FEMA P646 5: Siting, Spacing, Sizing, and Elevation Considerations 51
Chapter 5
Siting, Spacing, Sizing, and
Elevation Considerations
Tsunami risk is unique in that some communities may be susceptible to farsource-
generated tsunamis (longer warning time), near-source-generated
tsunamis (shorter warning time), or both. Far-source-generated tsunamis
generally allow sufficient warning time so that emergency response plans can
be based on evacuation out of the inundation zone. Near-source-generated
tsunamis may not allow sufficient time for evacuation, so emergency
response plans may need to include vertical evacuation refuge. This chapter
provides guidance on how to locate vertical evacuation refuges within a
community, and how to determine the size of a vertical evacuation structure.
5.1 Siting Considerations
Vertical evacuation structures should be located such that all persons
designated to take refuge can reach the structure within the time available
between tsunami warning and tsunami inundation. Travel time must also
take into consideration vertical circulation within the structure to levels
above the tsunami inundation elevation. Structures located at one end of a
community may be difficult for some users to reach in a timely fashion.
Routes to the structure should be easily accessible and well marked.
Location of vertical evacuation structures within a community should take
into account potential hazards in the vicinity of a site that could jeopardize
the safety of the structure, and should consider that natural behaviors of
persons attempting to avoid coastal flooding.
5.1.1 Warning, Travel Time, and Spacing
The West Coast and Alaska Tsunami Warning Center (WC/ATWC) in
Alaska, and the Pacific Tsunami Warning Center (PTWC) in Hawaii monitor
potential tsunamis, and warn affected populations of an impending tsunami.
Table 5-1 summarizes approximate warning times associated with the
distance between a tsunami-genic source and the site of interest. A farsource-
generated tsunami originates from a source that is far away from the
site, and could have 2 hours or more of advance warning time. A nearsource-
generated tsunami originates from a source that is close to the site,
and could have 30 minutes or less of advance warning time. Sites
Vertical evacuation structures
should be located such that all
persons designated to take refuge
can reach the structure within the
time available between tsunami
warning and tsunami inundation.
52 5: Siting, Spacing, Sizing, and Elevation Considerations FEMA P646
experiencing near-source-generated tsunamis will generally feel the effects of
the triggering event (e.g. shaking caused by a near-source earthquake), and
these effects will likely be the first warning of the impending tsunami. A
mid-source-generated tsunami is one in which the source is somewhat close
to the site of interest, but not close enough for the effects of the tsunami
generating event to be felt at the site. Mid-source-generated tsunamis would
be expected to have between 30 minutes and 2 hours of advance warning
time.
Table 5-1 Tsunami Sources and Approximate Warning Times
Location of Source Approximate Warning Time (t)
Far-source-generated tsunami t > 2 hrs
Mid-source-generated tsunami 30 min < t < 2 hrs
Near-source-generated tsunami t < 30 min
Consideration must be given to the time it would take for designated
occupants to reach a refuge. To determine the maximum spacing of tsunami
vertical evacuation structures, the critical parameters are warning time and
ambulatory capability of the surrounding community. Once maximum
spacing is determined, size must be considered, and population becomes an
important parameter. Sizing considerations could necessitate an adjustment
in the number and spacing of vertical evacuation structures if it is not feasible
to size the resulting structures large enough to accommodate the surrounding
population at the maximum spacing. Sizing considerations are discussed in
Section 5.2.
The average, healthy person can walk at approximately 4-mph. Portions of
the population in a community, however, may have restricted ambulatory
capability due to age, health, or disability. The average pace of a mobilityimpaired
population can be assumed to be about 2-mph.
Assuming a 2-hour warning time associated with far-source-generated
tsunamis, vertical evacuation structures would need to be located a maximum
of 4 miles from any given starting point. This would result in a maximum
spacing of approximately 8 miles between structures. Similarly, assuming a
30 minute warning time, vertical evacuation structures would need to be
located a maximum of 1 mile from any given starting point, or 2 miles
between structures. Shorter warning times would require even closer
spacing. Table 5-2 summarizes maximum spacing of vertical evacuation
structures based on travel time associated with a mobility-impaired
population.
Recommended Maximum
spacing of vertical evacuation
structures depends on warning time,
ambulatory speed, and the
surrounding population density.
FEMA P646 5: Siting, Spacing, Sizing, and Elevation Considerations 53
Table 5-2 Maximum Spacing of Vertical Evacuation Structures Based
on Travel Time
Warning Time
Ambulatory
Speed Travel Distance Maximum Spacing
2 hrs 2 mph* 4 miles 8 miles
30 min 2 mph* 1 mile 2 miles
15 min 2 mph* ½ mile 1 mile
* Based on the average pace for a mobility-impaired population
5.1.2 Ingress and Vertical Circulation
Tsunami vertical evacuation structures should be spaced such that people
will have adequate time not only to reach the structure, but to enter and move
within the structure to areas of refuge that are located above the anticipated
tsunami inundation elevation.
Increased travel times may need to be considered if obstructions exist, or
could occur, along the travel or ingress route. Unstable or poorly secured
structural or architectural elements that collapse in and around the entrance,
or the presence of contents associated with the non-refuge uses of a structure,
could potentially impede ingress. Allowance for parking at a vertical
evacuation refuge may decrease travel time to the refuge, but could
complicate access when the potential traffic for jams is considered.
Stairs or elevators are traditional methods of ingress and vertical circulation
in buildings, especially when designated users have impaired mobility.
Ramps, such as the ones used in sporting venues, however, can be more
effective for moving large numbers of people into and up to refuge areas in a
structure. Estimates of travel time may need adjustment for different
methods of vertical circulation. Disabled users may need to travel along a
special route that accommodates wheelchairs, and those with special needs
may require assistance from others to move within the structure.
When locating vertical evacuation structures, natural and learned behaviors
of evacuees should be considered. Most coastal communities have educated
their populations to “go to high ground” in the event of a tsunami warning.
Also, a natural tendency for evacuees will be to migrate away from the shore.
Vertical evacuation structures should therefore be located on the inland side
of evacuation zones and should take advantage of naturally occurring
topography that would tend to draw evacuees towards them. Figure 5-1
54 5: Siting, Spacing, Sizing, and Elevation Considerations FEMA P646
illustrates an arrangement of vertical evacuation structures in a community
based on these principles.
Figure 5-1 Vertical evacuation refuge locations considering travel distance,
evacuation behavior, and naturally occurring high ground.
Arrows show anticipated vertical evacuation routes.
5.1.3 Consideration of Site Hazards
Special hazards in the vicinity of each site should be considered in locating
vertical evacuation structures. Potential site hazards include breaking waves,
sources of large waterborne debris, and sources of waterborne hazardous
materials. When possible, vertical evacuation structures should be located
away from potential hazards that could result in additional damage to the
structure and reduced safety for the occupants. Due to limited availability of
possible sites, and limitations on travel and mobility of the population in a
community, some vertical evacuation structures may need to be located at
sites that would be considered less than ideal. Figure 5-2 illustrates adjacent
site hazards that could exist in a typical coastal community.
Potential site hazards include
breaking waves, sources of large
waterborne debris, and sources of
waterborne hazardous materials.
FEMA P646 5: Siting, Spacing, Sizing, and Elevation Considerations 55
Figure 5-2 Site hazards adjacent to vertical evacuation structures
(numbered locations). Arrows show anticipated vertical
evacuation routes.
Wave breaking takes place where the water depth is sufficiently finite. In the
design of usual coastal structures (e.g., breakwaters, seawalls, jetties), critical
wave forces often result from breaking waves. In general, tsunamis break
offshore. In the case of very steep terrain, however, they can break right at
the shoreline, which is known as a collapsing breaker.
Forces from collapsing breakers can be extremely high and very uncertain.
Location of vertical evacuation structures within the tsunami wave-breaking
zone poses unknown additional risk to the structure. While the possibility of
tsunami wave breaking at an on-shore location is not zero, it is considered to
be very rare. For these reasons, recommended sites for vertical evacuation
structures are located inland of the wave-breaking zone, and wave breaking
forces are not considered in this document.
In Figure 5-2, vertical evacuation structures are located some distance inland
from the shoreline. Structure No. 1 is located adjacent to a harbor and
container terminal. Impact forces from ships, barges, boats, and other
56 5: Siting, Spacing, Sizing, and Elevation Considerations FEMA P646
waterborne debris have the potential to become very large. Locations with
additional sources of large, possibly buoyant debris increase the chances of
impact by one or more waterborne missiles, and increase the potential risk to
the structure. If possible, it would be better if this structure was sited away
from the harbor and container terminal. If there is no alternate location
available to serve this area of the community, this structure would need to be
designed for potential impact from the shipping containers and boats likely to
be present during tsunami inundation.
Structure No. 2 is located off to the side of the harbor and adjacent to a
parking lot. This structure would need to be designed for debris consistent
with the use of the parking lot and surrounding areas, which could include
cars, trucks, and recreational vehicles.
Structure No. 3 is immediately adjacent to a gas station. In past tsunamis,
ignition of flammable chemicals or other floating debris has resulted in
significant risk for fire in partially submerged structures. Depending on the
potential for fuel leakage from this station in the event of a tsunami (or a
preceding earthquake), this structure would need to be designed with fire
resistive construction and additional fire protection.
Structure No. 4 is adjacent to a waterfront park facility. This location can be
ideal, as the potential for waterborne debris can be relatively low. Possible
hazards could include debris from park structures, naturally occurring
driftwood, or larger logs from downed trees. This area has a higher potential
for tourists and visitors unfamiliar with the area. It would require additional
signage to inform park users what to do and where to go in the event of a
tsunami warning.
Structure No. 5 is adjacent to an emergency response facility. Co-locating at
such facilities can provide opportunities for direct supervision by lawenforcement
and monitoring and support of refuge occupancies by other
emergency response personnel.
At two locations, Structure No. 6 is intended to aid evacuees in taking
advantage of naturally occurring high ground.
5.2 Sizing Considerations
Sizing of a vertical evacuation structure depends on the intended number of
occupants, the type of occupancy, and the duration of occupancy. The
number of occupants will depend on the surrounding population and the
spacing and number of vertical evacuation structures located in the area.
FEMA P646 5: Siting, Spacing, Sizing, and Elevation Considerations 57
Duration of occupancy will depend on the nature of the hazard and the
intended function of the facility.
5.2.1 Services and Occupancy Duration
A vertical evacuation structure is typically intended to provide a temporary
place of refuge during a tsunami event. While tsunamis are generally
considered to be short-duration events (i.e., pre-event warning period and
event lasting about 8 to 12 hours), tsunamis include several cycles of waves.
The potential for abnormally high tides and coastal flooding can last as long
as 24 hours.
A vertical evacuation structure must provide adequate services to evacuees
for their intended length of stay. As a short term refuge, services can be
minimal, including only limited space per occupant and basic sanitation
needs. Additionally, a vertical evacuation structure could be used to provide
accommodations and services for people whose homes have been damaged
or destroyed. As a minimum, this would require an allowance for more
space for occupants, supplies, and services. It could also include
consideration of different post-event rescue and recovery activities, and
evaluation of short- and long-term medical care needs. Guidance on basic
community sheltering needs is not included in this document, but can be
found in FEMA 361, Design and Construction Guidance for Community
Shelters (FEMA, 2000a).
Choosing to design and construct a vertical evacuation structure primarily for
short-term refuge, or to supply and manage it to house evacuees for longer
periods of time, is an emergency management issue that must be decided by
the state, municipality, local community, or private owner.
5.2.2 Square Footage Recommendations from Available
Sheltering Guidelines
Square footage recommendations are available from a number of different
sources, and vary depending on the type of hazard and the anticipated
duration of occupancy. The longer the anticipated stay, the greater the
minimum square footage recommended.
A shelter for mostly healthy, uninjured people for a short-term event would
require the least square footage per occupant. A shelter intended to house
sick or injured people, or to provide ongoing medical care, would require
more square footage to accommodate beds and supplies. For longer duration
stays, even more square footage is needed per occupant for minimum privacy
and comfort requirements, and for building infrastructure, systems, and
services needed when housing people on an extended basis.
58 5: Siting, Spacing, Sizing, and Elevation Considerations FEMA P646
Table 5-3, Table 5-4 and Table 5-5 summarize square footage
recommendations contained in International Code Council/National Storm
Shelter Association, ICC-500, Standard on the Design and Construction of
Storm Shelters (ICC/NSSA, 2007), FEMA 361 Design and Construction
Guidance for Community Shelters (FEMA, 2000a), and American Red Cross
Publication No. 4496, Standards for Hurricane Evacuation Shelter Selection
(ARC, 2002).
Table 5-3 Square Footage Recommendations – ICC-500 Standard
on the Design and Construction of Storm Shelters
(ICC/NSSA, 2007)
Hazard or Duration
Minimum Required Usable Floor
Area in Sq. Ft. per Occupant
Tornado
Standing or seated
Wheelchair
Bedridden
5
10
30
Hurricane
Standing or seated
Wheelchair
Bedridden
20
20
40
Table 5-4 Square Footage Recommendations – FEMA 361 Design
and Construction Guidance for Community Shelters
(FEMA, 2000a)
Hazard or Duration
Recommended Minimum Usable
Floor Area in Sq. Ft. per Occupant
Tornado 5
Hurricane 10
Table 5-5 Square Footage Recommendations – American Red Cross
Publication No. 4496 (ARC, 2002)
Hazard or Duration
Recommended Minimum Usable
Floor Area in Sq. Ft. per Occupant
Short-term stay (i.e., a few days) 20
Long-term stay (i.e., days to weeks) 40
FEMA P646 5: Siting, Spacing, Sizing, and Elevation Considerations 59
The number of standing, seating, wheelchair, or bedridden spaces should be
determined based on the specific occupancy needs of the facility under
consideration. When determining usable floor area, ICC-500 includes the
following adjustments to gross floor area:
• Usable floor area is 50 percent of gross floor area in shelter areas with
concentrated furnishings or fixed seating.
• Usable floor area is 65 percent of gross floor area in shelter areas with
un-concentrated furnishings and without fixed seating.
• Usable floor area is 85 percent of gross floor area in shelter areas with
open plan furnishings and without fixed seating.
5.2.3 Recommended Minimum Square Footage for Short-
Term Refuge from Tsunamis
For short-term refuge in a tsunami vertical evacuation structure, the duration
of occupancy should be expected to last between 8 to 12 hours, as a
minimum. Because tsunami events can include several cycles of waves,
there are recommendations that suggest evacuees should remain in a tsunami
refuge until the second high tide after the first tsunami wave, which could
occur up to 24 hours later.
Based on square footage recommendations employed in the design of shelters
for other hazards, the recommended minimum square footage per occupant
for a tsunami refuge is 10 square feet per person. It is anticipated that this
density will allow evacuees room to sit down without feeling overly crowded
for a relatively short period of time, but would not be considered appropriate
for longer stays that included sleeping arrangements. This number should be
adjusted up or down depending on the specific occupancy needs of the refuge
under consideration.
5.3 Elevation Considerations
In order to serve effectively as a vertical evacuation structure, it is essential
that the area of refuge be located well above the maximum tsunami
inundation level anticipated at the site. Determination of a suitable elevation
for tsunami refuge must take into account the uncertainty inherent in
estimation of the tsunami runup elevation, possible splash-up during impact
of tsunami waves, and the anxiety level of evacuees seeking refuge in the
structure.
To account for this uncertainty, the magnitude of tsunami force effects is
determined assuming a maximum tsunami runup elevation that is 30% higher
than values predicted by numerical simulation modeling or obtained from
Recommended minimum square
footage is 10 square feet
per occupant.
60 5: Siting, Spacing, Sizing, and Elevation Considerations FEMA P646
tsunami inundation maps. Because of the high consequence of potential
inundation of the tsunami refuge area, it is recommended that the elevation of
tsunami refuge areas in vertical evacuation structures include an additional
allowance for freeboard above this elevation.
The recommended minimum freeboard is one story height, or 10 feet (3
meters) above the tsunami runup elevation used in tsunami force
calculations. The recommended minimum elevation for a tsunami refuge
area is, therefore, the maximum tsunami runup elevation anticipated at the
site, plus 30%, plus 10 feet (3 meters).
5.4 Size of Vertical Evacuation Structures
Given the number and spacing of vertical evacuation structures, and the
population in a given community, the minimum size can be determined based
on square footage recommendations for the intended duration and type of
occupancy. Consideration of other functional needs, such as restrooms,
supplies, communications, and emergency power, should be added to the
overall size of the structure.
