details on how
to
use this manual are presented.
5.1 Foundation Design Types
The homebuilder, contractor, and local engineering professional can utilize
the
designs in this chapter and Appendix A to construct residential foundations
in
coastal areas. The selection of appropriate foundation designs for the
construction of residences is dependent upon the coastal zone, wind speed,
and
elevation requirements, all of which have been discussed in the previous
chapters. The following types of foundation designs are presented in this
manual:
Open/Deep Foundations
- Braced timber pile (Case A)
- Steel pipe pile with concrete column and grade beam (Case B)
- Timber pile with concrete column and grade beam (Case C)
- Timber pile with concrete grade and elevated beams and concrete columns
(Case
H)
Open/Shallow Foundations
- Concrete column and grade beam (Case D)
- Concrete column and grade beam with slab (Case G)
Closed/Shallow Foundations
- Reinforced masonry – crawlspace (Case E)
- Reinforced masonry – stem wall (Case F)
Each of these foundation types designed for coastal areas have advantages and
disadvantages that must be taken into account. Modifications to the details
and
drawings might be needed to incorporate specific home footprints, elevation
heights, and wind speeds to a given foundation type. Consultation with a
licensed professional engineer is encouraged prior to beginning construction.
The foundation designs and materials specified in this document are based on
principles and practices used by structural engineering professionals with
years
of coastal construction experience. This manual has been prepared to make the
information easy to understand.
Guidance on the use of the foundation designs recommended herein is provided
in
Appendix B. Examples of how the foundation designs can be used with some of
the
homes in the publication A Pattern Book for Gulf Coast Neighborhoods are
presented in Appendix B. Design drawings for each of the foundation types are
presented in Appendix A, and any assumptions used in these designs are in
Appendix C.
5.2 Foundation Design Considerations
The foundation designs proposed are suitable for homes with dimensions,
weights,
and roof pitches within certain ranges of values. A licensed professional
engineer should confirm the appropriateness of the foundation design of homes
with dimensions, weights, or roof pitches that fall outside of those defined
ranges.
Most of the foundation designs are based on a 14-foot wide (maximum) by 24-
foot
deep (minimum) “module” (Figure 5-1). From this basic building block,
foundations for specific homes can be developed. For example, if a 30-foot
deep
by 42-foot wide home is to be constructed, the foundation can be designed
around
three 14-foot wide by 30-foot deep sections. If a 24-foot deep by 50-foot
wide
home is desired, four 12.5-foot wide by 24-foot deep sections can be used. If
a
22-foot deep home is desired, the foundation designs presented here should
only
be used after a licensed professional engineer determines that they are
appropriate since the shallow depth of the building falls outside the range
of
assumptions used in the design.
[Begin figure]
Figure 5-1 is a schematic of a basic module and two footprints. The basic
module
is 24’ minimum, by 14’ maximum. Footprint 1 is for a 30’x42’ home. Footprint
2
is for a 28’x50’ home
[End figure]
The licensed professional engineer should also consider the following:
- Local soil conditions. The pile foundations have been developed for
relatively
soft subsurface soils. For driven treated lumber piles, the presumptive
allowable working load values of 7 tons per pile gravity, 4.65 tons per pile
uplift, and 2 tons per pile lateral were used. For steel pipe piles, the
presumptive allowable working load piles were greater (10 tons per pile for
gravity loading, 6.7 tons per pile for uplift, and 4 tons per pile for
lateral
loading). Soil testing on the site should also be considered to validate the
assumptions made.
In some areas of the coastal U.S. (e.g., portions of Louisiana), soils may
exist
that will not provide the presumptive pile values. In those areas, aspects of
the FEMA 550 deep foundation designs are still valid, but geotechnical
engineers
will need to be involved in portions of the design to determine required pile
parameters. In poor soils, additional piles may need to be installed or long
piles may need to be driven; however, the portions of the designs from the
grade
beams upward should remain valid.
The FEMA 550 shallow foundations are based on a presumptive bearing capacity
of
1,500 psf. This value is consistent with the presumptive bearing capacity of
Section 1806 of the 2009 IBC for clay, sandy clays, clayey silts, and sandy
silts (CL, ML, MH, and CH soils). In areas where soils will not provide this
presumed bearing capacity, the shallow FEMA 550 designs should not be used
until
their ability to support the required loads can be confirmed by design
professionals.
- Building weight. The foundations have been designed to resist uplift forces
resulting from a relatively light structure. If the actual home is heavier
(e.g., from the use of concrete composite siding or steel framing), it may be
cost-effective to reanalyze and redesign the footings. This is particularly
true
for a home that doesn’t need to be elevated more than several feet or has
short
foundation walls that can help resist uplift.
- Footprint complexity. By necessity, the foundations have been designed for
relatively simple rectangular footprints. If the actual footprint of the home
is
relatively complex, the engineer may need to consider torsional wind loading,
differential movement among the “modules” that make up the home, concentrated
loading in the home’s floor and roof diaphragms, and shear wall placement.
5.3 Cost Estimating
Cost information that homebuilders can use to estimate the cost of installing
the foundation systems proposed in this manual are presented in Appendix E.
These cost estimates are based on May 2006 prices from information provided
by
local contractors for the First Edition of this manual.
5.4 How to Use This Manual
The rest of this chapter is designed to provide the user with step by step
procedures for the information contained in this manual.
1. Determine location of the dwelling on a general map. Identify the location
relative to key features such as highways and bodies of water. An accurate
location is essential for using flood and wind speed maps in subsequent steps
of
the design process.
2. Determine location of dwelling on the appropriate FIRM
- Determine the flood insurance risk zone from the FIRM (Select V zone,
Coastal
A zone, non-Coastal A zone, or other). Refer to FEMA 258, Guide to Flood
Maps,
How to Use Flood Maps to Determine Flood Risk for a Property, for
instructions.
- Determine the BFE or the interim Advisory Base Flood Elevations (ABFEs) for
the location from the FIRM. If the dwelling is outside of flood-prone areas,
flood loads do not need to be considered.
3. Identify the local building code. Several states and municipalities in
coastal areas are adopting new building codes to govern residential
construction. This manual assumes that the IRC governs the design and
construction requirements.
4. Identify the local freeboard requirements and DFE. Using either the local
building codes, local floodplain ordinances, data obtained from local
building
officials, or personal preferences (only if greater than minimum
requirements),
determine the minimum freeboard above the BFE or ABFE. The DFE is the sum of
the
BFE or ABFE and freeboard values.
5. Determine the required design wind velocity. The 2006 and 2009 IRCs
reference
ASCE 7-05 as the source of the wind speed information.
6. Establish the topographic elevation of the building site and the dwelling.
Elevations can be obtained from official topographic maps published by the
National Geodetic Survey (NGS) and/or as established or confirmed by a
surveyor.
- If the dwelling and its surrounding site are above the DFE, no flood forces
need to be considered.
- If the desired topographic elevation is below the DFE, the dwelling must be
elevated above the BFE or ABFE.
7. Determine the height of the base of the dwelling above grade. Subtract the
lowest ground elevation at the building from the lowest elevation of the
structure (i.e., bottom of lowest horizontal structural member).
8. Determine the general soil classification for the site. For shallow
foundations, confirm that the soils on site have a minimum bearing capacity
of
1,500 psf. If soils lack that minimum capacity, contact a geotechnical
engineer
and/or a structural engineer to confirm that the FEMA 550 foundation
solutions
are appropriate.
For deep foundations, confirm that the presumptive pile capacities (for
gravity
loads, uplift loads, and lateral loads) are achievable. If soils present on
site
will not support the presumed pile capacities, contact a geotechnical
engineer
and/or a structural engineer to determine appropriate pile plans.
9. Estimate erosion and scour. Estimate accumulated erosion and episodic
scour
over the life of the structure. Use accumulated erosion to determine eroded
grade elevation and use accumulated erosion and episodic scour to determine
the
foundation depth required to ensure shallow foundations will not be
undermined.
10. Determine the type of foundation to be used to support the structure.
Depending on the location of the dwelling, design wind speed, and local soil
conditions documented above, select the desired or required type of
foundation.
Note that more than one solution may be possible. Refer to Chapter 4 for the
potential foundation designs that can be used within the flood zones
determined
from the FIRM maps. Drawings in Appendix A illustrate the construction
details
for each of the foundations. Refer to the drawings for further direction and
information about the needs for each type of unit.
11. Evaluate alternate foundation type selections. The choice of foundation
type
may be on the basis of least cost or to provide a personal choice,
functional,
or aesthetic need at the site. Refer to Appendix E for guidance on preparing
cost estimates. Functional needs such as provisions for parking, storage, or
other non-habitable uses for the area beneath the living space should be
considered in the selection of the foundation design. Aesthetic or
architectural
issues (i.e., appearance) also must be included in the evaluation process.
Guidance for the architectural design considerations can be obtained from A
Pattern Book for Gulf Coast Neighborhoods by the Mississippi Governor’s
Commission on Recovery, Rebuilding and Renewal (see Appendix B) and from many
other sources.
As part of the final analysis, it is strongly recommended that the selection
and
evaluation process be coordinated with or reviewed by knowledgeable
contractors
or design professionals to arrive at the best solution to fulfill all of the
regulatory and functional needs for the construction.
12. Select the foundation design. If the home’s dimensions, height, roof
pitch,
and weight are within the ranges used to develop these designs, the
foundation
designs can be used “as is.” However, if the proposed structure has
dimensions,
height, roof pitch, or weights that fall outside of the range of values used,
a
licensed professional engineer should be consulted. The materials presented
in
the appendices should help reduce the engineering effort needed to develop a
custom design. Figure 5-2 is a foundation selection decision tree for
determining which foundation design to use based on the requirements of the
home. Tables 5-1a and 5-1b show which foundation design cases can be used for
one- and two-story homes, respectively, based on height of elevation and wind
velocity.
Because the designs are good for a range of buildings, they will be
conservative
for some applications. A licensed professional engineer will be able to
provide
value engineering and may produce a more efficient design that reduces
construction costs.
5.5 Design Examples
The foundation designs were developed to allow a “modular approach” for
developing foundation plans. In this approach, individual rectangular
foundation
components can be assembled into non-rectangular building footprints (see
Figures 5-3 through 5-5). Appendix D provides detailed calculations and
analysis
for open and closed foundation designs. There are, however, a few rules that
must be followed when assembling the modules:
1. The eave-to-ridge dimension of the roof is limited to 23 feet. The upper
limit on roof height is to limit the lateral forces to those used in
developing
the designs.
2. Roof slopes shall not be shallower than 3:12 or steeper than 12:12. For a
12:12 roof pitch, this corresponds to a 42-foot deep home with a 2-foot eave
overhang.
3. The “tributary load depth” of the roof framing shall not exceed 23 feet,
including the 2-foot maximum roof overhang. This limit is placed to restrict
uplift forces on the windward foundation elements to those forces used in
developing the design. As a practical matter, clear span roof trusses are
rarely
used on roofs over 42 feet deep; therefore, this limit should not be unduly
restrictive. The roof framing that consists of multiple spans will require
vertical load path continuity down through the interior bearing walls to
resist
uplift forces on the roof. Load path continuity can be achieved in interior
bearing walls using many of the same techniques used on exterior bearing
walls.
