RECOMMENDED RESIDENTIAL CONSTRUCTION FOR COASTAL AREAS
Building on Strong and Safe Foundations
FEMA P-550, Second Edition / December 2009
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 = ˝ 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 = ˝C 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]