Given the maximum tsunami runup elevation anticipated at the site, the
minimum elevation of the area of refuge within a vertical evacuation
structure can be determined based on minimum freeboard recommendations.
Recommended minimum refuge
elevation is the maximum
anticipated tsunami runup
elevation, plus 30%, plus 10 feet
(3 meters).
FEMA P646 6: Load Determination and Structural Design Criteria 61
Chapter 6
Load Determination and
Structural Design Criteria
This chapter summarizes current code provisions as they relate to tsunami
load effects, describes intended performance objectives for vertical
evacuation structures, specifies equations for estimating tsunami forces, and
provides guidance on how tsunami forces should be combined with other
effects.
6.1 Currently Available Structural Design Criteria
Very little guidance is provided in currently available structural design
codes, standards, and guidelines on loads induced by tsunami inundation.
Established design information focuses primarily on loads due to rising water
and wave action associated with riverine flooding and storm surge. While
little specific guidance is provided, the presumption is that currently
available flood design standards are to be used in designing for tsunami load
effects.
6.1.1 Current U.S. Codes, Standards, and Guidelines
International Building Code. The International Code Council International
Building Code (ICC, 2006) Appendix G provides information on flood
design and flood-resistant construction by reference to ASCE/SEI Standard
24-05, Flood Resistant Design and Construction (ASCE 24, 2006a).
ASCE/SEI Standard 24-05. The American Society of Civil
Engineers/Structural Engineering Institute (ASCE/SEI) Standard 24-05
Flood Resistant Design and Construction (ASCE, 2006a) provides minimum
requirements for flood-resistant design and construction of structures located
in flood-hazard areas. Topics include basic requirements for flood-hazard
areas, high-risk flood-hazard areas, coastal high-hazard areas, and coastal A
zones. This standard complies with FEMA National Flood Insurance
Program (NFIP) floodplain management requirements.
ASCE/SEI Standard 7-05. ASCE/SEI Standard 7-05 Minimum Design
Loads for Buildings and Other Structures (ASCE, 2006b) provides
expressions for forces associated with flood and wave loads on specific types
of structural components. This standard covers important definitions that
Very little guidance is provided in
currently available structural design
codes, standards, and guidelines on
loads induced by tsunami
inundation.
Established design information
focuses primarily on loads due to
rising water and wave action
associated with riverine flooding
and storm surge.
62 6: Load Determination and Structural Design Criteria FEMA P646
relate to flooding and coastal high-hazard areas related to tides, storm surges,
riverine flooding, seiches or tsunamis.
FEMA 55 Coastal Construction Manual. The FEMA 55 Coastal
Construction Manual (FEMA, 2005) includes FEMA’s most recent study of
coastal seismic and tsunami loads. This manual was developed to provide
design and construction guidance for low-rise (less than three stories), oneand
two-family residential structures built in coastal areas throughout the
United States. The Coastal Construction Manual addresses seismic loads for
coastal structures, and contains expressions for flood loads, wave loads, and
load combinations for specific types of structural components.
The Manual also provides information on tsunami hazard. Section 7.2.2
states that:
“Tsunamis are long-period water waves generated by undersea shallowfocus
earthquakes or by undersea crustal displacements (subduction of
tectonic plates), landslides, or volcanic activity. Tsunamis can travel
great distances, undetected in deep water, but shoaling rapidly in coastal
waters and producing a series of large waves capable of destroying
harbor facilities, shore protection structures, and upland buildings…
Coastal construction in tsunami hazard zones must consider the effects of
tsunami runup, flooding, erosion, and debris loads. Designers should
also be aware that the “rundown” or return of water to the sea can also
damage the landward sides of structures that withstood the initial runup.”
The Manual also notes that tsunami effects at a particular site will be
determined by the following four basic factors:
• the magnitude of the earthquake or triggering event
• the location of the triggering event
• the configuration of the continental shelf and shoreline
• the upland topography
With regard to designing to resist tsunami loads, Section 11.7 of the Manual
states that:
“Tsunami loads on residential buildings may be calculated in the same
fashion as other flood loads; the physical processes are the same, but the
scale of the flood loads is substantially different in that the wavelengths
and runup elevations of tsunamis are much greater than those of waves
caused by tropical or extratropical cyclones … When the tsunami forms
a borelike wave, the effect is a surge of water to the shore. When this
occurs, the expected flood velocities are substantially higher…and if
FEMA P646 6: Load Determination and Structural Design Criteria 63
realized at the greater water depths, would cause substantial damage to
all buildings in the path of the tsunami. Designers should collect as
much data as possible about expected tsunami depths to more accurately
calculate tsunami flood forces.”
Although authors of the Coastal Construction Manual conclude that it is
generally not feasible or practical to design normal structures to withstand
tsunami loads, it should be noted that this study was for conventional
residential construction, and did not take into account the possibility of
special design and construction details that would be possible for vertical
evacuation structures.
City and County of Honolulu Building Code. The City and County of
Honolulu Building Code (CCH, 2000), Chapter 16, Article 11, provides
specific guidance for “structural design of buildings and structures subject to
tsunamis” in Section 16-11.5(f). The loading requirements in this section are
based on the 1980 Dames & Moore study, but with the velocity of flow in
feet per second estimated as equal in magnitude to the depth in feet of water
at the structure. Estimates are also given for anticipated scour around piles
and piers based on distance from the shoreline and the soil type at the
building site.
6.1.2 Summary of Current Design Requirements
Coastal areas that are subject to high-velocity wave action from storms or
seismic sources are designated Coastal V-Zones (ASCE, 2006a). Areas
inland of Coastal V-Zones that are subject to smaller waves caused by storm
surges, riverine flooding, seiches or tsunamis are designated Coastal A-Zones
(ASCE, 2006a).
In design for coastal flooding due to storm surge or tsunamis, buildings or
structures are proportioned to resist the effects of coastal floodwaters. Design
and construction must be adequate to resist the anticipated flood depths,
pressures, velocities, impact, uplift forces, and other factors associated with
flooding, as defined by the code. Habitable space in building structures must
be elevated above the regulatory flood elevation by such means as posts,
piles, piers, or shear walls parallel to the expected direction of flow. Spaces
below the design flood elevation must be free from obstruction. Walls and
partitions in a coastal high-hazard area are required to break away so as not
to induce excessive loads on the structural frame.
The effects of long-term erosion, storm-induced erosion, and local scour are
to be included in the design of foundations of buildings or other structures in
coastal high-hazard areas. Foundation embedment must be far enough below
64 6: Load Determination and Structural Design Criteria FEMA P646
the depth of potential scour to provide adequate support for the structure.
Scour of soil from around individual piles and piers must be provided for in
the design. Shallow foundation types are not permitted in V-Zones unless
the natural supporting soils are protected by scour protection, but are
permitted in A-Zones subject to stability of the soil and resistance to scour.
The main building structure must be adequately anchored and connected to
the elevating substructure system to resist lateral, uplift, and downward
forces.
6.1.3 Limitations in Available Flood Design Criteria Relative
to Tsunami Loading
Although many of the hydrostatic and hydrodynamic loading expressions in
currently available codes, standards and guidelines are well-established, there
are significant differences between tsunami inundation and riverine or storm
surge flooding. For a typical tsunami, the water surface fluctuates near the
shore with amplitude of several meters during a period of a few minutes to
tens of minutes. A major difference between tsunamis and other coastal
flooding is increased flow velocity for tsunami waves, which results in
significant increases in velocity-related loads on structural components.
Application of existing loading expressions to tsunami loading conditions
requires an estimate of the tsunami flood depth and velocity, neither of which
is provided with great accuracy by currently available information on
tsunami hazard.
Although impact of floating debris is considered in current codes, impact
force produced by a change in momentum is dependent on estimates of the
debris mass, velocity, and the time taken for the mass to decelerate. No
accommodation is made for added mass of the water behind the debris, or the
potential for damming if debris is blocked by structural components. More
significant forms of debris, such as barges, fishing boats, and empty storage
tanks may need to be considered for tsunamis, depending on the location of
the building under consideration. The size, mass, and stiffness of this type of
debris are not considered in currently available criteria.
No consideration is given to upward loads on the underside of structures or
components that are submerged by the flood or tsunami flow. These vertical
hydrodynamic loads, different from buoyancy effects, are considered by the
offshore industry in design of platforms and structural members that may be
submerged by large waves.
There are two primary scour mechanisms that occur during a tsunami event.
Shear-induced scour is similar to that observed during storm surge flooding,
and consists of soil transport due to the flow velocity. Liquefaction-induced
Although many of the hydrostatic
and hydrodynamic loading
expressions in currently available
codes, standards and guidelines are
well-established, there are
significant differences between
tsunami inundation and riverine or
storm surge flooding.
FEMA P646 6: Load Determination and Structural Design Criteria 65
scour results from rapid drawdown as the water recedes. Without sufficient
time to dissipate, pore pressure causes liquefaction of the soil resulting in
substantially greater scour than would otherwise occur. Although current
codes require consideration of scour, little guidance (other than rough
estimates) is given as to the potential extent of scour.
6.2 Performance Objectives
While specific performance objectives for various forms of rare loading can
vary, acceptable structural performance generally follows a trend
corresponding to:
• little or no damage for small, more frequently occurring events;
• moderate damage for medium-size, less frequent events; and
• significant damage, but no collapse for very large, very rare events.
In the case of earthquake hazards, current model building codes, such as the
International Building Code, assign seismic performance objectives to
buildings based on their inherent risk to human life (e.g., very large
occupancies) or their importance after an earthquake (e.g., emergency
operation centers or hospitals). Buildings and other structures are classified
into Occupancy Categories I through IV, in order of increasing risk to human
life or importance, and code prescriptive design criteria are correspondingly
increased, with the intention of providing improved performance. For
Occupancy Category IV, design rules are intended to result in a high
probability of buildings remaining functional after moderate shaking, and
experiencing considerably less damage than normal buildings in very rare
shaking.
Currently available performance-based seismic design procedures are
intended to explicitly evaluate and predict performance, instead of relying on
the presumed performance associated with prescriptive design rules.
However, performance-based design is an emerging technology and the
targeted performance cannot be delivered with 100% certainty. The current
standard-of-practice for performance-based seismic design contained in
ASCE/SEI 41-06 Seismic Rehabilitation of Existing Buildings (ASCE,
2006c) defines discrete performance levels with names intended to connote
the expected condition of the building: Collapse, Collapse Prevention, Life
Safety, Immediate Occupancy, and Operational. Seismic performance
objectives are defined by linking one of these building performance levels to
an earthquake hazard level that is related to the recurrence interval (return
period) and the intensity of ground shaking, as shown in Figure 6-1.
66 6: Load Determination and Structural Design Criteria FEMA P646
Figure 6-1 Seismic performance objectives linking building performance
levels to earthquake hazard levels (adapted from SEAOC,
1995).
When determining performance objectives for natural hazards, the most
difficult issue is deciding how rare (or intense) the design event should be.
For seismic design in the United States, this issue has been resolved through
the adoption of a national earthquake hazard map defining the Maximum
Considered Earthquake (MCE) and the intensity of shaking associated with
such an event.
6.2.1 Tsunami Performance Objective
In this document, the design tsunami event is termed the Maximum
Considered Tsunami (MCT). Unfortunately, there are no national maps
available for defining this hazard. In addition, due to the complexity of the
tsunami hazard, which must consider near and distant tsunami-genic sources
and highly uncertain relationships between earthquake events and subsequent
tsunami, no firm policy has been established defining a methodology for
setting a Maximum Considered Tsunami at a consistent hazard level.
Current methods for tsunami hazard assessment are described in Chapter 3.
Vertical evacuation structures designed in accordance with the guidance
presented in this document would be expected to provide a stable refuge
when subjected to a design tsunami event consistent with the Maximum
Considered Tsunami identified for the local area.
In general, the Maximum Considered Tsunami will be a rare, but realistic
event with large potential consequences. Consistent with the general trend of
The Tsunami Performance
Objective includes the potential for
significant damage while
maintaining a reliable and stable
refuge when subjected to the
Maximum Considered Tsunami.
Most structures would be expected
to be repairable, although the
economic viability of repair will be
uncertain
FEMA P646 6: Load Determination and Structural Design Criteria 67
acceptable performance for “Maximum Considered” loadings, the
performance of vertical evacuation structures in this event would include the
potential for significant damage while maintaining a reliable and stable
refuge. Most structures would be expected to be repairable after a large
event, although the economics of repair versus replacement will be uncertain,
depending on the specifics of the situation including the magnitude of the
actual event, interaction with the local bathymetry, and the design and
construction of the facility.
6.2.2 Seismic and Wind Performance Objectives
The performance objective for vertical evacuation structures subjected to
seismic and wind hazards should be consistent with that of code-defined
essential facilities such as hospitals, police and fire stations, and emergency
operation centers. Following the prescriptive approach in the International
Building Code, vertical evacuation structures are assigned to Occupancy
Category IV, triggering design requirements that provide enhanced
performance relative to typical buildings for normal occupancies.
In the specific case of earthquakes, design for enhanced performance is
necessary to assure that the structure is still usable for a tsunami following a
local seismic event. To obtain a higher level of confidence that a vertical
evacuation structure will achieve enhanced seismic performance, the design
developed by prescriptive code provisions can be evaluated using currently
available performance-based seismic design techniques and verification
analyses. Utilizing the approach in ASCE/SEI 41-06, the performance
objective for code-defined essential facilities would be Immediate
Occupancy performance for the Design Basis Earthquake (DBE) and Life
Safety performance for the Maximum Considered Earthquake (MCE).
6.3 Earthquake Loading
The recommended basis for seismic design of vertical evacuation structures
is the International Building Code, which references ASCE/SEI 7-05
Minimum Design Loads for Buildings and Other Structures for its seismic
requirements. These requirements are based on the NEHRP Recommended
Provisions for Seismic Regulations for New Buildings and Other Structures
(FEMA, 2004a) and additional information provided in the Commentary
(FEMA, 2004b). Vertical evacuation structures should be designed using
rules for Occupancy Category IV buildings.
The recommended basis for seismic evaluation and rehabilitation of existing
buildings that are being considered for use as vertical evacuation structures is
the SEI/ASCE Standard 31-03 Seismic Evaluation of Existing Buildings
Seismic and Wind Performance
Objectives are consistent with the
code-defined performance of
essential facilities such as hospitals,
police and fire stations, and
emergency operation centers.
68 6: Load Determination and Structural Design Criteria FEMA P646
(ASCE, 2003b), using the Immediate Occupancy performance objective, and
ASCE/SEI Standard 41-06 Seismic Rehabilitation of Existing Buildings,
using the performance objectives specified in Section 6.2.2.
6.3.1 Near-Source-Generated Tsunamis
A vertical evacuation structure located in a region susceptible to near-sourcegenerated
tsunamis is likely to experience strong ground shaking
immediately prior to the tsunami. As a properly designed essential facility, it
is expected that sufficient reserve capacity will be provided in the structure to
resist the subsequent tsunami loading effects. The reserve capacity of the
structure, which will be some fraction of the original, needs to be evaluated.
It is recommended that the condition of the structure after the Design Basis
Earthquake (DBE) be used to determine the adequacy for tsunami loading. If
inadequate, the resulting design would then need to be modified as necessary
to address tsunami effects. For areas that are subject to near-sourcegenerated
tsunamis, this sequential loading condition will clearly control the
design of the structure. To help ensure adequate strength and ductility in the
structure for resisting tsunami load effects, Seismic Design Category D, as
defined in ASCE/SEI 7-05, should be assigned to the structure, as a
minimum.
A properly designed essential facility is also expected to have improved
performance of non-structural components including ceilings, walls, light
fixtures, fire sprinklers, and other building systems. For evacuees to feel
comfortable entering a vertical evacuation structure following an earthquake,
and remaining in the structure during potential aftershocks, it is important
that visible damage to both structural and non-structural components be
limited. Particular attention should be focused on non-structural components
in the stairwells, ramps, and entrances that provide access and vertical
circulation within the structure.
6.3.2 Far-Source-Generated Tsunamis
Although a vertical evacuation structure may not experience earthquake
shaking directly associated with a far-source tsunami, seismic design must be
included as dictated by the seismic hazard that is present at the site. Even in
regions of low seismicity, however, it is recommended that Seismic Design
Category D be assigned to the structure, as a minimum, to help ensure
adequate strength and ductility for resisting tsunami load effects.
A vertical evacuation structure
located in a region susceptible to
near-source-generated tsunamis is
likely to experience strong ground
shaking immediately prior to the
tsunami.