4. On the perimeter foundation wall designs (Cases E and F), foundation shear
walls must run the full depth of the building module, and shear walls can not
be
spaced more than 42 feet apart.
5. All foundation modules shall be at least 24 feet deep and at least 24 feet
long. Although the basic module is limited to 42 feet long, longer home
dimensions can be developed, provided that the roof does not extend beyond
the
building envelope as depicted in Figure 2 of the Introduction.
[Begin figures]
Figure 5-3. “T” shaped modular design. Note A: Overall building dimensions
can
exceed 42 feet. The vertical dimensions from the eave to the ridge roof shall
not exceed 23 feet.
Figure 5-4. “L” shaped modular design. Note A: Overall building dimensions
can
exceed 42 feet. The vertical dimensions from the eave to the ridge roof shall
not exceed 23 feet.
Figure 5-5. “Z” shaped modular design. Note A: Overall building dimensions
can
exceed 42 feet. The vertical dimensions from the eave to the ridge roof shall
not exceed 23 feet.
[End figures]
Appendix A. Foundation Designs
This appendix contains the Case A through Case G drawings originally
developed
for the July 2006 edition of FEMA 550 and the new Case H drawings developed
for
this Second Edition of FEMA 550.
Case H drawings did not appear in the First Edition of FEMA 550. The drawings
developed for the Case H designs vary in style and format from the original
FEMA
550 designs for Cases A through G. Because of the differences in style and
format, and the fact that thousands of copies of the July 2006 edition of
FEMA
550 have been distributed, the Case H drawings are presented as standalone
drawings. Separate title sheets and general notes for the Case H drawings are
presented that are not intended to be used with the original drawings for
Cases
A through G.
Appendix B. Pattern Book Design
The illustrations in this appendix are from A Pattern Book for Gulf Coast
Neighborhoods prepared for the Mississippi Governor’s Rebuilding Commission
on
Recovery, Rebuilding, and Renewal by Urban Design Associates (UDA) of
Pittsburgh, Pennsylvania, in November 2005.
http://www.mississippirenewal.com/documents/Rep_PatternBook.pdf
Appendix C. Assumptions Used in Design
Gulf Coast Foundation Designs
The foundation designs proposed in Appendix A are based on the following
standards and codes:
ASCE 7-05
Minimum Design Loads for Buildings and Other Structures American Society of
Civil Engineers (ASCE)
ASCE 24
Flood Resistant Design and Construction American Society of Civil Engineers
(ASCE)
ACI 318-02
Building Code Requirements for Structural Concrete American Concrete
Institute
(ACI)
ACI 530-02/ASCE 5-02/TMS 402-02
Building Code Requirements for Masonry Structures American Concrete Institute
(ACI) American Society of Civil Engineers (ASCE) The Masonry Society (TMS)
ANSI/AF&PA NDS-2005
National Design Specifications for Wood Construction American Forest & Paper
Association (AF&PA) American National Standards Institute American National
Standards Institute (ANSI)American Wood Council (AWC)
IRC-2003
2003 International Residential Code for One- and Two-Family Dwellings
International Code Council (ICC)
FEMA 550 has been checked and found to be consistent with both the 2006 and
the
2009 IRC.
To provide flexibility for the builder, a range of dead loads and building
dimensions was used for calculating reactions on the foundation elements. For
uplift and overturning analyses, the structure was assumed to be relatively
light and narrow, and constructed with a relatively low-sloped roof. For
sliding
analyses, the home was considered relatively deep and constructed with a
steeper
roof slope. For the gravity loading analysis, a heavier structure was
assumed.
Dead Loads
For Use in ASCE 7-05 ASD Uplift/Overturning Load Combination #7 (0.6D + W +H)
First Floor 8 psf Vinyl flooring, 5/8-inch plywood sub-floor and 2 by 8
joists
16 inches on centers
Second Floor 10 psf First floor components plus 1 layer of ½-inch gypsum
drywall
Wall 9 psf Wood siding, 2 by 4 studs 16 inches on centers, ½-inch plywood
wall
sheathing, and one layer of ½-inch gypsum drywall
Roof12 psf200 lb/sq asphalt roofing, 15 lb/sq felt, ½-inch plywood decking, 2
by
4 top and bottom truss chords 24 inches on centers, ½-inch gypsum drywall
ceiling finish
Dead Loads
For Use in ASCE 7-05 ASD Gravity Load Combination #2 (D + H + F + L + T)
First Floor 16 psf Dead loads increased 8 lb/sf to account for additional
finishes like hardwood flooring (4 lb/sf), ½-inch slate (7 lb/sf), or thin
set
tile (5 lb/sf)
Second Floor 18 psf
Wall 10 psf Wall weight increased to account for cement composite siding
Roof 12 psf
Concrete 150 psf Normal weight concrete. Footings for continuous perimeter
walls
were also sized to support full height brick veneer at 40 psf.
Masonry 115 psf Medium weight block
Grout 105 psf
Brick Veneer 40 psf
Wind Loads
Designs provided for 120-mph, 130-mph, 140-mph, and 150-mph zones (3-second
gust
wind speeds per ASCE 7-05). Wind analysis used Method 2 for buildings of all
heights.
Exposure
Category C Open terrain with scattered obstructions generally less than 30
feet
in height; shorelines in hurricane-prone areas
K sub zt = 1 No topographic effects (i.e., no wind speedup effects from
hills,
ridges, or escarpments)
K sub d = 0.85 Wind directionality factor (for use with ASCE 7-02 load
combinations)
K sub z, K sub h Velocity pressure coefficients for Exposure Category C
Flood Loads
V zone Breaking wave load from a wave with height 78 percent of still- water
depth (d sub s) Flood velocity (fps) equal to (gd sub s)1/2 up to a maximum
of
10 fps (FEMA 55 Upper Bound)
Coastal A zone Breaking wave load from 1½ foot up to a 3-foot high wave
Flood
velocity (fps) equal to (gds)1/2 up to a maximum of 5 fps (FEMA 55 Upper
Bound)
Non-Coastal A zone Breaking wave load up to a 1½-foot high wave Flood
velocity
(fps) equal to stillwater flood depth (d sub s) (in feet)(FEMA 55 Lower
Bound)
Lateral Loads (on stem walls
Lateral earth pressures from saturated soils 100 pounds per cubic foot (pcf)
Surcharge for slab weight and first floor live load65 pounds per square foot
(psf)
Live Loads
First Floor 40 psf
Second Floor 30 psf
Roof 20 psf
Soil Bearing Capacity
1,500 psf presumptive value for clay, sandy clay, silty clay, clayey silt,
and
sandy silt (CL, ML, MH, and CH soils) (2009 IRC)
Building Dimensions
Building Width 14 ft (per module)
Building Depth max 42 ft min 24 ft
Shear Wall Spacing max 42 ft
Floor Height 10 ft (floor to floor dimension)
Roof Pitch Ratio min 3:12 Uplift and overturning calculation max 12:12
Sliding
calculation
Roof Overhang 2 ft
Appendix D. Foundation Analysis and Design Examples
Chapter 3 described the types of loads considered in this manual. This
appendix
demonstrates how these loads are calculated using a sample building and
foundation. The reactions from the loads imposed on the example building are
calculated, the loads on the foundation elements are determined, and the
total
loads are summed and applied to the foundation.
There is a noteworthy difference between the approach taken for designing the
foundations included in this manual and the analyses that a designer may
undertake for an individual building. The analyses used for the designs in
this
manual were based on the “worst case” loading scenario for a “range of
building
sizes and weights.” This approach was used to provide designers and
contractors
with some flexibility in selecting the home footprint and characteristics for
which these pre-engineered foundations would apply. This also simplifies
application of the pre-engineered foundations, reducing the number of pre-
engineered foundations, and results in conservative designs.
For example, the designs presented were developed to resist uplift and
overturning for a relatively light structure with a flat roof (worst case
uplift
and overturning) while gravity loads were based on a relatively heavy
structure
to simulate worst case gravity loads. Sliding forces were determined for a
relatively deep home with a steep roof to simulate the largest lateral loads.
The range of building weights and dimensions applicable to the designs are
listed in Appendix C.
Since the designs are inherently conservative for some building dimensions
and
weights, a local design professional may be consulted to determine if
reanalyzing to achieve a more efficient design is warranted. If a reanalysis
is
determined to be cost-effective, the sample calculations will aid the
designer
in completing that analysis.
D.1 Sample Calculations
The sample calculations have been included to show one method of determining
and
calculating individual loads, and calculating load combinations.
Type of Building
The sample calculation is based on a two-story home with a 28-foot by 42-foot
footprint and a mean roof height of 26 feet above grade. The home is located
on
Little Bay in Harrison County, Mississippi, approximately 1.5 miles southwest
of
DeLisle (see Figure D-1). The site is located on the Harrison County Flood
Recovery Map in an area with an Advisory Base Flood Elevation (ABFE) of 18
feet,
located between the 1.5-foot wave contour and the 3-foot wave contour
(Coastal A
zone). The local grade elevation is 15 feet North American Vertical Datum
(NAVD). The calculations assume short- and long-term erosion will occur and
the
ground elevation will drop 1 foot during the life of the structure. This
places
the home in a Coastal A zone with the flood elevation approximately 4 feet
above
the eroded exterior grade. Based on ASCE 7-05, the 3-second gust design wind
speed at the site is 128 miles per hour (mph) Exposure Category C. To reduce
possible damage and for greater resistance to high winds, the home is being
designed for a 140-mph design wind speed.
[Begin figure]
[End figure]
The home has a gabled roof with a 3:12 roof pitch. The home is wood-framed,
contains no brick or stone veneer, and has an asphalt shingled roof. It has a
center wood beam supporting the first floor and a center load bearing wall
supporting the second floor. Clear span trusses frame the roof and are
designed
to provide a 2-foot overhang. No vertical load path continuity is assumed to
exist in the center supports, but vertical and lateral load path continuity
is
assumed to exist elsewhere in the structure.
The proposed foundation for the home is a system of steel pipe piles, a
reinforced concrete grade beam, and concrete columns extending from the grade
beam to the elevated structure.