FEMA P646 6: Load Determination and Structural Design Criteria 69
6.4 Wind Loading
The recommended basis for wind design of a vertical evacuation structure is
the International Building Code, which references ASCE/SEI 7-05 Minimum
Design Loads for Buildings and Other Structures for the majority of its wind
requirements. In many locations affected by tsunami risk, earthquake
loading will likely govern over wind loading, but this is not necessarily true
for all regions.
At locations where wind loading controls the design, the use of special
seismic detailing for structural components should be considered. It is
recommended that Seismic Design Category D be assigned to the structure,
as a minimum, to help ensure adequate strength and ductility for resisting
tsunami load effects.
6.5 Tsunami Loading
The following tsunami load effects should be considered for the design of
vertical evacuation structures: (1) hydrostatic forces; (2) buoyant forces; (3)
hydrodynamic forces; (4) impulsive forces; (5) debris impact forces; (6)
debris damming forces; (7) uplift forces; and (8) additional gravity loads
from retained water on elevated floors.
In this document, wave-breaking forces are not considered in the design of
vertical evacuation structures. In general, tsunamis break offshore, and
vertical evacuation structures should be located some distance inland from
the shoreline. The term ‘wave-breaking’ is defined here as a plunging-type
breaker in which the entire wave front overturns. When waves break in a
plunging mode, the wave front becomes almost vertical, generating an
extremely high pressure over an extremely short duration. Once a tsunami
wave has broken, it can be considered as a bore because of its very long
wavelength. Further justification for not considering wave-breaking forces
can be found in Yeh (2008).
Wave-breaking forces could be critical for vertical evacuation structures
located in the wave-breaking zone, which is beyond the scope of this
document. If it is determined that a structure must be located in the wavebreaking
zone, ASCE/SEI 7-05 Minimum Design Loads for Buildings and
Other Structures and the Coastal Engineering Manual, EM 1110-2-1100,
(U.S. Army Coastal Engineering Research Center, 2002) should be consulted
for additional guidance on wave-breaking forces.
6.5.1 Key Assumptions for Estimating Tsunami Load Effects
Tsunami load effects are determined using the following key assumptions:
Tsunami Load Effects include:
(1) hydrostatic forces;
(2) buoyant forces;
(3) hydrodynamic forces;
(4) impulsive forces;
(5) debris impact forces;
(6) debris damming forces;
(7) uplift forces; and
(8) additional gravity loads from
retained water on elevated floors.
70 6: Load Determination and Structural Design Criteria FEMA P646
• Tsunami flows consist of a mixture of sediment and seawater. Most
suspended sediment transport flows do not exceed 10% sediment
concentration. Based on an assumption of vertically averaged sedimentvolume
concentration of 10% in seawater, the fluid density of tsunami
flow should be taken as 1.2 times the density of freshwater, or .s
= 1,200
kg/m3 = 2.33 slugs/ft3.
• There is significant variability in local tsunami runup heights, based on
local bathymetry and topographic effects, and uncertainty in numerical
simulations of tsunami inundation. Based on empirical judgment from
past tsunami survey data, it is recommended that the design runup
elevation, R, be taken as 1.3 times the predicted maximum runup
elevation, R*, to envelope the potential variability.
• Because of uncertainties in modeling tsunami inundation, design
parameters (e.g., flow velocity, depth, and momentum flux) derived from
numerical simulations should not be taken as less than 80% of the values
obtained from the analytical solutions described in Appendix E, and
provided in Equation 6-6, Equation 6-9, and Figure 6-7.
6.5.2 Hydrostatic Forces
Hydrostatic forces occur when standing or slowly moving water encounters a
structure or structural component. This force always acts perpendicular to the
surface of the component of interest. It is caused by an imbalance of
pressure due to a differential water depth on opposite sides of a structure or
component. Hydrostatic forces may not be relevant to a structure with a
finite (i.e., relatively short) breadth, around which the water can quickly flow
and fill in on all sides. Hydrostatic forces are usually important for long
structures such as sea walls and dikes, or for evaluation of an individual wall
panel where the water level on one side differs substantially from the water
level on the other side.
Hydrostatic and buoyant forces must be computed when the ground floor of a
building is watertight, or is sufficiently insulated and airtight to prevent or
delay the intrusion of water. In this situation, the hydrostatic force should be
evaluated for individual wall panels. The horizontal hydrostatic force on a
wall panel can be computed using Equation 6-1:
2
2 max
F p A 1 gbh h c w s = = . , (6-1)
where pc is the hydrostatic pressure, Aw is the wetted area of the panel, .s
is
the fluid density including sediment (1200 kg/m3 = 2.33 slugs/ft3), g is the
gravitational acceleration, b is the breadth (width) of the wall, and hmax is the
FEMA P646 6: Load Determination and Structural Design Criteria 71
maximum water height above the base of the wall at the structure location. If
the wall panel with height hw is fully submerged, then the horizontal
hydrostatic force can be written as Equation 6-2:
Fh = pcAw = .s
g hmax - hw
2
.
. .
.
. .
b hw (6-2)
where hmax is the vertical difference between the design tsunami runup
elevation R and the base elevation of the wall at the structure, zw, as shown in
Equation 6-3:
hmax = 1.3R * - zw = R - zw (6-3)
where R* is the maximum tsunami runup elevation taken as the estimated
maximum inundation elevation at the structure from a detailed numerical
simulation model, or the ground elevation at maximum penetration of the
tsunami from available tsunami inundation maps. The design runup
elevation, R, is taken as 1.3 times the predicted maximum runup elevation,
R*. The moment about the base of the wall can be evaluated using the line of
action of the hydrostatic force resultant, as shown in Figure 6-2.
Figure 6-2 Hydrostatic force distribution and location of resultant.
6.5.3 Buoyant Forces
Buoyant or vertical hydrostatic forces will act vertically through the centroid
of the displaced volume on a structure or structural component subjected to
partial or total submergence. The total buoyant force equals the weight of
water displaced. Buoyant forces on components must be resisted by the
weight of the component and any opposing forces resisting flotation.
Buoyant forces are a concern for structures that have little resistance to
upward forces (e.g. light wood frame buildings, basements, empty tanks
72 6: Load Determination and Structural Design Criteria FEMA P646
located above or below ground, swimming pools, components designed
considering only gravity loads).
For a watertight structure, the total buoyant force is given by Equation 6-4:
Fb = .s
gV (6-4)
where .s
is the fluid density including sediment (1200 kg/m3 = 2.33
slugs/ft3), and V is the volume of water displaced by the building, i.e., the
volume below the level of hmax as determined by Equation 6-3. Bouyant
forces on an overall building are shown in Figure 6-3. If there is insufficient
building weight to resist buoyant forces, tension piles may be used to
increase the resistance to flotation, but reduction in pile side friction due to
anticipated scour around the tops of the piles must be considered.
R
DATUM
DESIGN RUNUP HEIGHT
Building
Weight
Fb
Pile Tension
Total Displaced
Volume, V
hmax
Figure 6-3 Buoyant forces on an overall building with watertight lower
levels.
6.5.4 Hydrodynamic Forces
When water flows around a structure, hydrodynamic forces are applied to the
structure as a whole and to individual structural components. These forces
are induced by the flow of water moving at moderate to high velocity, and
are a function of fluid density, flow velocity and structure geometry. Also
known as drag forces, they are a combination of the lateral forces caused by
the pressure forces from the moving mass of water and the friction forces
generated as the water flows around the structure or component.
Hydrodynamic forces can be computed using Equation 6-5:
2
max
1
( )
2 Fd = .sCdB hu (6-5)
FEMA P646 6: Load Determination and Structural Design Criteria 73
where .s
is the fluid density including sediment (1200 kg/m3 = 2.33
slugs/ft3), Cd is the drag coefficient, B is the breadth of the structure in the
plane normal to the direction of flow (i.e. the breadth in the direction parallel
to the shore), h is flow depth, and u is flow velocity at the location of the
structure. For forces on components, B is taken as the width of the
component. It is recommended that the drag coefficient be taken as Cd = 2.0.
The resultant hydrodynamic force is applied approximately at the centroid of
the wetted surface of the component, as shown in Figure 6-4.
Figure 6-4 Hydrodynamic force distribution and location of resultant.
The combination h u2 represents the momentum flux per unit mass. Note that
(h u2)max does not equal hmax u2
max. The maximum flow depth, hmax , and
maximum flow velocity, umax , at a particular site may not occur at the same
time. The hydrodynamic forces must be based on the parameter (h u2)max,
which is the maximum momentum flux per unit mass occurring at the site at
any time during the tsunami.
The maximum value of (h u2) can be obtained by running a detailed
numerical simulation model or acquiring existing simulation data. The
numerical model in the runup zone must be run with a very fine grid size to
ensure adequate accuracy in the prediction of h u2.
The value (h u2)max can be roughly estimated using Equation 6-6:
( ) = - +
. . . .
.. . . .. . . . .
2
2 2
max
0.125 0.235 0.11
z z
hu gR
R R
(6-6)
where g is the acceleration due to gravity, R is the design runup elevation,
and z is the ground elevation at the base of the structure. The design runup
elevation, R, is taken as 1.3 times the maximum runup elevation, R*, which is
the maximum inundation elevation at the structure from a detailed numerical
simulation model, or the ground elevation at maximum penetration of the
74 6: Load Determination and Structural Design Criteria FEMA P646
tsunami from available tsunami inundation maps. To use this formula, the
sea level datum must be consistent with that used in the inundation maps.
The basis of Equation 6-6 is described in Appendix E. Although the
analytical solution is based on one-dimensional nonlinear shallow-water
theory for a uniformly sloping beach, with no lateral topographical variation
and no friction, the maximum value of (h u2) obtained from Equation 6-6 can
be used for: (1) preliminary design; (2) approximate design in the absence of
other modeling information; and (3) to evaluate the reasonableness of
numerical simulation results.
R* and z can be obtained from tsunami inundation maps. Because of
uncertainties in modeling tsunami inundation, numerically predicted values
of (h u2) should not be taken less than 80% of the values computed using
Equation 6-6.
6.5.5 Impulsive Forces
Impulsive forces are caused by the leading edge of a surge of water
impacting a structure. Ramsden (1993) performed comprehensive
experiments on impulsive forces. Laboratory data show no significant initial
impact force (impulse force) in dry-bed surges, but an overshoot in force is
observed in bores that occur when the site is initially flooded. The maximum
overshoot is approximately 1.5 times the subsequent hydrodynamic force,
consistent with independent laboratory data obtained by Arnason (2005).
Since impact momentum increases with the sudden slam of the steep front of
a bore (Yeh, 2007), the lack of overshoot in dry-bed surge can be attributed
to the relatively mild slope of the front profile of the water surface. If the
runup zone is flooded by an earlier tsunami wave, subsequent waves could
impact buildings in the form of a bore. Since the subsequent bore loading is
greater than the initial dry-bed surge impact, dry-bed surge loading may not
be critical.
For conservatism, it is recommended that the impulsive forces be taken as 1.5
times the hydrodynamic force, as shown in Equation 6-7:
Fs =1.5Fd (6-7)
Impulsive forces will act on members at the leading edge of the tsunami bore,
while hydrodynamic forces will act on all members that have already been
passed by the leading edge, as shown in Figure 6-5.
FEMA P646 6: Load Determination and Structural Design Criteria 75
h
z
max R
DATUM
DESIGN RUNUP HEIGHT
Fs,c2
Fs,b2
Fd,c1 Fs,c1
Fd,b2
Fd,c2
F - Impulsive forces on columns and beams at leading edge of bore
d,c1
s,c1
F - Drag forces on columns and beams behind leading edge of bore
c1 and c2 - Columns at first and second levels. b2 - Beams at second level
Figure 6-5 Hydrodynamic impulsive and drag forces on components of a
building subjected to inundation by a tsunami bore.
6.5.6 Debris Impact Forces
The impact force from waterborne debris (e.g., floating driftwood, lumber,
boats, shipping containers, automobiles, buildings) can be a dominant cause
of building damage. Unfortunately, it is difficult to estimate this force
accurately. Background information on the development of the
recommended impact force calculation is provided in Appendix D.
The debris impact force can be estimated using Equation 6-8:
i m max F = C u k m (6-8)
where Cm is the added mass coefficient, umax is the maximum flow velocity
carrying the debris at the site, and m and k are the mass and the effective
stiffness of the debris, respectively. It is recommended that the added mass
coefficient be taken as Cm = 2.0. Unlike other forces, impact forces are
assumed to act locally on a single member of the structure at the elevation of
the water surface, as shown in Figure 6-6.
z
R
DATUM
DESIGN RUNUP HEIGHT
W Fi
d
Figure 6-6 Waterborne debris impact force.
76 6: Load Determination and Structural Design Criteria FEMA P646
Debris impact forces should be evaluated considering the location of the
vertical evacuation structure and potential debris in the surrounding area.
For example, it is likely that floating debris would consist primarily of
driftwood, logs and pier pilings for most coastal towns, whereas for some
large port areas, the debris could be shipping containers. Locations near
yacht marinas or fishing harbors should consider possible impact from boats
that break their moorings.
Use of Equation 6-8 requires the mass and stiffness properties of the debris.
Approximate values of m and k for common waterborne debris are listed in
Table 6-1. Mass and stiffness properties for other types of debris will need to
be derived or estimated as part of the design process.
Table 6-1 Mass and Stiffness Properties of Common Waterborne Debris
Location of Source Mass (m) in kg
Effective stiffness
(k) in N/m
Lumber or Wood Log 450 2.4 x 106
40-ft Standard Shipping Container 3800 (empty) 6.5x108
20-ft Standard Shipping Container 2200 (empty) 1.5x109
20-ft Heavy Shipping Container 2400 (empty) 1.7x109
The magnitude of the debris impact force depends on mass and velocity.
Smaller (lighter) debris requiring little or no draft to float can travel at higher
velocities than larger (heavier) debris requiring much larger depths to float.
Use of maximum flow velocity without consideration of the depth required to
float large debris would be unnecessarily conservative. The appropriate
maximum flow velocity umax for a given flow depth can be obtained by
running a detailed numerical simulation model or by acquiring existing
simulation data. It is noted, however, that numerical predictions of flow
velocities are less accurate than predictions of inundation depths, and the grid
size for numerical simulations in the runup zone must be very fine in order to
obtain sufficient accuracy in velocity predictions.
When a suitable numerical simulation model is unavailable, the maximum
flow velocity carrying lumber or a wooden log (with essentially no draft) can
be estimated using the analytical solution for tsunami runup on a uniformly
sloping beach with no lateral topographical variation, given by Equation 6-9:
max 2 1
z
u gRR
= . - . . .
. .
. (6-9)
FEMA P646 6: Load Determination and Structural Design Criteria 77
where g is the acceleration due to gravity, R is the design runup height that is
1.3 times the ground elevation R* at the maximum tsunami penetration, and z
is the ground elevation at the structure (the datum must be at the sea level).
Background information on the development of this equation is provided in
Appendix E.
For a shipping container or other similar large debris with draft d, the ratio of
the draft d to the maximum runup height R can be computed, and Figure 6-7
can be used to estimate the maximum flow velocity. Draft d can be estimated
using Equation 6-10:
d = W
.s
gAf
(6-10)
where W is the weight of the debris, .s
is the fluid density including sediment
(1200 kg/m3 = 2.33 slugs/ft3), g is the acceleration due to gravity, and Af is
the cross-sectional area parallel to the water surface such that the product d ×
Af represents the volume of water displaced by the debris.
Figure 6-7 Maximum flow velocity of depth, d, at the ground elevation, z, and
maximum runup elevation, R. The bottom curve represents the lower
limit of maximum flow velocity.
Based on the appropriate curve for d/R, and ratio between the elevation of the
structure relative to the design runup elevation (z/R), Figure 6-7 will provide
an estimate of the maximum flow velocity. It should be understood that
Figure 6-7 is based on an analytical solution for tsunami runup on a
uniformly sloping beach, with no lateral topographical variation, and no
friction. Computed values may differ from the actual velocities, and
78 6: Load Determination and Structural Design Criteria FEMA P646
additional engineering evaluation and judgment should be considered.
Background information on the development of Figure 6-7 is provided in
Appendix E.
When numerical models are used to determine the maximum flow velocity,
umax, values should not be taken as less than 80% the analytical values
predicted using Equation 6-9 or Figure 6-7.