Methodology
1. Determine the loads based on the building’s parameters (Section D.1.1)
2. Calculate wind and flood loads using ASCE 7-05 (Section D.1.2)
3. Consider the structure as a rigid body, and use force and moment
equilibrium equations to determine reactions at the perimeter foundation
elements (Section D.2)
D.1.1 Determining Individual Loads on a Structure
Building Dimensions and Weights (pulled from the text of the example problem)
B = 42 Building width (ft)
L = 28 Building depth (ft)
F sub 1 = 10 First floor height (ft)
F sub 2 = 10 Second floor height (ft)
R = 3 3:12 roof pitch
W sub ovhg = 2 Width of roof overhang (ft)
Dead Loads
W sub rfDL = 12 Roof dead load including upper level ceiling finish (in
pounds
per square foot [psf])
W sub 1stDL = 8 First floor dead load (psf)
W sub 2ndDL = 10 Second floor dead load, including first floor ceiling finish
(psf)
W sub wlDL = 9 Exterior wall weight (psf of wall area)
Live Loads
W sub 1stLL = 40 First floor live load (psf)
W sub 2ndLL = 30 Second floor live load (psf)
W sub rfLL = 20 Roof live load (psf)
Wind Loads
Building Geometry
h/L = 1
L/B = 0.7
Site Parameters (ASCE 7-05, Chapter 6)
K sub zt = 1 Topographic factor (no isolated hills, ridges, or escarpments)
K sub d = 0.85 Directionality factor (for use with ASCE 7-05, Chapter 2, Load
Combinations)
K sub h = 0.94 For simplicity, Velocity Pressure Coefficient used at the 26
foot
mean roof height was applied at all building surfaces. See ASCE 7-05, Table
6-3,
Cases 1 and 2.
I = 1 Importance factor (residential occupancy)
V = 140 3-second gust design wind speed (mph)
G = 0.85 Gust effect factor (rigid structure, ASCE 7-05, Section 6.5.8.1)
External Pressure Coefficients (Cp) (ASCE 7-05, Figure 6-6)
C sub pwwrf = -1.0 Windward roof (side facing the wind)
C sub plwrf = -0.6 Leeward roof
C sub pwwl = 0.8 Windward wall
C sub plwwl = -0.5 Leeward wall (side away, or sheltered, from the wind)
C sub peave = -0.8 Windward eave
Positive coefficients indicate pressures acting on the surface. Negative
coefficients indicate pressures acting away from the surface.
Velocity Pressure (q sub h) (ASCE 7-05, Section 6.5.10)
Velocity pressure at mean roof height:
q sub h = 0.00256 KhKzt Kd V squared I
= 0.00256 (0.94)(1)(0.85)(140) squared (1)
q sub h = 40 psf
Wind Pressures (P)
Determine external pressure coefficients for the various building surfaces.
Internal pressures, which act on all internal surfaces, do not contribute to
the
foundation reactions. For sign convention, positive pressures act inward on a
building surface and negative pressures act outward.
P sub wwrfV = q sub h GC sub pwwrf Windward roof
= (40)(0.85)(-1)
P sub wwrfV = -34 psf
Likewise
P sub lwrfV = q sub h GC sub plwrf Leeward roof
P sub lwrfV = -20 psf
P sub wwwl = q sub h GC sub pwwwl Windward wall
P sub wwwl = 27 psf
P sub lwwl = q sub h GC sub plwwl Leeward wall
P sub lwwl = -17 psf
P sub wweave = q sub h GC sub peave Windward roof overhang
P sub wweave = -27 psf - eave
= -34 psf - upper surface
= -61 psf - total
Wind Forces (F) (on a 1-foot wide section of the home)
F sub wwrfV = P sub wwrfV L/2 Windward roof vertical force
= (-34 psf)(14 sf/lf)
= -476 lb/lf
F sub lwrfV = PlwrfV L/2 Leeward roof vertical force
= (-20 psf)(14 sf/lf)
= -280 lb/lf
F sub wwrfH = P sub wwrfH L/2 (r/12) Windward roof horizontal force
= (-34 psf)(14 sf/lf)(3/12)
= -119 lb/lf
F sub lwrfH = P sub lwrfH L/2 (r/12) Leeward roof horizontal force
= (-20 psf)(14 sf/lf)(3/12)
= -70 lb/lf
F sub wwwl1st = P sub wwwl (F1) Windward wall on first floor
= (27 psf)(10 sf/lf)
= 270 lb/lf
F sub wwwl2nd = P sub wwwl (F2) Windward wall on second floor
= (27 psf)(10 sf/lf)
= 270 lb/lf
F sub lwwl1st = P sub lwwl (F1) Leeward wall on first floor
= (-17 psf)(10 sf/lf)
= -170 lb/lf
F sub lwwl2nd = P sub lwwl (F2) Leeward wall on second floor
= (-17 psf)(10 sf/lf)
= -170 lb/lf
F sub wweave = P sub wweave w sub owhg Eave vertical force (lb)
= -(61 psf)(2 sf/lf) horizontal projected areas is negligible so horizontal
force is neglected
= -122 lb/lf
D.1.2 Calculating Reactions from Wind, and Live and Dead Loads
Sum overturning moments (M sub wind) about the leeward corner of the home.
For
sign convention, consider overturning moments as negative. Since vertical
load
path continuity is assumed not to be present above the center support, the
center support provides no resistance to overturning (see Figure D-2).
Mwind = (-476 lb/lf)(21 ft) + (-280 lb/lf)(7 ft) + (119 lb/lf)(21.75 ft) + (-
70
lb/lf)(21.75 ft) + (-270 lb/lf)(5 ft) + (-270 lb/lf)(15 ft) + (-170 lb/lf)(5
ft)
+ (-170 lb/lf)(15 ft) + (-122 lb/lf)(29 ft)
= -23,288 ft-lb/lf
Solving for the windward reaction, therefore:
W sub wind = M sub wind divided by L
W sub wind = -23,228 ft (lb/lf divided 28 ft)
W sub wind = -830 lb/lf
The leeward reaction is calculated by either summing vertical loads or by
summing moments about the windward foundation wall. Leeward reaction equals -
48
lb/lf.
Lateral Wind Loads
Sum horizontal loads (Flat) on the elevated structure. (Forces to the left
are
positive. See Figure D-2.)
F sub lat = (-119 lb/lf) + (70 lb/lf) + (270 lb/lf) + (270 lb/lf) + (170
lb/lf)
+ (170 lb/lf)
= 831 lb/lf
[Begin figure]
Figure D-2 illustrates paths for wind, live, and dead loads.
[End figure]
Dead Loads
In this example, dead load reactions (W sub dead) are determined by summing
loads over the tributary areas. Since the roof is framed with clear-span
trusses
and there is a center support in the home, each exterior foundation wall
supports ½ of the roof load, all of the exterior wall load, and ¼ of the
first
and second floor loads. This approach to analysis is somewhat conservative
since
it does not consider the entire dead load of the structure to resist
overturning. Standard engineering practice often considers the entire weight
of
the structure (i.e., not just the portion supported by the perimeter
foundation
walls) available to resist overturning. The closed foundations in this
guidance
were developed considering only the tributary dead load to resist uplift. The
open foundations were developed considering all dead loads to resist uplift.
W sub dead = L/2 (w sub rfDL)+ L/4 (w sub 1stDL + w sub 2ndDL) + (F sub 1 + F
sub 2) w sub wlDL
= [14 sf/lf (12 psf)] + [7 sf/lf (8 psf + 10 psf)] + [(10 sf/lf + 10 sf/lf) 9
psf]
= 474 lb/lf
Live Loads
Floor
Live loads (W sub live) are calculated in a similar fashion
W sub live = L/4(W sub 1stLL + W sub 2ndLL)
= (7 sq ft/lf)(40 + 30) psf
= 490 lb/lf
Roof
W sub liveroof = L/2(W sub rfLL)
= (14 sq ft/lf)(20 psf)
= 280 lb/lf
D.2 Determining Load Combinations
Combine loads as specified in Chapter 2 of ASCE 7-05. For this example, an
allowable stress design approach was used. A strength-based design is equally
valid.
Other loads (such as snow, ice, and seismic) are listed in the ASCE 7-05 Load
Combinations, but were considered to be too rare in the Gulf Coast of the
United
States to be considered in the design. ASCE 7-05 also lists rain loads that
are
appropriate for the Gulf Coast region. Since a minimum roof slope ratio of
3:12
was assumed for the homes, rain loading was not considered. Table D-1 lists
foundation wall reactions for each load case. Critical reactions that contain
the foundation design are highlighted.
[Begin table]
Table D-1. Design Reactions on Base of Elevated Home
ASCE 7-05 Load Combination: #1 D
Vertical (lb/lf): 474
ASCE 7-05 Load Combination: #2 D + L
Vertical (lb/lf): 964
ASCE 7-05 Load Combination: #3 D + L sub r
Vertical (lb/lf): 754
ASCE 7-05 Load Combination: #4 D + 0.75(L) + 0.75(L sub r)
Vertical (lb/lf): {1,052 critical load}
ASCE 7-05 Load Combination: #5 D + W
Vertical (lb/lf): -356
Horizontal (lb/lf): 831
ASCE 7-05 Load Combination: #6 D + 0.75(W) + 0.75(L) + 0.75(L sub r)
Vertical (lb/lf): 1,016
Horizontal (lb/lf): 623
ASCE 7-05 Load Combination: #7 0.6D + W
Vertical (lb/lf): {-546 critical load}
Horizontal (lb/lf):{831 critical load}
ASCE 7-05 Load Combination: #8 0.6D
Vertical (lb/lf): 284
[End table]
Where
D = dead load
L = live load
L sub r = roof live load
W = wind load
Flood Load Effects on a Foundation
In this example, since the foundation selected is a system of steel pipe
piles,
the equations used to calculate flood loads are based on open foundations.
Some
of the equations used to calculate flood loads will be different if the
building
has a closed foundation system.
Many flood calculations depend on the stillwater flooding depth (d sub s).
While
not listed on FIRMs, d sub s can be calculated from the BFE by knowing that
the
breaking wave height (H sub b) equals 78 percent of the stillwater depth and
that 70 percent of the breaking wave exists above the stillwater depth (see
Figure 11-3 of FEMA 55). Stated algebraically:
BFE = GS + d sub s + 0.70 H sub b
= GS + d sub s + 0.70(0.78 d sub s)
= GS + 1.55 d sub s
GS = 15 ft NAVD (initial elevation) – 1 ft (short- and long- term erosion)
= 14 ft NAVD
d sub s = (BFE – GS) ÷ 1.55
= (18 feet NAVD – 14 feet NAVD) ÷ 1.55
= 4 ft ÷1.55
= 2.6 ft
Hydrostatic Loads
Hydrostatic loads act laterally and vertically on all submerged foundation
elements. On open foundations, lateral hydrostatic loads cancel and do not
need
to be considered but vertical hydrodynamic forces (buoyancy) remain. The
buoyancy forces reduce the effective weight of the foundation by the weight
of
the displaced water and must be considered in uplift calculations. For
example,
normal weight concrete which typically weighs 150 lb/ft3 only weighs 86 lb/ft
cubed when submerged in saltwater (slightly more in freshwater).
In this example, calculations are based on an 18-inch square normal weight
concrete column that extends 4 feet above eroded ground elevation. The column
weighs 1,350 pounds dry ((1.5 ft)(1.5 ft)(4 ft)(150 lb/ft cubed). Under flood
conditions, the column displaces 9 ft3 of saltwater that, at 64 lb/ft cubed3,
weighs 576 pounds so the column weighs only 774 pounds when submerged.
Hydrodynamic Loads
Flood Velocity
Since a Coastal A zone is close to the flood source, flood velocity is
calculated using the ASCE 7-05 Equation C5-2:
V = (g d÷s)superscript 1/2 Upper bound flood velocity
Where
G = Gravitational acceleration (32.2 ft/sec squared)
d sub s = Design stillwater depth (ft)
Hence
V = [(32.2)(2.6)]superscript 1/2
= 9.15 feet per second (fps)
Hydrodynamic Forces
A modified version of ASCE 7-05 Equation C5-4 can be used to calculate the
hydrodynamic force on a foundation element as
F sub dyn = ½ C sub d r V squared A
Where
F sub dyn = Hydrodynamic force (lb) acting on the submerged element
C sub d = 2.0 Drag coefficient (equals 2.0 for a square or rectangular
column)
r = 2 Mass density of salt water (slugs/cubic foot)
A = 1.5 d sub s Surface area of obstruction normal to flow (ft squared)
For open foundation, “A” is the area of pier or column perpendicular to flood
direction (calculated for an 18-inch square column).
Hence
F sub dyn = (½) (2)(2)(9.15)2(1.5)(2.6)
= 653 lb/column
The force is assumed to act at a point ds/2 above the eroded ground surface.
The formula can also be used for loads on foundation walls. The drag
coefficient, however, is different. For foundation walls, C sub d is a
function
of the ratio between foundation width and foundation height or the ratio
between
foundation width and stillwater depth. For a building with dimensions equal
to
those used in this example, C sub d for a closed foundation would equal 1.25
for
full submersion (42 feet by 4 feet) or 1.3 if submersed only up to the 2.6-
foot
stillwater depth.
Dynamic loads for submersion to the stillwater depth for a closed foundation
are
as follows:
F sub dyn = ½ C sub d r V squared A
F sub dyn = (½) (1.3)(2)(9.15) squared (1)(2.6)
= 283 lb/lf of wall
Floodborne Debris Impacts
In this example, the loads imposed by floodborne debris were approximated
using
formula 11-9 contained in FEMA 55.
The Commentary of ASCE 7-05 contains a more sophisticated approach for
determining debris impact loading, which takes into account the natural
period
of the impacted structure, local debris sources, upstream obstructions that
can
reduce the velocity of the floodborne debris, etc.
It is suggested that designers of coastal foundations review the later
standards
to determine if they are more appropriate to use in their particular design.
The FEMA 55 Formula 11.9 estimates debris impact loads as follows:
F sub I = wV divided by (g Delta t)
Where
F sub i = impact force (lb)
w = weight of the floodborne debris (lb)
V = velocity of floodborne debris (ft/sec)
g = gravitational constant = 32.2 ft/sec squared
Delta t = impact duration (sec)
Floodborne debris velocity is assumed to equal the velocity of the moving
floodwaters and acting at the stillwater level. For debris weight, FEMA 55
recommends using 1,000 pounds when no other data are available. The impact
duration depends on the relative stiffness of the foundation and FEMA 55
contains suggested impact durations for wood foundations, steel foundations,
and
reinforced concrete foundations. For this example, the suggested impact
duration
of 0.1 second was used for the reinforced concrete column foundation.
F sub i = wV divided g Delta t
F sub i = [1,000 lb (9.15 ft/sec)] divided by [(32.2 ft/sec squared)(0.1
sec)]
F sub i = 2,842 lb
Breaking Wave Loads
When water is exposed to even moderate winds, waves can build quickly. When
adequate wind speed and upstream fetch exist, floodwaters can sustain wave
heights equal to 78 percent of their stillwater depths. Depending on wind
speeds, maximum wave height for the stillwater depth at the site can be
reached
with as little as 1 to 2 miles of upwind fetch.
Breaking wave forces were calculated in this example using ASCE 7-05 formulae
for wave forces on continuous foundation walls (ASCE 7-05 Equations 5-5 and
5-6)
and on vertical piles and columns (ASCE 7-05 Equation 5-4).
The equation for vertical piles and columns from ASCE 7-05 is
F sub brkp = ½ C sub db gama DH sub b squared
Where
F sub brkp = Breaking wave force (acting at the stillwater level) (lb)
C sub db = Drag coefficient (equals 2.25 for square or rectangular
piles/columns)
Gamma = Specific weight of water (64 lb/ft cubed for saltwater)
D = Pile or column diameter in ft for circular sections, or for a square pile
or
column, 1.4 times the width of the pile or column (ft). For this example,
since
the column is 18 inches square, D = (1.4)(1.5 ft) = 2.1 ft.
H sub b = Breaking wave height (0.78 d sub s)(ft) = (0.78)(2.6) = 2.03 ft
Note: The critical angle of a breaking wave occurs when the wave travels in a
direction perpendicular to the surface of the column. Waves traveling at an
oblique angle (a) to the surface of the waves are attenuated by the factor
sin
squared alpha.
F sub brkp = ½ C sub db g DH sub b squared
= ½ (2.25)(64 lb/ft cubed)(2.1 ft)(2.03 ft) squared
= 623 lb
For closed foundations, use equations in Section 5.4.4.2 of ASCE 7-05 to
calculate F sub brkp. FEMA 55 contains the following two equations for
calculating loads on closed foundations:
F sub brkw = 1.1 C sub p rho d sub s squared + 2.4 gamma d sub s squared Case
1
and
F sub brkw = 1.1 C sub p rho d sub s squared 2 + 1.9 gamma d sub s squared
Case
2
Where gamma and d sub s are the specific weights of water and design
stillwater
depths as before. C sub p is the dynamic pressure coefficient that depends on
the type of structure. C sub p equals 2.8 for residential structures, 3.2 for
critical and essential facilities, and 1.6 for accessory structures where
there
is a low probability of injury to human life. The term F sub brkw is a
distributed line load and equals the breaking wave load per foot of wall
length
where F sub brkw is assumed to act at the stillwater elevation.
Case 1 of Formula 11.6 represents a condition where floodwaters are not
present
on the interior of the wall being designed or analyzed. Case 1 is appropriate
for foundation walls that lack flood vents (see Figure D-3). The less
stringent
Case 2 is appropriate for walls where NFIP required flood vents are in place
to
equalize hydrostatic loads and reduce forces (see Figure D-4).
[Begin figures]
Figure D-3. Case 1. This is an illustration that shows normally incident
breaking wave pressures against a vertical wall (space behind vertical wall
is
dry). Source: ASCE 7-05
Figure D-4. Case 2. This is an illustration that shows normally incident
breaking wave pressures against a vertical wall (stillwater level equal on
both
sides of wall). Source: ASCE 7-05
[End figures]
In non-Coastal A Zones, the maximum wave height is 1.5 feet. This corresponds
to
a stillwater depth (d sub s) of approximately 2 feet (i.e., 1.5 foot/0.78 for
a
depth limited wave). For closed foundations in coastal areas with flood
vents, a
1.5-foot breaking wave creates 1,280 lbs per linear foot of wall and 1,400
lbs
per linear foot of wall on foundations that lack flood vents.
Wind Load on Columns
Wind loads have been calculated per ASCE 7-05, Section 6.5.13 (Wind Loads on
Open Buildings and with Monoslope, Pitched, or Troughed Roofs). The velocity
pressure (q sub h) calculated previously was used, although this is a
conservative figure based on the 26-foot mean roof height. The force
coefficient
(C sub f) was determined from ASCE 7-05, Figure 6-21 (chimneys, tanks,
rooftop
equipment, and similar structures); ASCE 7-05, Figure 6-20 (walls and solid
signs) could have been used as well.
From ASCE 7-05 Equation 6-28:
F sub wind = q sub z G C sub f A sub f
= 40 psf (0.85)(1.33 (interpolated C sub f))(1.5 ft)(4 ft)
= 270 lb
Wind loads on the foundation elements are not considered in combination with
flood loads since the elements are submerged during those events.
Flood Load Combinations
Section 11.6.12 of FEMA 55 provides guidance on combining flood loads. In
Coastal A zones, FEMA 55 suggests two scenarios for combining flood loads.
Case
1 involves analyzing breaking wave loads on all vertical supports and impact
loading on one corner or critical support and Case 2 involves analyzing
breaking
wave loads applied to the front row of supports (row closest to the flood
source), and hydrodynamic loads applied to all other supports and impact
loads
on one corner or critical support.
Depending on the relative values for dynamic and breaking waves, Case 1 often
controls for designing individual piers or columns within a home. Case 2
typically controls for the design of the assemblage of piers or columns
working
together to support a home. Because of the magnitude of the load, it is not
always practical to design for impact loads. As an alternative, structural
redundancy can be provided in the elevated home to allow one pier or column
to
be damaged by floodborne debris impact without causing collapse or excessive
deflection.
For the sample calculations, Case 1 was used (see Figure D-5) with a breaking
wave load of 623 pounds applied to a non-critical column. The loads were then
determined and summarized. Since the calculations must combine distributed
loads on the elevated structure and discrete loads on the columns themselves,
a
column spacing of 7 feet is assumed in the calculations. For lateral loads on
the structure, calculations are based on three rows of columns sharing
lateral
loads (Table D-2).
[Begin figure]
Figure D-5 is an illustration showing flood loads.
[End figure]
[Begin table]Table D-2. Loads on Columns Spaced 7 Feet On Center (for three
rows
of columns)
ASCE 7-05 Load Combination: #1 D + F
Vertical (lb): 1,350 + [7(474)] = 4,668
ASCE 7-05 Load Combination: #2 D + F + L
Vertical (lb): 1,350 + [7(964)] = 8,098
ASCE 7-05 Load Combination: #3 D + F + L sub r
Vertical (lb): 1,350 + [7(754)] = 6,628
ASCE 7-05 Load Combination: #4 D + F + 0.75(L) + 0.75(L sub r)
Vertical (lb): 774 + 7(1,052) = {8,138}
ASCE 7-05 Load Combination: #5 D + F + W + 1.5(F sub a)
Vertical (lb): 774 + [7(474 - 48)] = 3,756
Horizontal (lb): [(1/3)7(831)] + [1.5(623)] = {2,874}
ASCE 7-05 Load Combination: #6 D + F + 0.75(W) + 0.75(L) + 0.75(L sub r) +
1.5(F sub a)
Vertical (lb): 774 + 7[474 + 0.75(-48+490+280)]= 7,883
Horizontal (lb): [(1/3)7(0.75)(831)] + [1.5(623)]= 2,388
ASCE 7-05 Load Combination: #7 0.6D + W + 1.5(F sub a)
Vertical (lb): 0.6[774 + 7(474)] + [7(-830)]= {-3,555}
Horizontal (lb): [(1/3)7(831)] + [1.5(623)] = {2,874}
ASCE 7-05 Load Combination: #8 0.6D
Vertical (lb): [0.6(1,350)] + [7(306)] = 3,000
[End table]
Where
D = dead load
F = load due to fluids with well-defined pressures and maximum heights (see
Section D.2 for additional information)
F sub a = flood load
L = live load
L sub r = roof live load
W = wind load
Note: Critical loads are in bold with brackets ({ }).
Results
Each perimeter column needs to support the following loads:
Vertical Load = 8,138 lb
Uplift = 3,555 lb
Lateral Load = 2,874 lb
With the critical loads determined, the foundation elements and their
connections to the home can be designed.