6.5.7 Damming of Waterborne Debris
The damming effect caused by accumulation of waterborne debris can be
treated as a hydrodynamic force enhanced by the breadth of the debris dam
against the front face of the structure. Equation 6-11 is a modification of
Equation 6-5 to include the breadth of the debris dam:
Fdm = 1
2
.s
CdBd (hu2 )max (6-11)
where .s
is the fluid density including sediment (1200 kg/m3 = 2.33
slugs/ft3), Cd is the drag coefficient, Bd is the breadth of the debris dam, h is
flow depth, and u is flow velocity at the location of the structure. It is
recommended that the drag coefficient be taken as Cd = 2.0.
The momentum flux (h u2)max can be obtained by running a detailed
numerical simulation model, acquiring existing simulation data, or estimated
using Equation 6-6. Values of (h u2) obtained from numerical simulation
should not be taken as less than 80% of the values computed using Equation
6-6.
Since debris damming represents an accumulation of debris across the
structural frame, the total debris damming force will likely be resisted by a
number of structural components, depending on the framing dimensions and
the size of debris dam. The debris damming force, Fdm, should be assumed to
act as a uniformly distributed load over the extent of the debris dam. It
should be assigned to each resisting structural component by an appropriate
tributary width, and distributed uniformly over the submerged height of each
resisting component. A minimum debris dam width of Bd = 40 feet (or 12
m), representing a sideways shipping container or a mass of floating lumber,
is recommended. The effects of debris damming should be evaluated at
various locations on the structure to determine the most critical location.
6.5.8 Uplift Forces on Elevated Floors
Uplift forces will be applied to floor levels of a building that are submerged
by tsunami inundation. In addition to standard design for gravity loads, these
FEMA P646 6: Load Determination and Structural Design Criteria 79
floors must also be designed to resist uplift due to buoyancy and
hydrodynamic forces. When computing the buoyant forces on a floor slab,
consideration must be given to the potential for increased buoyancy due to
the additional volume of water displaced by air trapped below the floor
framing system. In addition, exterior walls at the upper floor level will
exclude water until their lateral resistance is exceeded by the applied
hydrostatic pressure. This can significantly increase the displaced volume of
water contributing to the buoyancy, as shown in Figure 6-8.
The total upward buoyant force exerted on a floor system can be estimated
using Equation 6-12:
Fb = .s
gAf hb (6-12)
where .s
is the fluid density including sediment (1200 kg/m3 = 2.33
slugs/ft3), g is the acceleration due to gravity, Af is the area of the floor panel
or floor framing component, and hb is the water height displaced by the floor
(including potentially entrapped air). The value of hmax indicated in Figure 6-
8 should be determined using Equation 6-3.
The upward buoyant force per unit area exerted to the floor system can be
estimated using Equation 6-13:
fb = .s
ghb (6-13)
h
h
a
h
u s
max
b
A f = BxL
Fb
Figure 6-8 A definition sketch for upward buoyant force exerted on an
elevated floor.
Hydrodynamic forces can also act vertically on floor slabs. During rapid
inundation, rising water will apply uplift to the soffit of horizontal structural
components, adding to the buoyancy uplift. The presence of structural walls
and columns in a building will obstruct the tsunami flow passing through the
building, and recent experiments have shown that this can result in
80 6: Load Determination and Structural Design Criteria FEMA P646
significant uplift forces on the floor slab immediately in front of the
obstruction. It is recommended that the building structural layout be
designed to minimize obstruction of tsunami flow through the lower levels of
the building.
Until further research results become available, the total uplift force on the
floor system can be estimated using Equation 6-14:
Fu = 1
2
Cu .s
Af uv
2 (6-14)
where Cu is a coefficient (taken as 3.0), .s
is the fluid density including
sediment (1200 kg/m3 = 2.33 slugs/ft3), Af is the area of the floor panel or
floor framing component, and uv is the estimated vertical velocity or water
rise rate (adapted from American Petroleum Institute, 1993).
The hydrodynamic uplift per unit area can be determined from Equation
6-15:
fu = 1
2
Cu .s
uv
2 (6-15)
Unless a detailed hydrodynamic study is performed, the value of uv for the
condition of sloping terrain below the building can be estimated using
Equation 6-16:
uv = utana (6-16)
where u is the horizontal flow velocity corresponding to a water depth, hs
equal to the elevation of the soffit of the floor system, and a is the average
slope of grade at the site, as shown in Figure 6-8. Using the maximum
horizontal flow velocity, umax, in Equation 6-15 would be unnecessarily
conservative since it may not correspond to a flow depth equal to the floor
soffit elevation. The maximum horizontal velocity u in Equation 6-16 can
also be estimated using Figure 6-7 by replacing d/R with hs/R.
6.5.9 Additional Gravity Loads on Elevated Floors
During drawdown, water retained on the top of elevated floors, as shown in
Figure 6-9, will apply additional gravity loads that can exceed the loads for
which the floor system was originally designed. The depth of water retained,
hr, will depend on the maximum inundation depth at the site, hmax, and the
lateral strength of the wall system at the elevated floor. It should be assumed
that the exterior wall system will be compromised at some point so that water
will inundate submerged floor levels. Because of the rapid rate of
FEMA P646 6: Load Determination and Structural Design Criteria 81
drawdown, it is likely that much of this water will be retained in the upper
levels (at least temporarily) resulting in significant additional gravity load on
the floor system. The maximum potential downward load per unit area, fr,
can be estimated using Equation 6-17:
= . fr s ghr (6-17)
where .s
is the fluid density including sediment (1200 kg/m3 = 2.33
slugs/ft3), g is the acceleration due to gravity, and hr is the maximum
potential depth of water retained on the elevated floor determined using
Equation 6-18:
hr = hmax - h1 = hbw (6-18)
where hmax is the maximum inundation level predicted at the site, h1 is the
floor elevation above grade, and hbw is the maximum water depth that can be
retained before failure of the wall due to internal hydrostatic pressure.
For elevated floors without walls (such as a parking structure with open
guardrails) water may remain on elevated floors until it has had time to drain
off the structure. Drainage systems should be provided to ensure that the
weight of retained water does not exceed the live load for which the floor is
designed.
hr
hmax
h1
u
Fr
Figure 6-9 Gravity loads exerted on an elevated floor with water retained
by exterior walls during rapid drawdown.
6.6 Combination of Tsunami Forces
Not all tsunami load effects will occur simultaneously, nor will they all affect
a particular structural component at the same time. This section describes
combinations of tsunami forces that should be considered for the overall
structure and for individual structural components. Other potential
82 6: Load Determination and Structural Design Criteria FEMA P646
combinations should be considered as needed, based on the particular siting,
structural system, and design of the structure under consideration.
6.6.1 Tsunami Force Combinations on the Overall Structure
Tsunami forces are combined on the overall structure as follows:
• Uplift due to buoyancy, Fb, and hydrodynamic uplift, Fu, have the effect
of reducing the total dead weight of a structure, which may impact the
overturning resistance. Buoyancy and hydrodynamic uplift appropriate
for the design inundation level should be considered in all load
combinations.
• Impulsive forces, Fs, are very short duration loads caused by the leading
edge of a surge of water impinging on a structure. As the surge passes
through a structure, impulsive forces will be applied sequentially to all
structural components, but not at the same time. Once the leading edge
of the surge has passed a structural component, it will no longer
experience the impulsive force, but rather a sustained hydrodynamic drag
force, Fd. The total horizontal hydrodynamic force on a structure will
therefore be a combination of impulsive forces on members at the
leading edge of the surge, and drag forces on all previously submerged
members behind the leading edge. Figure 6-10 shows how this
combination would apply to a building with multiple columns and shear
walls. The worst case lateral load will likely occur when the leading
edge of the surge reaches the last components in the building frame.
• Debris impact forces, Fi, are short duration loads due to impact of large
floating objects with individual structural components. Since large
floating objects are not carried by the leading edge of the surge, the
effect of debris impact is combined with hydrodynamic drag forces, Fd,
but not impulsive forces, Fs. Although many floating objects may impact
a building during a tsunami event, the probability of two or more impacts
occurring simultaneously is considered small. Therefore, only one
impact should be considered to occur at any point in time. Both the
individual structural component and the overall structure must be
designed to resist the impact force in combination with all other loads
(except impulsive forces).
• Debris damming has the effect of increasing the exposed area for
hydrodynamic loading. The debris damming force, Fdm, should be
considered to act in the most detrimental location on a structure while
hydrodynamic forces act on all other components of the structure. Figure
6-11 shows typical debris dam locations that could be considered in
conjunction with drag forces on all other submerged structural
Not all tsunami load effects will
occur simultaneously, nor will they
all affect a particular structural
component at the same time.
FEMA P646 6: Load Determination and Structural Design Criteria 83
components. It is conservative to ignore any shielding effect provided
by the debris dam for components downstream of the dam.
Figure 6-10 Impulsive and drag forces applied to an example building
Figure 6-11 Debris dam and drag forces applied to an example building
84 6: Load Determination and Structural Design Criteria FEMA P646
• Breakaway walls are not part of the structural support of the building,
and are intended, through design and construction, to fail under specific
lateral loading. If lower level infill walls are designed as breakaway
walls, the maximum lateral load will be the load at which the walls will
“fail,” and the overall structure, as well as the structural components
supporting these walls, must be designed to resist this failure load.
Guidance on the design of break-away walls is provided in Chapter 7.
• Design of floor systems to withstand the effects of potential retained
water, Fr, can be performed independently of the lateral loading on the
structure.
6.6.2 Tsunami Force Combinations on Individual
Components
Tsunami forces are combined on individual structural components (e.g.,
columns, walls, and beams), as follows:
• Impulsive force, Fs, due to the leading edge of the tsunami bore, for
maximum h u2.
• Hydrodynamic drag force, Fd, plus debris impact, Fi, at the most critical
location on the member, for maximum h u2.
• Debris damming, Fdm, due to a minimum 40-foot wide debris dam
causing the worst possible loading on the member, for maximum h u2.
• Hydrostatic pressure, Fh, on walls enclosing watertight areas of a
structure, for maximum h.
For uplift on floor framing components, the following combinations of
tsunami forces should be considered:
• Buoyancy, Fb, of submerged floor framing components including the
effects of entrapped air and upturned beams or walls, for maximum h.
• Hydrodynamic uplift, Fu, due to rapidly rising flood waters, for flow
velocity at a depth equal to the soffit of the floor system, hs.
• Maximum uplift case: The larger of the above uplift loads combined with
90% dead load and zero live load on the floor system, for design against
uplift failure of floor slabs, beams, and connections.
For downward load on floor framing components due to retained water, the
following force combination should be considered:
• Downward load due to water retained by exterior walls, fr, combined
with 100% dead load.
FEMA P646 6: Load Determination and Structural Design Criteria 85
6.7 Load Combinations
The load combinations presented herein are based on the guidance given in
the Commentary of ASCE/SEI 7-05 Minimum Design Loads for Buildings
and Other Structures (ASCE, 2006b), but are different from those used in
model building codes or ASCE/SEI Standard 7-05. They have been
reviewed in the development of this document, but have not been extensively
studied. They should be considered in addition to all other load
combinations provided by the current building code in effect, or Section 2 of
ASCE/SEI 7-05.
Tsunami forces that will act on the entire structure and on individual
structural components should be calculated in accordance with Section 6.5
and Section 6.6. The resulting member forces (Ts) should then be combined
with gravity load effects using the following Strength Design Load
Combinations:
Load Combination 1: 1.2D + 1.0Ts + 1.0LREF + 0.25L
Load Combination 2: 0.9D + 1.0Ts
where D is the dead load effect, Ts is the tsunami load effect, LREF is the live
load effect in refuge area (assembly loading), and L is the live load effect
outside of the refuge area.
A load factor of 1.0 is used in conjunction with tsunami forces calculated in
accordance with this document for the following reasons: (1) it is anticipated
that the tsunami hazard level corresponding to the Maximum Considered
Tsunami will be consistent with the 2500-year return period associated with
the Maximum Considered Earthquake used in seismic design; (2) potential
variability in tsunami runup elevations is explicitly considered by applying a
30% increase to runup elevations used in tsunami force calculations; and (3)
design for tsunami forces considers only the elastic response of components,
without consideration of inelastic response and corresponding forcereduction
factors (as is used in seismic design).
Load Combination 1 considers the refuge area in the vertical evacuation
structure to be fully loaded with assembly live load (e.g., 100 psf). The
assembly live load represents a practical upper limit for the maximum
density of evacuees standing in the refuge area. In combination with tsunami
inundation, it is expected that all other floor areas will experience a reduced
live load equal to 25% of the design live load. This reduced live load is
consistent with live load reductions used in combination with earthquake
Tsunami Load Combinations
should be considered in addition to
all other load combinations
provided by the current building
code in effect, or ASCE/SEI 7-05.
86 6: Load Determination and Structural Design Criteria FEMA P646
forces. When gravity load effects oppose tsunami load effects, Load
Combination 2 applies.
No additional importance factor, I, is applied to tsunami loads in this
document. These design guidelines have been developed specifically for
tsunami evacuation structures, and the critical nature of these structures has
been considered throughout.
Seismic loads are not considered to act in combination with tsunami loads.
While aftershocks are likely to occur, the probability that an aftershock will
be equivalent in size to the design level earthquake, and will occur at the
same time as the maximum tsunami inundation, is considered to be low.
6.8 Member Capacities and Strength Design
Considerations
Model building code provisions and engineering standards for Strength
Design, also known as Load and Resistance Factored Design (LRFD),
provide material-specific member capacity calculations and strength
reduction factors for various force actions and different structural
components. Until further research shows otherwise, it is recommended that
capacity calculations and strength reduction factors be applied to design for
tsunami loading in the same way they are currently applied to design for
earthquake and wind loading.
6.9 Progressive Collapse Considerations
Reducing the potential for disproportionate (i.e., progressive) collapse due to
the loss of one or more structural components will increase the likelihood
that a vertical evacuation structure will remain standing if a column is
severely damaged due to waterborne debris. The decision to include
progressive collapse considerations in the design for a particular structure
will depend on the site and the nature of the debris that could potentially
impact the structure. Because the potential exists for localized severe
damage due to debris impact, design for progressive collapse prevention is
strongly encouraged. In the United States, primary design approaches for
progressive collapse include the “tie force” strategy and the “missing
column” strategy.
6.9.1 Tie Force Strategy
The Department of Defense has adopted an indirect tie force strategy to
address the potential for progressive collapse in the design of facilities using
UFC 4-023-03, Design of Buildings to Resist Progressive Collapse (2005).
The tie force strategy is illustrated in Figure 6-12.
Member Capacities and
Strength Reduction Factors
should be applied to design for
tsunami loading in the same way
they are currently applied to design
for earthquake and wind loading.
FEMA P646 6: Load Determination and Structural Design Criteria 87
Tension ties in reinforced concrete structures typically consist of continuous
reinforcing steel in beams, columns, slabs, and walls, as shown in Figure
6-13. Reinforcement required for tension ties can be provided in whole, or in
part, by steel already sized to resist other actions, such as shear or flexure.
In many cases, the quantity of steel provided to resist gravity and lateral
forces for typical reinforced concrete structures is also sufficient to develop
the necessary tie forces.
Figure 6-12 Tie force strategy
Figure 6-13 Detailing of reinforcing steel for potential loss of a supporting
column
Pos. Moment
Region
Neg. Moment
Region
Cont. Bottom Steel
Provides Positive
Moment Capacity
Closely Spaced
Stirrups Enhance
Ductility
Loss of Column
88 6: Load Determination and Structural Design Criteria FEMA P646
It is reasonable to check tie force compliance after a structure is initially
designed for gravity and lateral loading. Ties must be properly spliced and
adequately anchored at each end in order to develop their full capacity and
perform as anticipated. Reinforcing steel used as tension ties must have
lapped, welded, or mechanically joined (Type 1 or Type 2) splices per ACI
318, Building Code Requirements for Structural Concrete (ACI, 2005).
Splices should be staggered and located away from joints and regions of high
stress.
Anchorage is critical to the performance of ties and must be carefully
assessed, particularly in cases where building layout may be non-typical.
Seismic detailing should be used to anchor ties to other ties, or at points of
termination (such as at the perimeter of a building). This includes providing
seismic hooks and seismic development lengths, as defined in ACI 318.
6.9.2 Missing Column Strategy
The General Services Administration (GSA) missing column strategy is an
independent check performed without consideration of other loads. This
approach is based on the concept that loss of a single column, in this case due
to impact from waterborne debris, should not result in progressive collapse of
the surrounding structural components.
Current progressive collapse criteria are found in Progressive Collapse
Analysis and Design Guidelines for New Federal Office Buildings and Major
Modernization Projects (GSA, 2003). As illustrated in Figure 6-14, this
strategy requires evaluation of surrounding structural components to continue
to support anticipated gravity loads in a series of missing column scenarios.