The following two examples are to demonstrate designs using information
provided
in this manual. The first example is based on a closed foundation; the second
example is based on an open foundation.
D.3 Closed Foundation Example
A structure to be supported by the closed foundation is identical to the
structure analyzed in the example from Section D.1. The site, however, is
different. For the closed foundation design, the structure is to be placed in
a
non-Coastal A zone where breaking waves are limited to 1.5 feet. The design
stillwater depth is 2 feet, and the BFE is 3 feet above exterior grade. While
the structure could be placed on a 3-foot foundation, the property owner
requested additional protection from flooding and a 4-foot tall foundation is
to
be built. Since the home elevation is identical to that in the example, the
loads and load combinations listed in Table D-1 are identical. However, since
the foundation is closed, flood forces must first be analyzed.
Like the previous analysis example, flood forces consist of hydrodynamic
loads,
debris loads, and breaking wave loads. Since the home is located in a non-
Coastal A zone, it is appropriate to use lower bound flood velocities. This
will
significantly reduce hydrodynamic and debris loads. From FEMA 55, the
following
equation is used:
V sub lower = d sub s
= 2 ft/sec
Hydrodynamic Loads
F sub dyn = ½ C sub d rho V squared A
= ½ C sub d rho Vsquared (1) d sub s
F sub dyn = (½) (1.4)(2)(2) squared(1)(2)
= 11 lb/lf of wall
Where Cd of 1.4 is for a (width of wall/ds) ratio of 21 (42 ft/2 ft)
(From FEMA 55, Table 11.2)
The hydrodynamic load can be considered to act at the mid-depth of the
stillwater elevation. The hydrodynamic load is less than the 27 psf wind load
on
the windward wall.
Debris Loads
F sub I = wV divided by gt
F sub I = [1,000 lb (2 ft/sec)] divided [(32.2 ft/sec squared)(0.1 sec)]
F sub I = 620 lb
Due to load distribution, the impact load will be resisted by a section of
the
wall. Horizontal shear reinforcement will increase the width of the section
of
wall available to resist impact. For this example, a 3-foot section of wall
is
considered to be available to resist impact. The debris impact load becomes
F sub iwall = (1/3) 620 lb
F sub iwall = 210 lb/ft
Breaking Wave Loads
The home is to be constructed in an SFHA; hence, the NFIP required flood
vents
will be installed. The breaking wave load can be calculated using formulae
for
equalized flood depths (Case 2).
F sub brkw = 1.1 C sub p gamma d sub s squared + 1.91 gamma d sub s squared
F sub brkw = gamma d sub s squared (1.1 C sub p + 1.91)
F sub brkw = (64)(2 squared){(1.1)(2.8) + 1.91}
F sub brkw = 1,280 lb/lf
The breaking wave load can be considered to act at the 2-foot stillwater
depth
(d sub s) above the base of the foundation wall.
The foundation must resist the loads applied to the elevated structure plus
those on the foundation itself. Chapter 2 of ASCE 7-05 directs designers to
include 75 percent of the flood load in load combinations 5, 6, and 7 for
non-
Coastal A zones. Table D-1 lists the factored loads on the elevated
structure.
Critical loads from Table D-1 include 546 lb/lf uplift, 1,052 lb/lf gravity,
and
831 lb/lf lateral from wind loading. The uplift load needs to be considered
when
designing foundation walls to resist wind and flood loads and when sizing
footings to resist uplift; the gravity load must be considered when sizing
footings and the lateral wind and flood loads must be considered in designing
shear walls.
Extending reinforcing steel from the footings to the walls allows the
designer
to consider the wall as a propped cantilever fixed at its base and pinned at
the
top where it connects to the wood framed floor framing system. The foundation
wall can also be considered simply supported (pinned at top and bottom). The
analysis is somewhat simpler and provides conservative results.
The 1,280 lb/lf breaking wave load is the controlling flood load on the
foundation. The probability that floodborne debris will impact the foundation
simultaneously with a design breaking wave is low so concurrent wave and
impact
loading is not considered. Likewise, the dynamic load does not need to be
considered concurrently with the breaking wave load and the 27 lb/sf wind
load
can not occur concurrently on a wall submerged by floodwaters.
The breaking wave load is analyzed as a point load applied at the stillwater
level. When subjected to a point load (P), a propped cantilevered beam of
length
(L) will produce a maximum moment of 0.197 (say 0.2) PL. The maximum moments
occur when “P” is applied at a distance 0.43L from the base. For the 4-foot
tall
wall, maximum moment results when the load is applied near the stillwater
level
(ds). In this example, the ASCE 7-05 required flood load of 75 percent of the
breaking wave load will create a bending moment of:
M = (0.2) f sub brkw (L)
= (0.2)(0.75)(1,280 lb/ft) (4 ft)
= 768 ft-lb/lf or
= 9,200 in-lb/lf
The reinforced masonry wall is analyzed as a tension-compression couple with
moment arm “jd,” where “d” is the distance from the extreme compression fiber
to
the centroid of the reinforcing steel, and “j” is a factor that depends on
the
reinforcement ratio of the masonry wall. While placing reinforcing steel off
center in the wall can increase the distance (d) (and reduce the amount of
steel
required), the complexity of off-center placement and the inspections
required
to verify proper placement make it disadvantageous to do so. For this design
example, steel is considered to be placed in the center of the wall and “d”
is
taken as one half of the wall thickness. For initial approximation, “j” is
taken
as 0.85 and a nominal 8-inch wall with a thickness of 7-5/8 inches is
assumed.
Solving the moment equation is as follows:
M = T (jd)
T = M/(jd) = Tension force
= M/(j)(t/2) (t = thickness of wall)
T = (9,200 in-lb/lf) ÷ {(0.85)(7.63 in)(0.5)}
= 2,837 lb/lf
For each linear foot of wall, steel must be provided to resist 2,837 pounds
of
bending stress and 546 pounds of uplift.
F sub steel = 2,837 lb/lf (bending) + 546 lb/lf (uplift)
= 3,383 lb/lf
The American Concrete Institute (ACI) 530 allows 60 kips per square inch
(ksi)
steel to be stressed to 24 ksi so the reinforcement needed to resist breaking
wave loads and uplift is as follows:
A sub steel = 3,383 lb/lf divided 24,000 lb/in squared
= 0.14 in squared /lf
Placing #5 bars (at 0.31 in squared/bar) at 24 inches on centers will provide
the required reinforcement. To complete the analysis, the reinforcement ratio
must be calculated to determine the actual “j” factor and the stresses in the
reinforcing steel need to be checked to ensure the limits dictated in ACI 530
are not exceeded. The wall design also needs to be checked for its ability to
resist the lateral forces from flood and wind.
Footing Sizing
The foundation walls and footings must be sized to prevent overturning and
resist the 546 lb/lf uplift. ASCE 7-05 load combination 6 allows 60 percent
of
the dead load to be considered in resisting uplift. Medium weight 8-inch
masonry
cores grouted at 24 inches on center weigh 50 lb/sf or, for a 4-foot tall
wall,
200 lb/lf. Sixty percent of the wall weight (120 lb/lf) reduces the amount of
uplift the footing must resist to 426 lb/lf. At 90 lb/ft cubed (60 percent of
150 lb/ft cubed for normal weight concrete), the footing would need to have a
cross-sectional area of 4.7 square feet. Grouting all cores increases the
dead
load to 68 lb/sf and reduces the required footing area to 4.25 square feet.
The
bearing capacity of the soils will control footing dimensions. Stronger soils
can allow narrower footing dimensions to be constructed; weaker soils will
require wider footing dimensions.
The design also needs to be checked to confirm that the footings are adequate
to
prevent sliding under the simultaneous action of wind and flood forces. If
marginal friction resistance exists, footings can be placed deeper to benefit
from passive soil pressures.
D.4 Open Foundation Example
For this example, the calculations are based on a two-story home raised 8
feet
above grade with an integral slab-grade beam, mat-type foundation and a 28-
foot
by 42-foot footprint. The home is sited approximately 800 feet from the shore
in
a Coastal A zone. Subtracting the elevation of the site (determined from a
topographic map or preferably from a survey) from the ABFE and adding
estimated
erosion (in feet) determines that the floodwaters during a design event
(including wave effects and runup) will extend 6 feet above the eroded
exterior
grade. It is important to note, however, that submittal of an elevation
certificate and construction plans to local building code and floodplain
officials in many jurisdictions will require that the elevation be confirmed
by
a licensed surveyor referencing an established benchmark elevation.
The wood framed home has a 3:12 roof pitch with a mean roof height of 30
feet, a
center wood beam supporting the first floor, and a center load bearing wall
supporting the second floor. Clear span trusses frame the asphalt-shingled
roof
and are designed to provide a 2-foot overhang. This home is a relatively
light
structure that contains no brick or stone veneers.
The surrounding site is flat, gently sloping approximately 1 foot in 150
feet.
The site and surrounding property have substantial vegetation, hardwood
trees,
concrete sidewalks, and streets. A four-lane highway and a massive concrete
seawall run parallel to the beach and the established residential area where
the
site is located. The beach has been replenished several times in the last 50
years. Areas to the west of the site that have not been replenished have
experienced beach erosion to the face of the seawall. The ASCE 7-05, 3-second
gust design wind speed is 140 mph and the site is in an Exposure Category C.
The proposed foundation for the home incorporates a monolithic carport slab
placed integrally with a system of grade beams along all column lines (see
Figure D-6). The dimensions of the grade beam were selected to provide
adequate
bearing support for gravity loads, resistance to overturning and sliding, and
mitigate the potential of undermining of the grade beams and slab due to
scour
action. The home is supported by concrete columns, extending from the top of
the
slab to the lowest member of the elevated structure, spaced at 14 feet on
center
(see Figure D-7).
[Begin figures]
Figure D-6 illustrates a layout of Open Foundation Example.
Figure D-7 is a loading Diagram for Open Foundation Example.
[End figures]
Lateral Wind Loads
Sum horizontal loads (Flat) on the elevated structure (forces to the left are
positive)
F sub lat = (-126 lb/lf) + (74 lb/lf) + (280 lb/lf) + (280 lb/lf) + (180
lb/lf)
+ (180 lb/lf)
= 868 lb/lf
Dead Loads
Dead load reactions (W sub dead) are determined by summing loads over the
tributary areas. For the anterior columns:
W sub dead = L/2 (w sub rfDL) + L/4 (w sub 1stDL + w sub 2ndDL) + (F sub 1 +
F
sub 2) w sub wlDL
= [14 sf/lf (12 psf)] + [7 sf/lf (8 psf + 10 psf)] + [(10 sf/lf + 10 sf/lf) 9
psf]
= 474 lb/lf
Live Loads
Floor
Live loads (W sub live) are calculated in a similar fashion
W sub live = L/4(W sub 1stLL + W sub 2ndLL)
= (7 sq ft/lf)(40 + 30) psf
= 490 lb/lf
Roof
W sub liveroof = L/2(W sub rfLL)
= (14 sq ft/lf)(20 psf)
= 280 lb/lf
A minimum roof slope of 3:12 was assumed for the homes; rain loading was not
considered.
Flood Effects
Since the foundation selected is a system of concrete columns, the equations
used to calculate flood loads are based on open foundation. The stillwater
flooding depth (d sub s) is as follows:
d sub s = DFE divided by 1.55
= 6 ft divided by 1.55
= 3.9 ft
Hydrostatic Loads
Calculations are based on a 16-inch square normal weight concrete column that
extends 8 feet above the concrete slab.
The column weighs 2,123 pounds dry ((1.33 ft)(1.33 ft)(8 ft)(150 lb/ft
cubed).
Under flood conditions, the column displaces 10.6 ft cubed of saltwater
which,
at 64 lb/ft cubed, weighs 679 pounds so the column weighs 1,444 pounds when
submerged.
Hydrodynamic Loads
Flood Velocity
Since a Coastal A zone is close to the flood source, flood velocity is
calculated using the ASCE 7-05 Equation C5-2 as follows:
V = [(32.2 ft/sec squared)(3.9 ft)] superscriopt 1/2
= 11.21 feet per second (fps)
Flood Force
ASCE 7-05 Equation C5-4 is as follows:
F sub dyn = ½ C sub d rho V squared A
= (½) (2)(2)(11.21 fps) squared (1.33 ft)(3.9 ft)
= 1,303 lb/column
Floodborne Debris Impact
The flood debris impact can be estimated, per FEMA 55 Formula 11.9, as
follows:
F sub i = wV divided by¸ gt
= [1,000 lb (11.21 ft/sec)] ¸ [(32.2 ft/sec squared)(0.1 sec)]
= 3,478 lb
Breaking Wave Load
The equation for vertical piles and columns from ASCE 7-05 is as follows:
F sub brkp = ½ Cdb gamma DH sub b squared
= ½ (2.25)(64 lb/ft cubed)(1.82 ft)(3.04 ft 9*) squared
= 1,211 lb
* A wave height of 3.04 ft suggests a V zone but, in this example, the depth
of
water is increased by erosion, which is not considered in mapping A zones.
The
deeper water supports a bigger wave, which in this case exceeds the V-zone
wave
height minimum.
Wind Load on Columns
For a load case combining both wind and flood forces, the column would be
almost
completely submerged; therefore, the wind load on the column shall not be
included.
Calculating Reactions from Wind, Live, and Dead Loads
Sum overturning moments (M sub wind) and (M sub flood) about the leeward
corner
of the mat foundation. For sign convention, consider overturning moments as
negative. Note in this example the home is slightly higher above grade and
hence
the wind loads are slightly higher.
M sub wind = (-504 lb/lf)(21 ft) + (-294 lb/lf)(7 ft) + (126 lb/lf)(21.75 ft)
+
(-74 lb/lf)(21.75 ft) + (-280 lb/lf)(13 ft) + (-280 lb/lf)(23 ft) + (-180
lb/lf)(13 ft) + (-180 lb/lf)(23 ft) + (-130 lb/lf)(29 ft)
= -31,841 ft-lb/lf
The vertical components of the reaction caused by the wind overturning moment
is:
R sub x = 31,841 lb ÷ 28 ft = +/- 1,137 lb/ft
M sub flood = 1.5[((-1,211 lb)(3.9 ft)) + (2(-1,303 lb)(3.9 ft/2)) + ((-3,478
lb)(3.9 ft))] = 35,053 ft-lb/ft
The vertical component of the reaction caused by the flood overturning moment
is:
R sub x = 35,053 lb ÷ 28 ft = +/- 1,252 lb outboard columns
Load Combinations
Table D-3 summarizes loads on the open foundation example. Loads are listed
for
the eight load combinations and critical loads are highlighted.
Table D-3. Loads at Base of Columns Spaced 14 Feet On Center (for three rows
of
columns per bay)
ASCE 7-05 Load Combination: #1 D + F
Vertical (lb): 1,444 + 14(474) = 8,080
ASCE 7-05 Load Combination: #2 D + F + L
Vertical (lb): 1,444 + 14(964) = 14,940
ASCE 7-05 Load Combination: #3 D + F + L sub r
Vertical (lb): 1,444 + 14(754) = 12,000
ASCE 7-05 Load Combination: #4 D + F + 0.75(L) + 0.75(sub Lr)
Vertical (lb): 8,080 + (0.75)[(14)(490+280)] = 16,165
ASCE 7-05 Load Combination: #5 D + F + W + 1.5(F sub a)
Vertical (lb): 8,080 +/- 14(1,137) +/- 1,252 = 25,876; -9,716 windward;
leeward
Horizontal (lb): wind + (1.5)[F sub dyn + F sub i][(14(868)(1/3)] + (1.5)
[(1,303+3,478)] = 11,222
ASCE 7-05 Load Combination: #6 D + F + 0.75(W) + 0.75(L) + 0.75(L sub r) +
1.5(F sub a)
Vertical (lb): 8,080 +/- (0.75)(14)(1,137) +(0.75)(14)([(490+280)] +/- 1,252
=
2,348; 29,982 windward; leeward
Horizontal (lb): (0.75) wind + (1.5)[F sub dyn + F sub
i][(0.75)(14)(868)(1/3)]
+ (1.5)[(1,303+3,478)] = 10,210
ASCE 7-05 Load Combination: #7 0.6D + W + 1.5(F sub a)
Vertical (lb): 0.6 [2,123+14(474)] +/- 14(1,137) +/- (1.5)1,252 = -12,541;
23,051 windward; leeward
Horizontal (lb): wind + (1.5)[F sub dyn + F sub i][(14(868)(1/3)] +
(1.5)[(1,303+3,478)] = 11,222
ASCE 7-05 Load Combination: #8 0.6D
Vertical (lb): [0.6((2,123) + 14(474))] = 5,255
[End table]
Critical loads are in bold.
Where
D = dead load
F = fluid (buoyancy) load
L = live load
L sub r = roof live load
W = wind load
Ww = windward
Lw = leeward
Results
Each perimeter column needs to support the following loads:
Vertical Load = 29,982 lb
Uplift = 12,541 lb
Lateral Load = 11,222 lb
Moment wind + f sub dyn = [(1/3)(14)(1,314)(8) + (1,303)(3.9/2)] ÷ 1,000
lb/kip
= 51.6 ft-kip
Moment wind + f sub brkp = [(1/3)(14)(1,314)(8) + (1,113)(3.9)] ÷ 1,000
lb/kip
= 96.9 ft-kip
Moment debris = (3,478)(3.9) ÷ 1,000 lb/kip
= 13.6 ft-kip
Moment wind + f sub dyn+ debris = 51.6 ft-kip + 13.6 ft-kip
= 65.1 ft-kip
The force is assumed to act at a point ds/2 above the eroded ground surface.
For
concrete design, we use load factors per ASCE 7-05.
Ultimate Moment wind + f sub dyn = (48.6)(1.2)+(2.5)(2.0)
= 63.4 ft-kip
Ultimate Moment wind + f sub brkp = (48.6)(1.2)+(4.3)(2.0)
= 66.9 ft-kip
Ultimate Moment wind + f sub dyn + debris = (63.4)+13.6(2)
= 90.6 ft-kip
Foundation Design
Overturning
The overturning moment due to wind with a typical bay of 14 feet wide is as
follows:
M sub wind = (-31,841 ft-lb/lf)(14 ft)
= -445,774 ft-lb
M sub Fa (1.5) = (1.5)[(1,211 lb)(3.9 ft) + (2)(1,303 lb)(3.9 ft/2)]
= -14,707 ft-lb
M sub o = -445,774 ft-lb - 14,707 ft-lb
= -460,481 ft-lb
In this example, it is assumed that the home and the foundation slab are
reasonably symmetrical and uniform; therefore, it is assumed the center of
gravity for the dead loads is at the center of the bay.
Dead load at perimeter columns
D sub ext = (474 lb/ft)(14 ft)(2 columns)
= 13,272 lb
Dead load at an interior column:
D sub int = (14 ft)(14 ft)(8 psf)
= 1,568 lb
Dead load of 3 columns: (3)(8 ft)(1.33 ft x 1.33 ft)(150)lb cubic ft
= 6,368 lb
Assume that only the grade beams are sufficiently reinforced to resist
overturning (neglect weight of slab)
Dead load of the grade beams (area)(depth) (density of concrete)[(28 ft x 3
ft)
+ (3)((11 ft)(3 ft)](4 ft)(150 lb cubic ft) = 109,800 lb
Summing the dead loads = 13,272 + 1,568 + 6,368 + 109,800 = 131,008 lb
Allowable dead load moment of 60 percent
M sub d = (0.6)( 131,008 lb)(14 ft)
= 1,100,476 ft-lb
Since M sub ot = 460,481 ft-lb is very much less than 0.6 M sub d = 1,100,476
ft-lb, the foundation can be assumed to resist overturning.
Sliding
The maximum total lateral load of wind and flood acting on the entire typical
bay is as follows:
L sub wfa = W + 1.5 F sub a
= [(14 ft (868 lb/ ft) + 1.5(7,203 lb)]
= 23,375 lb
Sliding Resistance = (tan phi)N + Passive Resistance at Vertical Foundation
Surfaces
Phi = internal angle of soil friction, assume phi = 25 degrees
N = net normal force (building weight - uplift forces)
N = (131,008) + (14 ft)[((16 ft)(-21 psf )+(14 ft)(-36 psf)+(2 ft)(-65 psf)]
= 117,428 lb
Ignore passive soil pressure
Dead Load Sliding Resistance = (tan 25)(117,428 lb)
= 54,758 lb
Since L sub wfa = 23,371 lb is less than 60 percent of Dead Load Sliding
Resistance = 54,758, the foundation can be assumed to resist sliding.
Soil Bearing Pressure
The simplified approach for this mat foundation assumes that only the grade
beams carry loads to the soil; the slab between grade beams is not considered
to
contribute support. It is further assumed that the bearing pressure is
uniform
in the absence of wind and flood loading. The areas of the grade beams along
the
outboard column lines, in the direction of the flow of wind and flood, are
considered the “critical areas” of the grade beam. The load combination table
below indicates the bearing pressures for the ASCE 7- 05 load combinations
for
the critical grade beam area. These load combinations are calculated to
ensure
that downward forces of the wind and flood moment couple do not overstress
the
soil. The factored dead load moment that resists overturning is of a
magnitude
such that there is no net uplift along critical grade beams. Table D-4
presents
foundation bearing pressures for typical bays.