Live loads on the building are reduced to simulate those in place at the time
the column is damaged. In the case of vertical evacuation structures, full live
loads should be considered in the refuge area while reduced live loads can be
considered elsewhere in the building.
The missing column approach utilizes plastic design concepts in evaluating
the capability of surrounding structural components to continue to support
gravity loads, so some damage in these components is permitted as a result of
a missing column scenario. Given that waterborne debris is most likely to
impact an exterior or corner column, missing column scenarios should
consider the potential loss of any single exterior column. Loss of interior
columns need not be considered.
FEMA P646 6: Load Determination and Structural Design Criteria 89
Figure 6-14 Missing column strategy
FEMA P646 7: Structural Design Concepts and Additional Considerations 91
Chapter 7
Structural Design Concepts
and Additional Considerations
This chapter summarizes structural design concepts and other considerations
relevant to the design of vertical evacuation structures, including retrofit of
existing structures, permitting, peer review, quality control, planning issues,
and potential cost impacts.
7.1 Attributes of Tsunami-Resistant Structures
Structural system selection and configuration, from foundation to roof
framing, can have a significant effect on the ability of a vertical evacuation
structure to withstand anticipated tsunami, earthquake, and wind loading.
Many common structural systems can be engineered to resist tsunami load
effects.
Structural attributes that have demonstrated good behavior in past tsunamis
include: (1) strong systems with reserve capacity to resist extreme forces; (2)
open systems that allow water to flow through with minimal resistance; (3)
ductile systems that resist extreme forces without failure; and (4) redundant
systems that can experience partial failure without progressive collapse.
Systems exhibiting these attributes include reinforced concrete and steel
moment frame systems, and reinforced concrete shear wall systems.
7.2 Structural Considerations for Tsunami Load
Effects
Foundation design must consider the local effects of scour and liquefaction.
In many cases foundation support will consist of deep foundations (piles).
Pile design must consider increased demands due to downdrag and additional
lateral forces, and increased unbraced pile length due to scour. Potential
uplift from the overall buoyancy of the structure needs to be accounted for in
the foundation design.
Design of individual columns for tsunami lateral loads should be performed
assuming the appropriate degree of fixity at the column base and at each
floor level. For example, a reinforced concrete column in a multi-story
building supported by pile foundations can be assumed fixed at the base and
at each floor level. A steel column forming part of a moment-resisting frame
can be assumed pinned or fixed at the base and at each floor level.
Tsunami-Resistant Structures
have:
(1) strong systems with reserve
capacity to resist extreme forces;
(2) open systems that allow water
to flow through with minimal
resistance;
(3) ductile systems that resist
extreme forces without failure; and
(4) redundant systems that can
experience partial failure without
progressive collapse.
92 7: Structural Design Concepts and Additional Considerations FEMA P646
Column shape is also important. Round columns will result in lower drag
forces than square or rectangular shapes. In addition, waterborne debris will
be less likely to fully impact round columns.
If shear walls are used, the plan orientation of the walls is important. It is
recommended that the shear walls be oriented parallel to the anticipated
direction of tsunami flow to reduce associated hydrodynamic forces and
impact forces from waterborne debris.
Design of reinforced concrete walls for tsunami forces should consider the
full load on the wall, including hydrodynamic and debris impact forces,
spanning vertically between floor levels. Reinforced concrete beams poured
integral with the floor will be braced by the slab. Design of beams for
horizontal tsunami forces should take into account the lateral bracing
provided by the floor slab. Isolated beams must be designed for horizontal
shear and bending induced by tsunami loads.
Floor systems must be designed for the effects of buoyancy and
hydrodynamic uplift, which will induce shear and bending effects that are
opposite to those resulting from gravity loads. Even though lower levels of a
vertical evacuation structure are not intended for use during a tsunami,
failure could result in damage or collapse of columns supporting upper
levels, including the tsunami refuge area.
In structural steel floor systems, lateral torsional buckling of beam bottom
flanges must be considered when subjected to uplift loading. In reinforced
concrete floor systems, continuity of reinforcement should be provided in
beams and slabs for at least 50% of both the top and bottom reinforcement.
Prestressed concrete floor systems must be carefully checked for buoyancy
and hydrodynamic uplift effects when submerged. Internal prestressing
forces used to oppose dead loads add to these effects. Web elements of
typical prestressed joist systems are susceptible to compression failure under
uplift conditions, and many typical bearing connections are not anchored for
potential net uplift forces. Localized damage to the concrete in a prestressed
floor system can result in loss of concrete compressive capacity, and release
of the internal prestressing forces.
7.2.1 Foundation / Scour Design Concepts
Scour around shallow foundations can lead to failure of the supported
structural element. Foundations consisting of drilled shafts or driven piles
can be designed to avoid this failure; however, they must be able to resist all
FEMA P646 7: Structural Design Concepts and Additional Considerations 93
applied loads after scouring has exposed the pile cap and top of the shafts or
piles.
Dames and Moore (1980) suggest that scour depth is related to distance from
the shoreline and soil type. As indicated in Table 7-1, scour depth is
estimated as a percentage of the maximum tsunami flow depth, d.
Table 7-1 Approximate Scour Depth as a Percentage of Flow Depth, d
(Dames and Moore, 1980)
Soil Type
Scour depth (% of d)
(Shoreline Distance < 300 feet)
Scour depth (% of d)
(Shoreline Distance > 300 feet)
Loose sand 80 60
Dense sand 50 35
Soft silt 50 25
Stiff silt 25 15
Soft clay 25 15
Stiff clay 10 5
Observations after the Indian Ocean Tsunami indicate that scour can occur
significantly farther inland than 300 feet from the shoreline. Conservative
engineering judgment should be exercised in categorizing the soil type at the
site into the broad categories listed above.
7.2.2 Breakaway Wall Concepts
Solid enclosure walls below the tsunami inundation level will result in large
tsunami loads on the overall building. These walls will also increase the
potential for wave scour at grade beams and piles. Non-structural walls
below the anticipated tsunami flow depth can be designed as breakaway
walls to limit the hydrostatic, buoyancy, hydrodynamic, and impulsive forces
on the overall building and individual structural members. Breakaway wall
requirements are described in detail in the FEMA 55 Coastal Construction
Manual (FEMA, 2005), which complies with National Flood Insurance
Program (NFIP) requirements for construction in the mapped V-Zone.
Breakaway walls can create wave reflection and runup prior to failure as
indicated in Figure 7-1.
In accordance with ASCE/SEI Standard 24-05 Flood Resistant Design and
Construction (ASCE, 2006a), walls, partitions, and connections to the
structure that are intended to break away are designed for the largest of the
following loads acting perpendicular to the plane of the wall:
• The wind load specified in ASCE/SEI Standard 7-05 Minimum Design
Loads for Buildings and Other Structures (ASCE, 2006b).
94 7: Structural Design Concepts and Additional Considerations FEMA P646
Figure 7-1 Effect of breakaway walls on waves (FEMA, 2005).
• The earthquake load specified in ASCE/SEI Standard 7-05.
• 10 psf (0.48kN/m2).
• Not more than 20 psf (0.6 kN/m2) unless the design meets the following
conditions: (1) breakaway wall collapse is designed to result from a flood
load less than that which occurs during the base flood; and (2) the
supporting foundation and the elevated portion of the building is
designed to resist collapse, permanent lateral displacement, and other
structural damage due to the effects of flood loads in combination with
other loads.
Standard engineering practice can often result in considerable design
overstrength, which would be detrimental to a breakaway wall system and
the supporting structure. Care should be taken to avoid introducing
unnecessary conservatism into the design. All components, including
sheathing, siding, and window frame supports, must be considered in
determining the actual strength of the breakaway wall system, and the
resulting maximum load on the supporting structure. The most desirable
fusing mechanism includes failure of the top and side connections while the
bottom connection remains intact, allowing the wall panel to lay down under
the tsunami flow without becoming detached and part of the debris flow.
Metal Stud Walls. Metal stud infill walls are commonly used as part of the
building envelope. Unless properly galvanized, metal studs will corrode
rapidly in the coastal environment. Recent lateral load testing of typical
metal stud wall configurations shows that ultimate failure occurs when the
studs separate from either the top or bottom tracks. However, the load
required to produce this failure is as much as four times the wind load for
which the studs were initially designed. It is therefore necessary to introduce
FEMA P646 7: Structural Design Concepts and Additional Considerations 95
some sort of a “fuse” at the top track connection to ensure that the wall fails
at a predictable load. Such a fuse might include a reduced stud section at the
top of the studs. Testing of fuse mechanisms would be required to verify that
they have the capacity needed to resist design loads, but will fail at
predictably higher load levels.
Masonry Walls. Masonry walls are commonly used as enclosures in lower
levels of larger buildings. They can be restrained with the use of a dowel pin
fuse system around the top and sides of the wall, without bonded contact to
the structure. Such a system should be tested to verify that it will fail at
predictable load levels that exceed design loads. If properly fused, the
masonry wall will cantilever from the foundation and load will no longer be
applied to the surrounding structural frame, upon failure of the dowel pins.
To allow wall failure due to foundation rotation without damage to the
remaining structure, separation of the wall foundation from the building
foundation should be considered.
7.3 Concepts for Modifying and Retrofitting Existing
Structures
It may not always be feasible to construct new buildings in an area that
requires vertical evacuation refuge. Although retrofitting existing buildings
to perform as a vertical evacuation structure could be expensive and
disruptive to current users of the building, it may be the most viable option
available. Existing buildings considered for use as vertical evacuation
structures should possess the structural attributes listed in Section 7.1 that are
associated with tsunami-resistant structures, and should be evaluated for
tsunami load effects in accordance with Chapter 6. In the case of nearsource-
generated tsunamis, existing buildings should also be evaluated for
seismic effects. Because of the importance of vertical evacuation structures,
and the need for these facilities to function as a refuge when exposed to
extreme tsunami and seismic loading, reduced loading criteria for existing
buildings, as is the current state-of-practice for seismic evaluation of existing
buildings, is not recommended for evaluation of potential tsunami vertical
evacuation structures.
The following concepts can be considered in the modification and retrofit of
existing buildings for use as vertical evacuation structures:
• Roof system. Upgrade roof systems to support additional live loads
associated with refuge occupancy. Protect or relocate existing building
functions at the roof level (e.g., mechanical equipment) that would be at
risk or unsafe in the immediate vicinity of high occupancy areas. Modify
existing roof parapets for fall protection of refuge occupants.
Existing buildings considered for
use as vertical evacuation structures
should possess the attributes of
tsunami-resistant structures listed in
Section 7.1
96 7: Structural Design Concepts and Additional Considerations FEMA P646
• Wall system. Consider modifying walls and wall connections in the
lower levels of the building to perform as breakaway walls to minimize
tsunami hydrostatic, hydrodynamic, and surge forces on the building.
• Access. Modify ingress into the building and improve vertical
circulation through the use of new entrances, ramps, and stairs. Consider
placing access points on the outside of the building for ease of
construction and high visibility.
• Potential Debris. Remove or relocate building ground level functions
that may become potential water-borne debris.
• Existing hazards at the site. Consider and protect against other hazards
that might exist at the building site, including other adjacent buildings
that could collapse, and the presence of hazardous or flammable
materials near the site.
7.4 Permitting and Quality Assurance for Vertical
Evacuation Structures
7.4.1 Permitting and Code Compliance
Before construction begins, all necessary state, local, building, and other
permits should be obtained. Because model building codes and engineering
standards do not address the design of a tsunami refuge specifically, design
professionals should meet with building officials to discuss possible design
requirements.
In general, mechanical, electrical, and plumbing systems should be designed
for the normal daily use of the facility, unless otherwise directed by the
authority having jurisdiction. Designing these systems for the high
occupancy load that would occur only when the structure is serving as a
vertical evacuation refuge may not be necessary.
7.4.2 Peer Review
A vertical evacuation structure is a unique structure that must withstand
special loads and load combinations. While earthquake, wind, and flood
loading effects are well understood in the design and permitting process,
consideration of tsunami load effects includes some new concepts and
approaches. Considering the importance of vertical evacuation structures
and the extreme nature of tsunami loading, peer review by a qualified
individual or team is recommended.
The unique nature of vertical
evacuation structures may
require special allowances for:
(1) permitting and code compliance;
(2) peer review; and
(3) quality assurance.
FEMA P646 7: Structural Design Concepts and Additional Considerations 97
7.4.3 Quality Assurance / Quality Control
Because a vertical evacuation structure must perform well during extreme
loading conditions, quality assurance and quality control for the design and
construction of the structure should be at a level above that for normal
building construction. Design calculations and drawings should be
thoroughly scrutinized for accuracy.
The quality of both construction materials and methods should be ensured
through the development and application of a quality control program. A
quality assurance plan should be based on the Special Inspection
Requirements listed in Chapter 17 of the International Building Code (ICC,
2006). Special inspections and quality assurance provisions for primary
seismic- and wind-resisting systems should be applied to tsunami-resisting
elements of vertical evacuation structures. Exceptions that waive the need
for quality assurance when elements are prefabricated should not be allowed.
In addition to the building elements that are normally included special
inspection programs, the following items require special attention:
• Breakaway walls and their connections to structural components to avoid
unintended conservatism in construction.
• Other special components or details that are used to minimize tsunamiloading
effects.
• Piles, pilecaps and grade beams that will potentially experience the
effects of scour.
7.5 Planning Considerations for Vertical Evacuation
Structures
In addition to structural design, planning for vertical evacuation facilities
should consider a number of issues, including access, parking, pets,
occupancy limitations, and protection of critical functions.
• Access and Entry. Confusion and panic will occur if evacuees arrive at
a refuge facility, but cannot enter. Provisions should be made to ensure
access in the event of a tsunami, while providing adequate security
during times when the facility is unoccupied. Ideally, a vertical
evacuation refuge should be configured so that it is always accessible, or
can be entered without emergency personnel.
• Americans with Disabilities Act (ADA). Vertical evacuation
structures, when not operating as a refuge, must comply with Federal,
state, and local ADA requirements and ordinances for the normal daily
use of the facility. Design of ingress and vertical circulation within a
Planning for vertical
evacuation facilities should
allow for:
(1) access and entry;
(2) Americans with Disabilities Act;
(3) parking;
(4) pets;
(5) occupancy limitations; and
(6) protection of critical functions.
98 7: Structural Design Concepts and Additional Considerations FEMA P646
vertical evacuation structure should consider the needs of disabled
occupants to the extent possible, and the extent required by law, in the
case of emergency evacuation. Given potential limitations on
functionality of power sources and vertical conveyance systems (e.g.,
elevators and escalators) in the event of a near-source earthquake,
disabled occupants may need assistance accessing refuge areas in vertical
evacuation structures.
• Parking. Parking at evacuation facilities can be a problem. Traffic
congestion can adversely affect access to the facility, and parked vehicles
can become waterborne debris that can damage the structure. Planning
for vertical evacuation facilities should consider parking limitations.
• Pets. Refuge facilities are typically not prepared to accommodate pets.
Many people, however, do not want to leave their pets behind during a
disaster. Planning should carefully consider the policy regarding pets.
• Occupancy Limitations. Population density can be non-uniform, and
can vary by time of day, week, or year. In the event of a tsunami,
evacuation behavior of the surrounding population may result in an
unequal distribution of evacuees among available refuge facilities. In
determining the maximum occupancy for a refuge facility, the time of
day, day of the week, or season of the year that will result in the largest
number of possible evacuees should be considered. The maximum
occupancy might need to be increased in order to accommodate
unexpected additional occupants or visitors in the area.
• Protection of Critical Functions. A vertical evacuation facility must be
operational to serve its intended function in the event of a tsunami.
Functions that are critical for operation as a short-term refuge,
emergency response, medical care, or long-term sheltering facility must
be protected from tsunami inundation, or located within the area of
refuge. These might include emergency power, electrical equipment,
communications equipment, basic sanitation needs, medical and
pharmaceutical supplies, and emergency provisions (e.g., food, water,
and supplies).
7.6 Cost Considerations for Vertical Evacuation
Structures
Design of vertical evacuation structures for tsunami load effects will require
more strength, ductility, and robustness than is necessary for normal-use
structures. As recommended in this document, this can include the use of
seismic detailing provisions, progressive collapse preventative measures,
customized breakaway wall details, and deeper foundation systems. As such,
FEMA P646 7: Structural Design Concepts and Additional Considerations 99
it is expected that structural construction costs will be higher for vertical
evacuation structures than for other structures. While there are no direct
comparisons between the cost of a conventional structure versus the cost of a
tsunami-resistant structure, order-of-magnitude information on potential
structural construction cost increases can be obtained from currently
available information.
Structural costs, however, are only a fraction of total construction costs for a
building. Depending on the nature of building occupancy and use, structural
construction costs can range between 5% and 40% of total construction costs.
Structural costs are a lower percentage of the total for occupancies with
special uses (e.g., hospitals) requiring more expensive nonstructural systems
and contents, and are higher percentage of the total for occupancies with
standard uses (e.g., offices).
Anecdotal evidence from design and construction of essential facilities (e.g.,
hospitals) in California, Oregon, and Washington indicate that the cost
premium for seismic design requirements associated with essential facilities
versus ordinary occupancy facilities is on the order of 10% to 20% of
structural construction costs. This would represent an increase on the order
of 1% to 8% in terms of total construction costs.
In a recent study funded by the National Institute of Standards and
Technology, Engineering Design and Cost Data for Reinforced Concrete
Buildings for Next Generation Design and Economic Standards for
Structural Integrity (NIST, 2007), the cost premium for progressive collapseresistant
design was on the order of 10% to 20% of structural construction
costs. Similar to seismic design, this would represent an increase on the
order of 1% to 8% in terms of total construction costs.
Considering additional allowances for added strength to resist tsunami load
effects, it is reasonable to expect that a tsunami-resistant structure, including
seismic-resistant and progressive collapse-resistant design features, would
experience about a 10% to 20% order-of-magnitude increase in total
construction costs over that required for normal-use buildings. While each
project will be unique, and relative costs will depend on the specific tsunami
hazard and site conditions, it should not be assumed that incorporation of
tsunami-resistant design features in a vertical evacuation structure will be
cost prohibitive.
Structural construction costs are only
a fraction of total construction costs
for a building.
Tsunami-resistant structures could
experience about a 10% to 20%
order-of-magnitude increase in total
construction costs over that required
for normal-use buildings.
FEMA P646 A: Vertical Evacuation Structure Examples from Japan 101
Appendix A
Vertical Evacuation Structure
Examples from Japan
In Japan there are examples of structures that were designed and constructed
specifically for the purpose of tsunami refuge. It appears that no formal
guidance for design of these structures is available.
Life-Saving Tower: The Life-Saving Tower (Tasukaru Tower) developed
by Fujiwara Industries Company, Limited, Japan, is shown in Figure A-1.
This is a simple and economical structure that enables a temporary high
refuge for evacuees. The structure has a 5.4-meter span between the
supporting posts, a refuge elevation of 5.8 meters from ground level, and a
capacity of 50 people.
Figure A-1 Life-Saving Tower
102 A: Vertical Evacuation Structure Examples from Japan FEMA P646
Nishiki Tower: The Nishiki Tower, shown in Figure A-2, was constructed
in the town of Kise, Mie Prefecture, Japan. The five-story, 22-meter tall
reinforced concrete structure resembles a lighthouse, and has a spiral
staircase winding up the outside of the building. It was specifically designed
to serve as a tsunami refuge, but is used for other (non-refuge) purposes on
normal days. The first floor is used for public toilet and storage space for
fire equipment; the second floor for a meeting room; and the third floor for
an archival library for natural disasters. The fourth and fifth floors have 73
square meters of refuge space for evacuees.
Figure A-2 Nishiki Tower.
Nishiki Tower is a well-engineered structure that is designed to withstand a
seismic event commensurate to JMA VII on the Japanese earthquake
intensity scale that is comparable to a MMI XII (modified Mercalli scale).
The building is founded on a 4-meter deep sand-and-gravel layer, and is
supported on concrete piles extending 6 meters below grade. The possibility
of liquefaction is remote, considering the large particle size of the sand-andgravel
layer. Elastic design was employed for consideration of tsunami
forces. Based on historical data from the 1944 Tou-Nankaido Earthquake, a
design tsunami of 6 meters in height was used for design. It is designed to
withstand the impact of a 10-ton ship at a velocity of 10 m/sec. This
FEMA P646 A: Vertical Evacuation Structure Examples from Japan 103
criterion was based on size of ships moored in the neighboring port. The
intended performance level allows for partial damage of the building without
incurring loss of life.
Elevated Shelter at Shirahama Beach Resort: A rather aesthetic tsunami
refuge was constructed at a beach resort in the town of Shirahama,
Tokushima Prefecture, shown in Figure A-3. It is designed to accommodate
700 refugees in the area of 700 square meters. The design inundation
elevation is 7.5 meters, based on historical data from the 1854 Ansei-Tokai
Earthquake (M 8.4) and resulting tsunami. With a planned freeboard of 4
meters, the evacuation platform is located at elevation of 11.5 meters. This
reinforced concrete structure is designed to withstand a maximum base
acceleration of 780 gal. Because of a potential for soil liquefaction, pipe
piles were driven approximately 20 meters deep into bedrock. The facility is
also equipped with a solar-powered lighting system.
Figure A-3 Refuge at Shirahama Beach Resort (Photo courtesy of N. Shuto).
Other Tsunami Refuge Structures: There are other structures in Japan
specifically designed as tsunami refuges. A reinforced concrete structure in
the town of Kaifu, Tokushima Prefecture, Japan is shown in Figure A-4. An
artificial high ground (berm), shown in Figure A-5, was constructed in
Aonae, Okushiri-Island, Japan, where the 1993 tsunami struck the hardest.
After the 1993 Okushiri Tsunami, Aonae elementary school, shown in Figure
A-6, was reconstructed as a tsunami resistant structure. The upper floors can
104 A: Vertical Evacuation Structure Examples from Japan FEMA P646
be used as tsunami refuge spaces. The ground floor of the school is
constructed with breakaway walls to relieve tsunami forces.
Figure A-4 Tsunami refuge in Kaifu, Japan.
Figure A-5 Berm constructed for tsunami refuge in Aonae, Japan.
FEMA P646 A: Vertical Evacuation Structure Examples from Japan 105
Figure A-6 Aonae Elementary School. Upper floors are intended for use as
tsunami refuge space.
FEMA P646 B: Community Design Example 107
Appendix B
Community Design
Example
A hypothetical community is indicated in Figure B-1 below. In this
appendix, the initial design and configuration of a series of vertical
evacuation structures is illustrated.
The community has evaluated public and private sites that might be
appropriate for construction of new vertical evacuation structures and
identified existing facilities for possible renovation for use as vertical
evacuation structures. This evaluation includes consideration of the number
of sites required based on travel time and population, as discussed in Chapter
5.
Figure B-1 Hypothetical sketch of example community showing potential
vertical evacuation structure sites and evacuation routes.
108 B: Community Design Example FEMA P646
An assessment of the tsunami inundation depths and flow velocities is
necessary for assessing tsunami effects within the community and
determining tsunami design parameters. Predicted tsunami inundation depths
for this example community are shown in Figure B-2.
Figure B-2 Example community inundation map. Shaded areas show
various predicted tsunami inundation depth, d.
In this example community, the area of refuge at each site would need to be
elevated as indicated in Table B-1.
Table B-1 Design Elevations for Areas of Refuge
Site
Predicted
Inundation
Depth
Freeboard
(3 meters plus 30%)
Design
Elevation
Site 1 3 m 3 m + 0.9 m 6.9 m
Site 2 4 m 3 m + 1.2 m 8.2 m
Site 3 3 m 3 m + 0.9 m 6.9 m
Site 4 4 m 3 m + 1.2 m 8.2 m
Site 5 3 m 3 m + 0.9 m 6.9 m
FEMA P646 B: Community Design Example 109
Tsunami inundation depths indicated in Figure B-2 are increased by 30% to
account for local variability in numerical simulations. An additional
minimum freeboard of 3 meters (or one-story height) is recommended to
ensure that the area of refuge is not inundated from splash or wave action.
The velocity at a particular site is affected by the surrounding topography as
well as natural and man made obstructions to flow. Predicted flow velocities
for this example community are shown in Figure B-3 and summarized in
Table B-2.
Figure B-3 Example community inundation flow velocity map. Shaded
areas show various predicted tsunami flow velocities, u.
Table B-2 Tsunami Flow Velocity at Each Site
Site
Tsunami Flow Velocity
Site 1 9 m/s
Site 2 12 m/s
Site 3 9 m/s
Site 4 12 m/s
Site 5 9 m/s
110 B: Community Design Example FEMA P646
B.1 Site 1 Example: Escape Berm
Site 1 has several unique conditions to consider. The waterfront in this area
is somewhat industrial in nature and includes a container terminal facility at
the harbor. Areas adjacent to the site contain some residential development.
The evacuation population at this site would include both employees of the
harbor industrial area and adjacent residences.
The community has been struggling with finding ways to address other social
issues in this area, which have included a lack of recreational facilities for the
residents, some neglected and deteriorating properties, and a need to
revitalize and enhance the area. At this site a man-made berm, as shown in
Figure B-4, provides an opportunity to add new public open space in addition
to vertical evacuation refuge. This solution creates a unique elevated park
setting for the community, which addresses recreational needs, and provides
a scenic overlook for the waterfront.
With a location adjacent to a container terminal facility, there is a potential
for shipping containers to become waterborne debris. Construction of the
berm utilizing a sheet piles to contain the fill addresses this issue.
Figure B-4 Example escape berm design
The features of this escape berm, illustrated in Figure B-5, include the
following:
• Location 1 (Figure B-5). The semi circular configuration was selected to
help divert tsunami flood waters and potential waterborne debris around
the facility and away from the access stairs and ramp. The elevated area
is over 31,000 square feet, and can handle over 3,000 evacuees at 10
FEMA P646 B: Community Design Example 111
square feet per person. There is sufficient space in the elevated area to
accommodate a comfort station that could be used for both day to day
recreational purposes and emergency use.
Figure B-5 Example escape berm plan layout
• Location 2 (Figure B-5). The ocean facing side of the berm is essentially
vertical to prevent tsunami flood waters and potential floating debris
from moving upslope into the area of refuge. Trees and other
landscaping can be used to hide the vertical face and create an
aesthetically appealing feature.
• Location 3 (Figure B-5). The sides of the berm can be sloped to provide
additional access to the area of vertical refuge. Care should be taken to
orientate the slope so that water and debris are not inadvertently
channeled upslope.
• Locations 4 and 5 (Figure B-5). Stairs and ramps provide primary
access for both recreational and emergency purposes.
Additional considerations are illustrated in Figures B-6 and B-7 and
described below.
Figure B-6 Example escape berm section
112 B: Community Design Example FEMA P646
Figure B-7 Example escape berm rear elevation
• Location 1 (Figure B-6). Where the elevated area is adjacent to a steep
drop off, guard rails or walls of appropriate size and height should be
provided for fall protection. Using a solid wall for the guardrail will
have the added benefit of providing additional protection from tsunami
runup or splash onto the area of refuge. Walls can be configured to
divert splash away from the wall.
• Location 2(Figure B-6). Materials used to help create the berm will
need to be constructed deep enough below existing grade to ensure that
retaining system is not undermined by scour around the perimeter of the
berm.
• Location 3(Figure B-7). With sufficient length, both ADA compliant
ramps and stairs can be provided. This will address both the day to day
recreational use of the facility as well as emergency evacuation needs.
Sloped surfaces on the sides of the berm can be used to provide
additional access, and can also help channel floating debris away from
the base of the ramps and stairs to minimize the risk of blockage.
B.2 Site 2 Example: Multi-Use Structure
Site 2 is situated on property managed by the school district. The site is
located adjacent to an existing school and the surrounding area contains a
combination of residential and business use. The existing school is located
well within the inundation zone. The waterfront in this area includes an ongrade
parking lot that services businesses in the area, and a nearby oceanfront
park. The evacuation population at this site would include children attending
the school, neighbors in the adjacent residences, employees of nearby
businesses, and nearby users of the oceanfront park.
The school district has had an ongoing need for a covered gymnasium. At
this site, the community has decided to incorporate the roof of the proposed
gymnasium into its emergency planning. It is decided that this new structure
will be designed to meet the requirements for a vertical evacuation structure
FEMA P646 B: Community Design Example 113
to serve two important community needs. The structure is illustrated in
Figure B-8.
Located adjacent to an on-grade parking lot, the structure will need to be
designed for potential impacts from floating vehicles. If the community is
located in a climate that requires the gymnasium to be enclosed, special
attention should be paid to the design of the exterior wall system. Walls
should be detailed as breakaway walls to minimize tsunami loading on the
overall structure. Otherwise the structure will need to be designed to for the
corresponding increased hydrostatic, hydrodynamic, and impulse loads.
As a school facility, the building must also be designed to address typical
health and safety requirements for school facilities in normal use (when not
serving as a vertical evacuation refuge).
Figure B-8 Example gymnasium
Features of this multi use structure, illustrated in Figure B-9 and Figure
B-10, include the following:
• Location 1 (Figure B-9). The rectangular layout is selected based on the
gymnasium requirements for the school. The elevated area is over
10,000 square feet in size, and can handle over 1,000 evacuees at 10
square feet per person. Using available census information, it has been
determined that this should be sufficient for the surrounding area this
facility is intended to serve.
114 B: Community Design Example FEMA P646
• Location 2 (Figure B-9). Stair access is designed using a concrete
encased stair structure that will have its own inherent strength. The
shape is intended to channel tsunami flow and potential debris away
from both the structure and the stair system.
• Location 3 (Figure B-9). An additional ADA accessible ramp system is
considered for a future phase of the project. This could utilize sheet piles
and fill to further channel tsunami flow and waterborne debris away from
the structure.
Figure B-9 Example gymnasium plan
Figure B-10 Example gymnasium elevation
FEMA P646 B: Community Design Example 115
• Location 4 (Figure B-10). The structural system utilizes a concrete
moment frame to create an open lower level that will keep hydrodynamic
loads on the structure to a minimum. This includes using circular shaped
columns.
• Location 5 (Figure B-10). Additional strength can be provided in the
system by using walls that parallel the anticipated direction of the
tsunami inundation flow.
• Location 6 (Figure B-9). The stairs structures can be integrated with the
primary structure to provide additional strength, or they can be made
structurally independent.
FEMA P646 C: Example Calculations 117
Appendix C
Example Calculations
A rectangular-shaped tsunami evacuation structure, 10 m wide, is constructed
at a site 200 m from the shoreline, where the elevation is 4 m from the sea
level. The local beach slope is 1/50 and there is no significant alongshore
variation in the topography; hence, it is reasonable to assume a plane beach
with a 1/50 slope. The tsunami inundation map indicates the elevation R* =
10 m at the maximum inundation point (runup height of 10 m at the location
500 m from the shoreline). A log (8.53 m long, 0.35 m in diameter, and 450
kg mass) is considered as the design waterborne missile for the impact
loading. In addition, the impact loading of a 40-ft shipping container (40 ft L
x 8 ft W x 8-1/2 ft H: or 12.2 m x 2.44 m x 2.59 m) is estimated to be 30,000
kg (30 tons). A definition sketch for these example calculations is provided
in Figure C-1.
R* = 10 m
z = 4 m
datum
Figure C-1 Definition sketch for example calculations: R* is the maximum
runup elevation (the maximum inundation distance is 500 m) and
z is the elevation at the location of the tsunami evacuation
structure (located 200 m from the shoreline). Two horizontal lines
represent the initial water level and the maximum inundation
level, respectively.
C.1 Inundation Depth
The recommended design runup height, R, is 30% greater than the predicted
maximum runup elevation, R*, to account for local amplification and
uncertainty in the predicted value, i.e., R = 1.3 R* = 13 m. Therefore, the
design inundation depth at the structure is 13 – 4 = 9 m. A freeboard of 3 m
(10 ft) is recommended; therefore, the refuge area must be located higher
than 9 + 3 = 12 m above the ground level. If the typical floor height is 4 m,
then the refuge area should be located on the 4th floor or higher.
118 C: Example Calculations FEMA P646
C.2 Hydrostatic and Buoyant Forces
It is recommended that all nonstructural walls at the lower levels of the
building be designed as breakaway walls. In that case, the hydrostatic forces
and potential uplift of the overall building are not important. However, if the
structure, or any portion of the structure, is constructed watertight at the
lower levels, then the wall panels must be designed for the anticipated
hydrostatic pressure. The maximum force acting on a wall panel of 4-m wide
and 3-m tall on the ground floor can be computed using Equation 6-2. Since
the wall panel on the ground floor is fully submerged:
Fh = .s
g R- (z + .z) - hW
2
.
. .
.
. .
hW b
= (1200 kg m3)(9.81m sec2 ) 1.3 × 10m - (4m + 0.5m) - 3m
2
.