Self Weight of 1 square of foot Grade Beam = (4 ft)(150 lb/cubic ft) = 650
psf
Area of Critical Grade Beam = [(3ft)(14 ft)] + [(3 ft)(5.5 ft)]
= 58.5 ft squared
Weight of Critical Grade Beam = (58.5 ft squared)(4 ft)(150 lb/cubic ft)
= 35,100 lb
Critical Column Uplift = 12,541 lb (load combination 7)
Verification of Uplift Resistance = [35,100 lb(0.6)] – 12,541 lb
= 8,519 lb (positive load, no uplift)
Presumptive Allowable Bearing Pressure = 1,500 psf
[Begin table]
Table D-4. Foundation Bearing Pressures for Typical Bays (for three rows of
columns per bay)
ASCE 7-05 Load Combination: #1 D + F
Combined Loads (lb): 8,080
Bearing Pressures (psf): 8,090 ÷ 58.5 + 650 = 788
ASCE 7-05 Load Combination: #2 D + F + L
Combined Loads (lb): 14,940
Bearing Pressures (psf): 14,950 ÷ 58.5 + 650 = 905
ASCE 7-05 Load Combination: #3 D + F + L sub r
Combined Loads (lb): 12,000
Bearing Pressures (psf): 12,000 ÷ 58.5 + 650 = 855
ASCE 7-05 Load Combination: #4 D + F + 0.75(L) + 0.75(L sub r)
Combined Loads (lb): 16,165
Bearing Pressures (psf): 16,165 ÷ 58.5 + 650 = 926
ASCE 7-05 Load Combination: #5 D + F + W + 1.5(F sub a)
Combined Loads (lb): 28,104
Bearing Pressures (psf): 28,104 ÷ 58.5 + 650 = 1,130
ASCE 7-05 Load Combination: #6 D + F + 0.75(W) + 0.75(L) + 0.75(L sub r) +
1.5(F sub a)
Combined Loads (lb): 29,982
Bearing Pressures (psf): 29,982 ÷ 58.5 + 650 = 1,163
ASCE 7-05 Load Combination: #7 0.6D + W + 1.5(F sub a)
Combined Loads (lb): 23,051
Bearing Pressures (psf): 23,051 ÷ 58.5 + 650 = 1,044
ASCE 7-05 Load Combination: #8 0.6D
Combined Loads (lb): 5,255
Bearing Pressures (psf): 5,255 ÷ 58.5 + 650 = 740
[End table]
Where
D = dead load
F = load due to fluids with well-defined pressures and maximum heights (See
Section D.2 for additional information)
F sub a = flood load
L = live load
L sub r = roof live load
W = wind load
Note: The maximum bearing pressure is in bold (1,163 psf) and is less than
the
assumed 1,500 psf bearing pressure.
Design of Concrete members per ACI-318-02 Code and ASCE 7-05; Sections 2.3.2
and
2.3.3-1
Column Design
Verify that 16-inch x 16-inch column design is adequate.
Concrete strength = 4,000 psi
Reinforced with (4) #8 bars, grade 60 reinforcing, with 2½-inch clear cover
Note: 1,000 lb = 1.0 kip
Assume that the total wind load distributed through the floor uniformly to 3
columns.
Check combined axial and bending strength:
Ultimate Moment wind + f sub dyn = (1.6)[(8 ft)((14 ft)(868 lb/ft)/(3))] +
(2.0)[(3.9 ft/2)(1,303) lb)]
= 51,849 ft-lb + 5,082 ft-lb
= 56,931 ft-lb ÷ 1,000 lb/kip = 56.9 ft-kip
Ultimate Moment wind + f sub brkp = (51,849 ft-lb) + (2.0)[(1,211 lb)(3.9
ft)]
= 61,295 ft-lb ÷ 1,000 lb/kip = 61.3 ft-kip
Ultimate Moment wind + f sub brkp + debris = 61.3 ft-kip + (2.0)(3.5 kip)(3.9
ft)
= 88,6 ft-kip
Maximum Factored Moment = 88.6 ft-kip = 1,063 in kip
Refer to Table D-3, conservatively assume flood load factor of 2.0 for all
axial
loads
Maximum factored Axial Compression = (2.0)(30.0 kip) = 60.0 kip
Maximum factored Axial Tension = (2.0)(12.5 kip) = 25.0 kip
Based on a chart published by the Concrete Reinforced Steel Institute (CRSI),
the maximum allowable moment for the column = 1,092 in kip for 0 axial load
and
1,407 in kip for 102 kip axial load; therefore, the column is adequate.
Check shear strength:
Critical Shear = wind + F sub dyn + F sub i
Ultimate Shear = Vu = [(14 ft)(0.868 kip)(1/3)](1.6) + [(1.3 kip)+(3.5
kip](2.0)
= Vu = 16.0 kip
As the maximum unit tension stress is only 25.0 kip/16 in x 16 in = 0.098
kip/in
squared and the maximum axial compression stress is only 60.0 kip/16 in x 16
in
= 0.234 kip/in squared, we can conservatively treat the column as a flexural
member or beam. The allowable shear of the concrete section then, per ACI-
318-02
11.3.1.1, 11.3.1.3, and 11.5.5.1 with minimum shear reinforcing
(tie/stirrup),
would be as follows.
Allowable Shear = V sub c = (0.75)(16 in-2.5 in)(16 in )(2)(4,000
psi)superscript 1/2 (1/1,000) = 20.5 kip
The shear strength of the column is adequate with minimum shear
reinforcement.
The minimum area of shear, Av, per ACI 318-02, 11.5.5.3 would be:
A sub v = (50)(width of member)(spacing of reinforcing)/yield strength of
reinforcing
= (50)(16 in)(16 in)/(60,000 psi) = 0.21 in squared
2 pieces of #4 bar A sub s = (2)(0.2) = 0.40 in squared
Use of #4 bar for column ties (shear reinforcement) is adequate.
Check spacing per ACI -318-02, 7.10.4
16 diameter of vertical reinforcing bar = (16)(1 in) = 16 in
48 diameter of column tie bar = (48)(1/2 in) = 24 in
Least horizontal dimension = 16 in
Therefore, #4 ties at 16 inches on center are adequate.
The column design is adequate.
Grade Beam Design
The size of the grade beam was configured to provide adequate bearing area,
resistance to uplift, a reasonable measure of protection from damaging scour,
and to provide a factor of redundancy and reserve strength should the
foundation
be undermined. A grade beam 36 inches wide and 48 inches deep was selected.
Maximum Bearing Pressure = 1,163 psf = 1.2 ksf (kip/square foot)
Assume a combined load factor of 2.0 (for flood)
Check Shear strength:
Maximum Factored Uniform Bearing Pressure = w sub u = (2.0) (3.0 ft)(1.2 ksf)
=
7.2 kip/ft
Maximum Factored Shear = V sub u = (7.2 kip/ft)(14 ft/2) = 50.0 kip
Allowable Shear without minimum shear reinforcing (stirrups) = V sub c/2
V sub c/2= (0.75)(36 in)(48 - 3.5 in)(63 psi)(1/1,000) = 75.7 kip
Use nominal #4 two leg stirrups at 24 inches on center
Check flexural strength: Assume simple span condition
Maximum Factored Moment = M sub u = (7.2 kip/ft)(14 ft) squared (1/8) = 176
ft-
kip
Concrete strength = 4,000 psi
Reinforcement grade = 60,000 psi
Try (4) #6 reinforcing bar continuous top and bottom
A sub s = (4)(0.44 in squared) = 1.76 in squared top or bottom
total A sub s = (2)(1.76) = 3.52 in squared
Reinforcement ratio (r)
Rho = A sub s ÷ [(section width)(section depth- clear cover- ½ bar diameter)]
Rho = A sub s ÷ [(b sub w)(d)]
Rho = (1.76 in squared) ÷ [(36)(48 - 3 - 0.375)]
Rho = 0.01096
One method of calculating the moment strength of a rectangular beam, for a
given
section and reinforcement, is illustrated in the 2002 edition of the CRSI
Design
Handbook. Referencing page 5-3 of the handbook, the formula for calculating
the
moment strength can be written as follows:
Phi M sub n = (phi)[((A sub s)(f sub y)(d)) – (((A sub s)(f sub y)) ÷
((0.85)(f
'c)(width of member)(2)))]
Phi M sub n = (0.9)[((1.76)(60,000)(44.63)) – (((1.76)(60,000)) ÷
((0.85)(4,000)(36)(2)))]
= 4,241,634 in lb ÷ 12,000 = 353 ft-kip
phi = 0.9
Area of reinforcing steel (A sub s) minimum flexural
= (0.0033)(24)(44.63)
= 3.6 in squared
or
(1.33) (As required by analysis) = F sub Mn is much greater than M sub u
Area of steel for shrinkage and temperature required = (0.0018)(48)(36) = 3.1
in
squared
Total area of steel provided = (8) #6 = (8)(0.44) = 3.52 in squared adequate
Therefore, the grade beam design is adequate, use (4) #6 reinforcing bar
continuous on the top and bottom with #4 stirrups at 24-inch spacing.
Appendix E. Cost Estimating
The cost data provided in Appendix E were developed in 2006 for the First
Edition of FEMA 550 for select communities along the Gulf of Mexico. The
applicability or accuracy of the data to other coastal areas has not been
investigated. Although relative costs between foundation systems may apply in
other coastal regions, users of the manual should verify current actual costs
for any given location.
Appendix F. Pertinent Coastal Construction Information
FEMA 499 Fact Sheet No.
1 Coastal Building Successes and Failures
2 Summary of Coastal Construction Requirements and Recommendations
4 Lowest Floor Elevation
5 V-Zone Design and Construction Certification
6 How Do Siting and Design Decisions Affect the Owner's Costs?
7 Selecting a Lot and Siting the Building
8 Coastal Building Materials
9 Moisture Barrier Systems
11 Foundations in Coastal Areas
12 Pile Installation
13 Wood-Pile-to-Beam Connections
14 Reinforced Masonry Pier Construction
15 Foundation Walls
16 Masonry Details
26 Shutter Alternatives
27 Enclosures and Breakaway Walls
29 Protecting Utilities
All of the FEMA 499 Fact Sheets listed above can be viewed, downloaded, or
printed. Go to http://www.fema.gov/library/viewRecord.do?id=1570 and click on
the Resource File for the Fact Sheet(s) of interest.
FEMA P-757 Recovery Advisory
Erosion, Scour, and Foundation Design
The above Recovery Advisory can be viewed, downloaded, or printed. Go to
http://www.fema.gov/library/viewRecord.do?id=3577 and click on Appendix D,
pages
20-23.
Appendix G. FEMA Publications and Additional References
The American Concrete Institute (ACI). 2005. Building Code Requirements for
Structural Concrete and Commentary, ACI 318.
American Forest & Paper Association’s (AF&PA) American Wood Council (AWC).
2005.
National Design Specification for Wood Construction. ANSI/AF&PA NDS 2005.
American Forest & Paper Association. 2008. Wood Frame Construction Manual
(WFCM)
for One- and Two-Family Dwellings.
American Society for Testing and Materials (ASTM). 2000. Standard Test Method
for High-Strain Dynamic Testing of Piles, ASTM D 4945. November 2000.
American Society of Civil Engineers (ASCE) 7-05. 2005. Minimum Design Loads
for
Buildings and Other Structures, ASCE 7-05. ISBN: 0784408092.
ASCE 24-05. 2006. Flood Resistant Design and Construction. ISBN: 0784408181.
American Wood Preservers’ Association (AWPA). 1991. Care of Pressure-Treated
Wood Products.