. .
.
. .
(3m)(4m)
= 989 kN
where .z is the height at the toe of the wall panel from the ground level,
assumed to be 0.5 m. Note that the fluid density . = 1.2 .water is used
assuming a mixture of seawater and sediment.
With the water level at 9 m at the building location, the first and second
floors will be submerged. Assuming the nonstructural walls have broken
away at these two levels, but not yet at the third level, then the uplift due to
buoyancy acting on the third floor should be evaluated. Assuming plan
dimensions of 5 m by 5 m for a typical floor panel on the third floor, and a
floor elevation of 7 m above the ground level, as shown in Figure C-2, then
the upward buoyant force can be computed using Equation 6-4:
kN
kg m m m m m m m
F gA hb s f b
589
(1200 / 3 )(9.81 / sec2 )(5 5 )((1.3 10 4 ) 7 )
=
= × × - -
= .
where hb is the water height displaced by the floor including the effect of air
trapped below the floor, as shown in Figure C-2.
C.3 Hydrodynamic and Impulsive Forces
Hydrodynamic drag and impulse forces are exerted on the building as a
whole, assuming no breakaway walls at the lower levels. The maximum
FEMA P646 C: Example Calculations 119
7m
9m
2m
5m
Figure C-2 Condition resulting in buoyant forces
value of h u2 at the site can be computed using Equation 6-6, with z = 4 m, R
= 13 m and g = 9.81 m/sec2:
(hu2 )max
= gR2 0.125 - 0.235
z
R
+ 0.11
z
R
.
. .
.
. .
. 2
. .
.
. .
= 105m3 sec2
Hence, from Equation 6-5 the fluid force is:
kN
kg m m m
F C hu d s d
1260
2 (1200 / )(2.0)(10 )(105 / sec )
1
2 ( )
1
3 3 2
max
2
=
=
= .
where B = 10 m (shelter width), and Cd = 2.0. If the worst-case tsunami
arrives at a previously flooded site, then the tsunami front may form a bore.
The impulsive force for this condition would be 1.5 times the hydrodynamic
force, as in Equation 6-7:
Fs = 1.5Fd = 1890 kN
If the nonstructural walls at the lower level are designed to break away
during a tsunami, then the hydrodynamic drag and impulse forces would be
computed for all individual structural members (e.g., columns, shear walls)
and combined as shown in Figure 6-10.
120 C: Example Calculations FEMA P646
C.4 Impact Force
The maximum flow velocity at the site can be estimated using R = 13 m in
Equation 6-9:
umax = 2gR 1 - z
R
.
. .
.
. .
= 2 g(13m) 1- 4m
13m
.
. .
.
. .
= 13.3m sec.
Note that this flow velocity is at the leading tongue of the flow where the
flow depth is nil. Hence, this value of approximately 48 km/hr (30 mph) will
be conservative. Using this conservative velocity estimate, the impact force
due to a floating log can be computed by Equation 6-8, with Cm = 2.0, k = 2.4
x 106 N /m, and m = 450 kg:
Fi = Cm umax km
= 2.0(13.3m sec) (2.4 ×106 N m)(450 kg)
= 874 kN
This force would be applied locally at the assumed point of impact.
If the assumed draft, d, of the log is 0.25m, then the velocity is evaluated
using Figure 6-7. Using the ratios . = z/R = 0.31, and the flow depth, d/R =
0.019, at the location of the site:
umax
2gR
= 0.53
umax = 0.53 2(9.81)(13) = 8.5m sec
The impact force is then:
Fi = Cm umax km
= 2.0(8.5m sec) (2.4 ×106 N m)(450 kg)
= 560 kN
which is more realistic than the previous estimate (874 kN). The total force
on the structure at the time of the impact can be determined conservatively
by combining this impact force with the hydrodynamic drag force
determined earlier:
FEMA P646 C: Example Calculations 121
Fi + Fd = 560 + 1260 = 1820 kN
To compute the impact force due to a floating shipping container, the draft,
d, must be estimated:
d = W
. gAbox
=
(30000 kg)g
(1200 kg m3)g (12.2m × 2.44m) = 0.84m
where W is the weight and Abax is the cross sectional area of the box in the
horizontal plane, and the constant g cancels out. The maximum flow velocity
that supports draft, d = 0.84 m, can be found from Figure 6-7. At the location
of the site, . = z/R = 0.31, and the flow depth, d/R = 0.065. Figure 6-7 shows
umax along the limit curve at
. = 0.31. Hence, the maximum velocity is:
umax = 0.15 2gR = 2.4m sec.
The impact force due to the shipping container is computed by Equation 6-8
with Cm = 2.0, k = 2.4 x 106 N /m, and m = 30000 kg:
Fi = Cm umax km
= 2.0(2.4m sec) (2.4 ×106 N m)(30000 kg)
= 1290 kN
The total force on the structure at the time of the impact can be determined
conservatively by combining this impact force with the hydrodynamic drag
force determined earlier:
Fi + Fd = 1290 + 1260 = 2550 kN
C.5 Damming Effect of Waterborne Debris
The damming effect of debris can be computed using Equation 6-11, which
is readily obtained from the hydrodynamic force computed earlier,
substituting the recommended debris dam width of 12 m (40 ft):
Fdm = (1260 kN)× 12m
10m
.
. .
.
. .
= 1510 kN
122 C: Example Calculations FEMA P646
If the building were wider than 12 m, then the damming effect should be
considered at various locations as shown in Figure 6-11 to determine the
worst condition for loading on the structure as a whole, and on individual
structural elements.
C.6 Hydrodynamic Uplift Forces
The hydrodynamic uplift force can be computed using Equation 6-14.
Assuming that the water depth at the soffit of the second floor is hs = 3 m,
and at the location of the shelter site, . = z/R = 0.31, and the flow depth, d/R
= hs/R = 0.23, Figure 6-7 shows u along the limit curve at . = 0.31. Hence,
the maximum velocity is:
u = 0.15 2gR = 2.4m sec.
The vertical velocity can be computed using Equation 6-16, assuming the
slope at the site is 1/50:
uv = u tana = (2.4)(1 50)= 0.048m sec
Hence, the hydrodynamic uplift force given by Equation (6-14) is:
Fu = 1
2
Cu .s
Af uv
2
= 1
2
(3)(1200 kg m3)(5m × 5m)(0.048m sec)2
= 103N
which is insignificant for the beach slope assumed in this example. If a
beach slope of 1/5 is assumed, the hydrodynamic uplift force increases to
10.3 kN.
FEMA P646 D: Background Information on Impact Load Calculations 123
Appendix D
Background Information on
Impact Load Calculations
D.1 Available Models for Impact Loads
The impact force from waterborne missiles (e.g., floating driftwood, lumber,
boats, shipping box containers, automobiles, buildings) can be a dominant
cause of building destruction. Unfortunately, it is difficult to estimate this
force accurately. Unlike the other forces, the impact force occurs locally at
the point of contact when the debris is smaller than the building. Impact
forces can be assumed to act at or near the water surface level when the
debris strikes the building. Most available models are based on the impulsemomentum
concept, in which the impulse of the resultant force acting for an
infinitesimal time is equal to the change in linear momentum:
( ) 0
I F dt d mu ; 0
t = . = t . (D-1)
where:
I = impulse
F = resultant force
m = mass of water-borne missile
u = velocity of the missile
t = time
For actual computations, a small but finite time, .t (not infinitesimal), and
the average change in momentum are used as an approximation. There is
significant uncertainty in evaluating the duration of impact, .t. The
following are available formulae for missile-impact force estimation.
Matsutomi (1999). Matsutomi experimentally investigated the impulse
forces of driftwood. He performed two sets of experiments: one in a small
water tank and the other for full-scale impact in air. In his small water tank, a
bore and a surge were generated (a bore is a moving hydraulic jump onto a
quiescent shallower water in front of it, while a surge is a moving water body
onto a dry bed). A scaled-down driftwood model was placed 2.5 m upstream
from the receiving wall. The model driftwood was picked up by the
124 D: Background Information on Impact Load Calculations FEMA P646
generated bore (or surge) and impacted onto the receiving vertical wall. His
full-scale impact experiments were conducted to compensate for potential
scale effects in his small-scale experiments. A full-scale log was tied at the
end of a pendulum and was swung against the stationary stop equipped with
the load cell. It is noted that this impact condition in the air may significantly
differ from an actual waterborne case because of the absence of the added
mass effect of water: prior to the impact, the waterborne missile is carried by
the surrounding water flow and the momentum of the water must contribute
to the impact force. Matsutomi compensated for the added mass effect with
the data obtained from the small-scale water tank experiments. Based on a
regression analysis of the large amount of data, Matsutomi proposed
Equation D-2 for the impact force, F:
1.2 0.4
2 1.6 f
M
w w
F u
C
D L gD L
s
. .
. . . .
= .. .. .. .. . .
(D-2)
where:
.w = the specific weight of the log,
D and L = the diameter and the length of the log,
respectively,
CM = the added-mass coefficient,
u = the velocity of the log at impact, and
sf
= the yield stress of the wood.
Matsutomi recommended sf
= 20 × 106 Pa for a wet log. From small-scale
experimental data, he recommended a value of CM ˜ 1.7 for a bore or surging
condition, and CM ˜ 1.9 for a steady flow. Note that the recommended values
of CM are the upper limit when more than 60% of the receiving wall is open
and permeable. The value of CM is smaller when the receiving wall does not
allow the flow to pass through the receiving wall. For a solid (impermeable)
receiving wall, Matsutomi found that CM = 0.5 for a bore and CM = 1.1 for a
surging flow. Note that in the case of a bore striking an impermeable wall
(i.e., no flow-through), CM is less than unity (= 0.5). This is because the flow
reflection at the wall actually reduces the impact force.
In spite of a thorough study with a large amount of laboratory data, the
derived form of Equation D-2 is inconvenient due to the particular choice of
the scaling parameters, and it is only applicable to driftwood or logs.
Ikeno et al. (2001; 2003). Laboratory experiments similar to Matsutomi
(1999) were performed to examine the impact forces of the objects other than
driftwood or logs. They used cylindrical, square column, and spherically-
FEMA P646 D: Background Information on Impact Load Calculations 125
shaped drift bodies. Note that unlike Matsutomi’s experiments, Ikeno et al.
only examined the impact onto an impermeable vertical wall. The following
empirical formula was derived based on small-scale experiments
(approximately 1/100 model):
2.5
M
F u
SC
gm g DL
. .
= . .
. .
. .
(D-3)
where:
S = a constant (equal to 20 for a bore case),
CM = the added mass coefficient,
m = the mass of the drift body.
CM = 0.5 regardless the shape of the objects for a bore impact onto an
impermeable wall, which was adopted from Matsutomi’s results. For a drybed
surge, Ikeno and Tanaka (2003) suggested S = 5 and CM = 0.8 for
spherical-shaped objects and CM = 1.5 ~ 2.0 for cylinders and square-shaped
columns. The results by Ikeno et al. are valid only for the condition of an
impermeable wall (i.e., the entire incident flow reflects back to the offshore
direction). This is why the added mass coefficient has a value less than unity.
Haehnel and Daly (2002). At the U.S. Army Cold Regions Research and
Engineering Laboratory (CRREL), Haehnel and Daly performed experiments
similar to Matsutomi (1999). They considered reduced-scale logs in steady
flow in a small flume, and prototype logs in a large towing basin. It must be
noted that, just as potential errors were introduced in Matsutomi’s full-scale
pendulum impact experiments conducted in the air, the condition in the
towing basin also differs from the actual impact condition of a waterborne
missile. In the towing basin the water is stationary while in the actual
condition moving water carries the missile. Instead of the impulsemomentum
approach, Haehnel and Daly analyzed the data based on the
linear dynamic model with one degree of freedom. Since the collision occurs
over a short duration, damping effects are neglected. Assuming a rigid
structure, the model can be formulated by Equation D-4:
m..x.. + k x = 0 (D-4)
where:
m = the mass of the log,
x = the summation of the compression of the building and
the log during impact and rebound
the dot denotes the time derivative, and
126 D: Background Information on Impact Load Calculations FEMA P646
k = the effective constant stiffness associated with both the
log and the building.
Solving Equation D-4 yields the maximum force by Equation D-5:
Fmax = Max. k x = u k m (D-5)
where:
u = the impact velocity.
Based on their laboratory experiments, the effective constant stiffness k
between a log and a rigid building was estimated to be 2.4 × 106 N/m.
Haehnel and Daly demonstrated that the impulse-momentum approach could
be reduced to the constant-stiffness approach shown in Equation D-5 by
setting . t =
p
2
m
k
(note that, to be consistent to Equation D-4, the force is
considered a sinusoidal function in time). The work-energy approach can
also be made equivalent to Equation D-5 by setting the stopping distance as
S = u
m
k
. The work-energy approach is an impact force estimation that
equates the work done on the building with available kinetic energy of the
floating missile. Based on their laboratory data, the following formulae were
suggested by Haehnel and Daly:
Constant-stiffness approach:
. 1550 max F = Max k x = u k m ˜ u m (D-6)
Impulse-momentum approach:
90.9
2 max
um
F um
t
p
= ˜
. (D-7)
Work-energy approach:
2
125 2 8000 max
u m
F mu
x
= ˜ +
. (D-8)
Note that in Equations D-6, D-7, and D-8, the velocity, u, is in m/sec and the
mass, m, is in kg. It is emphasized that errors associated with the use of a
towing tank (instead of the realistic condition of a log being carried with
flow) may be significant in the results by Haehnel and Daly (2002).
SEI/ASCE Standard 7-02 (ASCE, 2003a). ASCE gives the following
design formula based on Equation D-1:
FEMA P646 D: Background Information on Impact Load Calculations 127
2
I O D B max muC C C C R
F
t
p
=
. (D-9)
where:
m = the water-borne-missile mass,
u = the impact velocity of the missile,
CI = the importance coefficient,
CO = the orientation coefficient,
CD = the depth coefficient,
CB = the blockage coefficient,
Rmax = the maximum response ratio for impulsive load, and
.t = the impact duration.
All of the C coefficients are based on non-peer-reviewed results of laboratory
testing and on engineering judgment. Rmax is a coefficient to compensate for
the effect of the degree of compliance of the building. A single value of the
impact duration, .t = 0.03 sec, is recommended, although there is wide
variation in the impact duration owing to, for example, the object material,
the flow blockage condition, and the compliance of the building. It is worth
noting that the City and County of Honolulu Building Code (CCH, 2000)
recommends .t values for wood construction as 1.0 sec, steel construction as
0.5 sec, and reinforced concrete as 0.1 sec. Furthermore, the FEMA 55
Coastal Construction Manual (FEMA, 2005) provides the .t values shown
in Figure D-1. Such an excessive variation in .t makes Equation D-9
unreliable.
Figure D-1 Ranges of duration of impact (FEMA, 2005).
D.2 Summary and Discussion
Review of previous work clearly demonstrates the immaturity and
uncertainty of the present understanding of missile-impact forces. The form
of Equation D-9 exhibits a struggle to obtain an engineering estimate of the
forces by adjusting five coefficients based on engineering judgment, together
128 D: Background Information on Impact Load Calculations FEMA P646
with the unreliable estimate for .t. All of the prediction formulae are based
on small-scale laboratory data by compensating with the full-scale
measurements in the compromised conditions. For example, Matsutomi’s
full-scale data were obtained by the impact study in air, and Haehnel and
Daly’s data were obtained in a towing tank. Since the added mass effect
appears important at the impact (the impact halts not only the waterborne
missile itself but also the water flowing around it), the results derived from
the compromised experimental conditions may contain significant errors. For
this reason information available from the auto industry related to automobile
crash tests were not considered in this review.
Even if the impact velocity, u, and the missile mass, m, were given, each
formula yields a different functional relation to predict the forces, which
indicates complexity and uncertainty inherent in the problem:
Constant-stiffness approach . F .u m,
Impulse-momentum approach . F .um,
Work-energy approach . F . u2 m, (D-10)
Ikeno and Tanaka (2003) . F . u2.5mn , n ˜ 0.58, and
Matsutomi (1999) . F . u1.2mn , n ˜ 0.66.
Although Equation D-2 by Matsutomi is based on his substantial analyses of
a large set of the laboratory data, the form of Equation D-2 is physically
ambiguous in terms of the choice of the scaling parameters, is limited only to
cylindrical shaped missiles, and is inconvenient for use in actual practice.