American Wood Preservers’ Association. 2008. AWPA M4-08 Standard for the Care
of
Preservative-Treated Wood Products.
Concrete Reinforcing Steel Institute (CRSI). 2002. CRSI Design Handbook.
FEMA. 1984. Elevated Residential Structures, FEMA 54. March 1984.
FEMA. 1995. Guide to Flood Maps: How to Use a Flood Map to Determine Flood
Risk
for a Property, FEMA 258. May 1995.
FEMA. 1996. Corrosion Protection for Metal Connectors in Coastal Areas for
Structures Located in Special Flood Hazard Areas, FEMA Technical Bulletin 8-
96.
August 1, 1996.
FEMA. 2000. Coastal Construction Manual, FEMA 55. June 2000.
FEMA. 2004. Design Guide for Improving School Safety in Earthquakes, Floods,
and
High Winds, FEMA 424. January 2004.
FEMA. 2005. Mitigation Assessment Team Report: Hurricane Charley in Florida,
FEMA 488. April 2005.
FEMA. 2005. Mitigation Assessment Team Report: Hurricane Ivan in Alabama and
Florida, FEMA 489. August 2005.
FEMA. 2005. Home Builders’ Guide to Coastal Construction Technical Fact
Sheets, FEMA 499. August 2005.
FEMA. 2006. Mitigation Assessment Team Report: Hurricane Katrina in the Gulf
Coast, FEMA 549. July 2006.
FEMA. 2006. Flood Insurance Manual.
http://www.fema.gov/business/nfip/manual.shtm. May 2006.
FEMA. 2008. NFIP Technical Bulletin 2, Flood Damage-Resistant Materials
Requirements for Buildings Located in Special Flood Hazard Areas in
accordance
with the National Flood Insurance Program. August 2008
FEMA. 2009. Hurricane Ike MAT Recovery Advisories: Enclosures and Breakaway
Walls and Erosion, Scour, and Foundation Design. FEMA P-757. April 2009.
International Building Code (IBC). 2009 International Building Code. 2009.
International Code Council (ICC). Standard for Residential Construction in
High
Wind Regions (ICC-600). November 2006.
International Residential Code (IRC). 2009 International Residential Code for
One- and Two-Family Dwellings. 2009.
Mississippi Governor’s Rebuilding Commission on Recovery, Rebuilding and
Renewal. 2005. A Pattern Book for Gulf Coast Neighborhoods. November 2005.
National Fire Protection Association (NFPA). 2003. NFPA 5000®: Building
Construction and Safety Code®, 2003 Edition.
Appendix H. Glossary
3-second peak gust – The wind speed averaging time used in ASCE 7 and the
IBC.
Allowable Stress Design (ASD) – A method of proportioning structural members
such that elastically computed stresses produced in the members by nominal
loads
do not exceed specified allowable stresses (also called working stress
design).
A Zone – A zones are the areas not listed as V zones, but also identified on
a
Flood Insurance Rate Map (FIRM) as being subject to inundation during a 100-
year
flood. The associated flood elevation has a 1-percent chance of being equaled
or
exceeded in any given year. There are several categories of A zones that may
be
identified on a FIRM with one of the following designations: AO, AH, A1-30,
AE,
and unnumbered A zones.
Base flood -– A flooding having a 1-percent chance of being equaled or
exceeded
in any given year; also known as the 100-year flood.
Base Flood Elevation (BFE) – Elevation of the 1-percent flood. This elevation
is
the basis of the insurance and floodplain management requirements of the
National Flood Insurance Program (NFIP).
Coastal A zone – The portion of the Special Flood Hazard Area (SFHA) landward
of
a V zone or landward of an open coast without mapped V zones (e.g., the
shorelines of the Great Lakes), in which the principal sources of flooding
are
astronomical tides, storm surges, seiches, or tsunamis, not riverine sources.
Like the flood forces in V zones, those in Coastal A zones are highly
correlated
with coastal winds or coastal seismic activity. Coastal A zones may therefore
be
subject to wave effects, velocity flows, erosion, scour, or combinations of
these forces. During base flood conditions, the potential for breaking wave
heights between 1.5 feet and 3.0 feet will exist. Coastal A zones are not
shown
on present day FIRMs or mentioned in a community’s Flood Insurance Study
(FIS)
Report.
Crawlspace foundation – Crawlspace foundations are typically low masonry
perimeter walls with interior piers supporting a wood floor system. These
foundations are usually supported by shallow footings and are prone to
failure
caused by erosion or scour.
Design flood – The design flood is often, but not always equal to the base
flood
for areas identified as SFHAs on a community’s FIRM.
Design Flood Elevation (DFE) – The elevation of the design flood, including
wave
height relative to the datum specified on a community’s Flood Hazard Map.
Design professional – A State licensed architect or engineer.
Erosion – Process by which floodwaters lower the ground surface in an area by
removing upper layers of soil.
Exposure Category B – A wind exposure identified in ASCE 7 and the
International
Building Code (IBC) as urban and suburban areas, wooded areas, or other
terrain
with numerous closely spaced obstructions having the size of single-family
dwellings or larger.
Exposure Category C – A wind exposure identified in ASCE 7 and the IBC as
open
terrain with scattered obstructions having heights generally less than 30
feet
(9.1 meters). This category includes flat open country, grasslands, and all
water surfaces in hurricane-prone regions.
Fixity – Resistance to flotation; stable or immovable.
Flood Insurance Rate Map (FIRM) – An official map of a community, on which
FEMA
has delineated both the SFHA and the risk premium zones applicable to the
community. The map shows the extent of the base floodplain and may also
display
the extent of the floodway and BFEs.
Freeboard – The height added to place a structure above the base flood to
reduce
the potential for flooding. The increased elevation of a building above the
minimum design flood level to provide additional protection for flood levels
higher than the 1-percent chance flood level and to compensate for inherent
inaccuracies in flood hazard mapping.
Hydrodynamic forces – The amount of pressure exerted by moving floodwaters on
an
object, such as a structure. Among these loads are positive frontal pressure
against the structure, drag forces along the sides, and suction forces on the
downstream side.
Hydrostatic forces – The amount of lateral pressure exerted by standing or
slowly moving floodwaters on a horizontal or vertical surface, such as a wall
or
a floor slab. The water pressure increases with the square of the water
depth.
Leeward – The side away, or sheltered, from the wind.
Limit of Moderate Wave Action (LiMWA) – The landward extent of coastal areas
designated Zone AE where waves higher than 1.5 feet can exist during a design
flood (also known as the Coastal A zone).
National Flood Insurance Program (NFIP) – The NFIP is a Federal program
enabling
property owners in participating communities to purchase insurance as a
protection against flood losses in exchange for State and community
floodplain
management regulations that reduce future flood damages. Participation in the
NFIP is based on an agreement between communities and the Federal Government.
If
a community adopts and enforces floodplain management regulations to reduce
future flood risks to new construction in floodplains, the Federal Government
will make flood insurance available within the community as a financial
protection against flood losses. This insurance is designed to provide an
insurance alternative to disaster assistance to reduce the escalating costs
of
repairing damage to buildings and their contents caused by floods. The
program
was created by Congress in 1968 with the passage of the National Flood
Insurance
Act of 1968.
National Geodetic Vertical Datum 1929 (NGVD 1929) – A vertical elevation
baseline determined in 1929 as a national standard. Used as the standard for
FIRMs until 2000.
North American Vertical Datum 1988 (NAVD 1988) – A vertical elevation
baseline
determined in 1988 as a more accurate national standard. The current vertical
elevation standard for new FIRMs.
Scour – Erosion by moving water in discrete locations, often as a result of
water impacting foundation elements.
Shore-normal – Perpendicular to the shoreline.
Slab-on-grade foundation – Type of foundation in which the lowest floor of
the
house is formed by a concrete slab that sits directly on the ground.
Slug – A unit of mass in the English foot-pound-second system. One slug is
the
mass accelerated at 1 foot per second (fps) by a force of 1 pound (lb). Since
the acceleration of gravity (g) in English units is 32.174 fps, the slug is
equal to 32.174 pounds (14.593 kilograms).
Special Flood Hazard Area (SFHA) – Portion of the floodplain subject to
inundation by the base flood.
Stem wall foundation – A type of foundation that uses masonry block and
reinforced with steel and concrete. The wall is constructed on a concrete
footing, back-filled with dirt, compacted, and the slab is then poured on
top.
Strength Design – A method of proportioning structural members such that the
computed forces produced in the members by the factored loads do not exceed
the
member design strength (also called load and resistance factor design).
V zones – V zones are areas identified on FIRMs as zones VE, V1-30, or V.
These
areas, also known as Coastal High Hazard Areas, are areas along the coast
that
have a 1 percent or greater annual chance of flooding from storm surge and
waves
greater than 3 feet in height, as well as being subject to significant wind
forces.
Wave trough – The lowest part of the wave between crests.
Windward – The side facing the wind.
Appendix I. Abbreviations and Acronyms
ABFE Advisory Base Flood Elevation
ACI American Concrete Institute
AF&PA American Forest & Paper Association
AISI American Iron and Steel Institute
AL Alabama
ANSI American National Standards Institute
ASCE American Society of Civil Engineers
ASD allowable stress design
ASTM American Society for Testing and Materials
AWC American Wood Council
AWPA American Wood Preservers’ Association
BFE Base Flood Elevation
C&C components and cladding
CMU concrete masonry unit
CRSI Concrete Reinforced Steel Institute
cy cubic yard
DFE Design Flood Elevation
ea each
FEMA Federal Emergency Management Agency
FIRM Flood Insurance Rate Map
FIS Flood Insurance Study
FL Florida
fps feet per second
FRP fiber reinforced polyester
Ft feet
g gravity
IBC International Building Code
ICC International Code Council
IRC International Residential Code
K 1,000 pounds
ksi kips per square inch
lb pound
LA Louisiana
lb/sq pounds per square foot
lf linear foot
LiMWA Limit of Moderate Wave Action
ls lump sum
MAT Mitigation Assessment Team
mph miles per hour
m/s meters per second
MS Mississippi
MWFRS main wind force resisting system
NAVD North American Vertical Datum
NDS National Design Specification
NFIP National Flood Insurance Program
NFPA National Fire Protection Association
NGS National Geodetic Survey
NGVD National Geodetic Vertical Datum
NR Not Recommended
NRCS National Resource Conservation Service
o.c.on center
PCA Portland Cement Association
pcf pounds per cubic foot
p/lf pounds per linear foot
psf pounds per square foot
psi pounds per square inch
RA Recovery Advisory
RISA Rapid Interactive Structure Analysis
ROM rough order of magnitude
SBC Standard Building Code
SCS Soil Conservation Service
SEI Structural Engineering Institute
SFHA Special Flood Hazard Area
SLOSH Sea, Lake, and Overland Surges from Hurricanes
Sq square foot
SWL stillwater level
TBTO tributylin oxide
TMS The Masonry Society
TX Texas
UDA Urban Design Associates
u.n.o.unless noted otherwise
USGS United States Geological Survey
WCFM Wood Frame Construction Manual for One- and Two-Family Dwellings
WWF welded wire fabric