The empirical Equation D-3 by Ikeno et al. is based on their small-scale
laboratory experiments with an impermeable wall; hence, its extrapolation is
unreliable for real-world applications. Proper estimates of .t and .x are
formidable for the impulse-momentum and work-energy approaches,
respectively. The value of the effective constant stiffness, k, is difficult to
evaluate for Haehnel and Daly’s Equation D-5. In reality, k is not constant; it
is likely a function of x during the impact. Hence, the linearized equation D-4
may be inadequate.
Until more comprehensive studies can be made, the constant stiffness
approach of Equation D-5, suggested by Haehnel and Daly, is recommended
because of its simple but rational formulation. In addition, as shown in the
foregoing comparisons in Equations D-10, the functional relation of m and u
to the force F is similar to Matsutomi’s empirical Equation D-2, which was
derived based on a very large amount of experimental data. Considering that
Matsutomi’s empirical treatment was based on the impulse-momentum
approach, the coincidental similarity with the constant-stiffness approach
FEMA P646 D: Background Information on Impact Load Calculations 129
provides additional confidence in the formulation. Since the added-mass
effect must be included, it is recommended that Equation D-5 be modified as
shown in Equation D-11:
Fmax = CMu km (D-11)
with CM ˜ 2 for conservatism (note that Matsutomi (1999) found that CM ˜
1.7 ~ 1.9 and Ikeno et al. (2001, 2003) used CM ˜ 1.5 ~ 2.0) and k must be
determined based on the model missile (as mentioned earlier, k = 2.4 × 106
N/m was recommended for a log by Haehnel and Daly). Note that a proper
estimate of k is the key for this method. An added advantage for the use of
Equation D-11 is that k is not as sensitive as .t and .x in the impulsemomentum
and work-energy approaches, which can be shown from the fact
that .t and .x are proportional to 1
k , as discussed earlier.
FEMA P646 E: Maximum Flow Velocity and Momentum Flux in the 131
Tsunami Runup Zone
Appendix E
Maximum Flow Velocity and
Momentum Flux in the
Tsunami Runup Zone
E.1 Flow Velocity
For prediction of flow velocities and depths at a site of interest for a given
design tsunami, the best practice available is to run a detailed numerical
simulation model with a very fine grid size (less than 10 meters) in the
tsunami runup zone. Such a numerical model is usually run with a nested
grid system with a grid size of several kilometers in the abyssal plain, a few
hundreds of meters on the continental shelf, a few tens of meters near the
shore, and less than 10 meters in the runup zone. A numerical simulation can
provide the complete time history of flow velocity and depth at the site of
interest.
Alternatively, the use of analytical solutions can be considered. Although
some simplifications and assumptions must be imposed, the results are useful
as a guideline for checking the reasonableness of results, or as estimate of
approximate values in the absence of other information. Available analytical
solutions are based on one-dimensional, fully nonlinear shallow-water-wave
theory for the condition with a uniformly sloping beach. With those
assumptions, the exact solution for the runup of an incident bore was given
by Shen and Meyer (1963), based on Ho and Meyer (1962). The maximum
fluid velocity occurs at the leading runup tip as calculated by Equation E-1:
u = 2 g x tana , (E-1)
where:
a = the beach slope,
g = the gravitational acceleration, and
x = the distance from the maximum runup location to the location of
interest; the location of interest must be above the initial
shoreline.
Results indicate that the flow close to the leading runup tip moves up the
beach under gravity, just like a particle with simple energy transfer between
132 E: Maximum Flow Velocity and Momentum Flux in the FEMA P646
Tsunami Runup Zone
its kinetic and potential energies. According to Yeh (2006), Equation E-1
provides the upper-limit envelope of the flow velocity for all incident
tsunami forms. Because a real beach is not uniformly sloped, it is more
convenient to present Equation E-1 as a function of the ground elevation,
instead of distance as follows:
max 2 1
z
u gRR
= . - . . .
. . (E-2)
where:
R = the ground elevation at the maximum penetration of tsunami
runup, measured from the initial shoreline, and
z = the ground elevation of the location of interest, measured from
the initial shoreline level.
It is emphasized that the model does not include the effects of friction and
the maximum flow velocity occurs at the leading runup tip, where the flow
depth is zero. Since debris requires some finite flow depth in order to float
(draft), use of Equations E-1 and E-2 to estimate velocity for impact load
calculations is somewhat overconservative.
Based on Shen and Meyer’s (1963) results, Peregrine and Williams (2001)
provided the formulae for the temporal and spatial variations in fluid velocity
and flow depth of the incident bore runup. With slightly different scaling,
Yeh (2007) expressed Peregrine and Williams’ formulae for the flow depth
and velocity, respectively as follows:
. = 1
36t 2 (2 2t -t 2 - 2. )2
(E-3)
and
( 2 ) 1
2 2
3
. t t .
t
= - + (E-4)
where, in the above equations:
d
R
. = ; 2
u
gR
. = ; tan
g
t
R
t = a ;
z
R
. =
d = the water depth,
R = the ground elevation at the maximum penetration of tsunami
runup, measured from the initial shoreline,
u = the flow velocity,
g = the gravitational acceleration,
a = the beach slope,
FEMA P646 E: Maximum Flow Velocity and Momentum Flux in the 133
Tsunami Runup Zone
t = the time: 0 when the bore passes at the initial shoreline, and
z = the ground elevation of the location of interest, measured from
the initial shoreline: this identifies the location of interest along a
uniformly sloping beach.
For a given maximum runup penetration, an incident bore should yield the
maximum flow velocity. Gradual flooding of non-breaking tsunamis should
result in slower flow velocity than that caused by the bore runup. Therefore,
Equations E-3 and E-4 can be used to estimate the maximum flow velocity at
a given location for a given flow depth. Combining Equations E-3 and E-4
and eliminating t, Figure E-1 can be derived. Each curve in the figure
represents the dimensionless flow velocity . versus the location . (in terms
of ground elevation, z) for a given local flow depth, d. This figure can be
used to evaluate the maximum flow velocity that can carry floating debris
with finite draft depth, since draft of the debris must be greater than the flow
depth to make the debris float.
Figure E-1 Maximum flow velocity of depth, d, at the ground elevation, z,
and maximum runup elevation, R. The bottom curve represents
the lower limit of maximum flow velocity.
The bottom curve in Figure E-1 is the lower limit of the maximum flow
velocity for a given depth, d. Note that the results in Figure E-1 are based on
the runup condition of uniform incident bore. Local inundation depth of
other tsunami forms usually exceeds that of a bore runup, and the maximum
flow velocity is lower than the limit curve in Figure E-1. Hence when a
floating-debris has a draft that exceeds the flow depth of the bore runup, the
134 E: Maximum Flow Velocity and Momentum Flux in the FEMA P646
Tsunami Runup Zone
design velocity umax can be estimated conservatively with the lower limit
curve.
E.2 Momentum Flux
Using the exact solution algorithm, Yeh (2006) developed an envelope curve
of the maximum momentum flux per unit water mass per unit width, hu2,
expressed in Equation E-5:
( ) ( ) 2 2
2 2 0.11 0.015
hu x x
ga
= .. + .. .. (E-5)
where:
hu2 = the momentum flux per unit mass per unit width,
a = the beach slope,
g = the gravitational acceleration,
x = the distance from the maximum runup location to the location of
interest (the location of interest must be above the initial
shoreline), and
.. = the maximum runup distance.
Once the maximum runup distance, .., is determined (e.g., from an available
inundation map), the momentum flux, . hu2 per unit breadth at a given
location x, can be computed by Equation E-5. It is emphasized that Equation
E-5 is for a uniform beach slope; therefore, some adjustments need to be
made to evaluate realistic conditions. Because a real beach is not uniformly
sloped, it is more convenient to express Equation E-5 as a function of ground
elevation instead of distance, as follows:
2 2
2 0.125 0.235 0.11
hu z z
gR R R
= - + .. ..
. . (E-6)
where:
hu2 = the momentum flux per unit mass per unit width,
g = the gravitational acceleration,
R = the ground elevation at the maximum penetration of tsunami
runup, measured from the initial shoreline, and
z = the ground elevation of the location of interest, measured from
the initial shoreline: this identifies the location of interest along a
uniformly sloping beach.
Although a real beach is not uniformly sloped and tsunami runup is not a
one-dimensional motion, Figure E-1 and Equations E-2 and E-6 provide an
analytical basis for runup conditions.
FEMA P646 Glossary 135
Glossary
The following definitions are provided to explain the terms and acronyms
used throughout this document. Many have been taken directly from the
FEMA 55, Coastal Construction Manual (FEMA, 2005).
A
ADA – Americans with Disabilities Act. Law requiring that design
accommodations be made for persons with certain disabilities.
A-Zone – Under the National Flood Insurance Program, the area subject to
inundation by a 100-year flood where waves are less than 3 feet high
[designated Zone A, AE, A1-A30, A99, AR, AO, or AH on a Flood
Insurance Rate Map (FIRM)].
Armor – Material used to protect slopes from erosion and scour by
floodwaters, such as riprap, gabions, or concrete.
ASCE – American Society of Civil Engineers.
ATC – Applied Technology Council.
B
Base flood – Flood that has a 1% probability of being equaled or exceeded in
any given year, also known as the 100-year flood.
Base Flood Elevation (BFE) – Elevation of the base flood in relation to a
specified datum, such as the National Geodetic Vertical Datum or the North
American Vertical Datum. The Base Flood Elevation is the basis of the
insurance and floodplain management requirements of the National Flood
Insurance Program.
Bathymetry – Underwater configuration of a bottom surface of an ocean,
estuary, or lake.
Berm – A mound of soil or other earthen material.
Bore – A long, broken wave propagating into a quiescent body of water, with
an abrupt increase in water depth at its front face covered with turbulent,
tumbling water.
136 Glossary FEMA P646
Breakaway wall – Under the National Flood Insurance Program, a wall that
is not part of the structural support of the building and is intended, through its
design and construction, to collapse under specific lateral loading forces
without causing damage to the elevated portion of the building or supporting
foundation system. Breakaway walls are required by the National Flood
Insurance Program regulations for any enclosures constructed below the Base
Flood Elevation beneath elevated buildings in coastal high-hazard areas (also
referred to as V-Zones). In addition, breakaway walls are recommended in
areas where floodwaters flow at high velocities or contain ice or other debris.
Building codes – Regulations adopted by local governments that establish
standards for construction, modification, and repair of buildings and other
structures.
Building official – An officer or other designated authority charged with the
administration and enforcement of the code, or a duly authorized
representative such as a building, zoning, planning, or floodplain
management official.
Bulkhead – A wall or other structure, often of wood, steel, stone, or
concrete, designed to retain or prevent sliding or erosion, and occasionally
used to protect against wave action.
C
CAEE – Canadian Association for Earthquake Engineering.
Cast-in-place concrete – Concrete that is formed, placed, and cured in its
final location in the structure.
Cladding – Exterior surface of the building envelope.
Coastal A-Zone – The portion of the Special Flood Hazard Area landward
of a V-Zone or landward of an open coast without mapped V-Zone in which
the principal sources of flooding are astronomical tides, storm surge, seiches,
or tsunamis (not riverine sources). The flood forces 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. (Note: National Flood Insurance
Program regulations do not differentiate between coastal A-Zones and noncoastal
A-Zones.)
Coastal barrier – Depositional geologic features such as a bay barrier,
tombolo, barrier spit, or barrier island that consists of unconsolidated
FEMA P646 Glossary 137
sedimentary materials; is subject to wave, tidal, and wind energies; and
protects landward aquatic habitats from direct wave attack.
Coastal High-Hazard Area – Under the National Flood Insurance Program,
an area of special flood hazard extending from offshore to the inland limit of
a primary frontal dune along an open coast, and any other area subject to
high-velocity wave action from storms or seismic sources. On a Flood
Insurance Rate Map, the coastal high-hazard area is designated Zone V, VE,
or V1–V30. These zones designate areas subject to inundation by the base
flood where wave heights or wave runup depths are greater than or equal to 3
feet. In Hawaii, the VE-Zones are generally determined where the depth of
water from a 100-year event (as determined from tsunami and/or hurricane
data) is greater than 4 feet.
Collapsing breaker – A type of breaking wave associated with a steep beach
slope and flat incident wave, which occurs right at the instantaneous
shoreline.
D
Dead load – Weight of all materials of construction incorporated into the
building, including but not limited to walls, floors, roofs, ceilings, stairways,
built-in partitions, finishes, cladding, and other similarly incorporated
architectural and structural items and fixed service equipment. See Loads.
Debris – Solid objects or masses carried by or floating on the surface of
moving water.
Debris impact loads – Loads imposed on a structure by the impact of
waterborne debris.
Debris line – Markings on a structure or the ground caused by the deposition
of debris, indicating the height or inland extent of floodwaters.
Design Basis Earthquake (DBE) – The earthquake hazard level that
structures are specifically proportioned to resist, taken as two-thirds of the
Maximum Considered Earthquake (MCE) hazard level.
DoD – Department of Defense.
Draft – The depth of water that a body needs in order to float.
F
FEMA – Federal Emergency Management Agency.
FEMA MAT Report – FEMA Mitigation Assessment Team Report.
138 Glossary FEMA P646
Fill – Material such as soil, gravel, or crushed stone placed in an area to
increase ground elevations or change soil properties. See Structural Fill.
FIRM – Flood Insurance Rate Map.
Far-source-generated tsunami – Tsunami resulting from a source located
far from the site such that it arrives in excess of a 2-hour timeframe.
500-year flood – Flood that has a 0.2% probability of being equaled or
exceeded in any given year.
Flood elevation – Height of the water surface above an established elevation
datum such as the National Geodetic Vertical Datum, the North America
Vertical Datum, or mean sea level.
Flood Insurance Rate Map – Under the National Flood Insurance Program,
an official map of a community upon which the Federal Emergency
Management Agency has delineated both the special hazard areas and the
risk premium zones applicable to the community. (Note: The latest FIRM
issued for a community is referred to as the effective FIRM for that
community.)
Flood-hazard area – The greater of the following: (1) the area of special
flood hazard, as defined under the National Flood Insurance Program, or (2)
the area designated as a flood-hazard area on a community's legally adopted
flood-hazard map, or otherwise legally designated.
Footing – The enlarged base of a foundation wall, pier, post, or column
designed to spread the load of the structure so that it does not exceed the soil
bearing capacity.
G
GSA – General Services Administration.
Grade beam – Section of a concrete slab that is thicker than the slab and acts
as a footing to provide stability, often under load-bearing or critical structural
walls.
H
Hydrodynamic loads – Loads imposed on an object, such as a building, by
water flowing against and around it. Among these loads are positive frontal
pressure against the structure, drag effect along the sides, and negative
pressure on the downstream side.
FEMA P646 Glossary 139
Hydrostatic loads – Loads imposed on a surface, such as a wall or floor
slab, by a standing mass of water. The water pressure increases linearly with
the water depth; hence, the hydrostatic loads increase with the square of the
water depth.
I
Impact forces – Loads that result from waterborne debris transported by
tsunami waves striking against buildings and structures or parts thereof.
Impulsive forces – Force induced against a vertical obstruction subjected to
the leading edge of a tsunami during runup, also termed “surge” forces.
Ingress – The act of entering a building.
Inland zone – For the purposes of this report, the area that is inland of the Aand
X-Zones (the limit of the 500-year flood).
L
Liquefaction – A phenomenon that occurs in saturated soils when the net
pore pressure exceeds the gravity force holding soil particles together. Soil
strength and stiffness decrease dramatically as the soil behaves similar to a
fluid.
Loads – Forces or other actions that result from the weight of all building
materials, occupants and their possessions, environmental effects, differential
movement, and restrained dimensional changes.
M
Masonry – Built-up construction of combination of building units or
materials of clay, shale, concrete, glass, gypsum, stone, or other approved
units bonded together with or without mortar, grout, or other accepted
methods of joining.
Maximum Considered Earthquake (MCE) – The most severe earthquake
effects considered by seismic design codes and standards. The MCE is based
on the United States Geological Survey seismic hazard maps, which are
based on a combination of: (1) 2500-year probabilistic earthquake ground
motion hazards; and (2) deterministic ground motion hazards in regions of
high seismicity, with the appropriate ground motion attenuation relationships
defined for each region.
Maximum Considered Tsunami (MCT) – A design tsunami event based on
a probabilistic assessment considering all possible tsunami sources, or a
140 Glossary FEMA P646
deterministic assessment considering the maximum tsunami that can
reasonably be expected to affect a site.
Mid-source-generated tsunami – Tsunami generated by a source that is
near the site of interest, but not close enough so that the effects of the
triggering event is felt at the site.
Mitigation – Any action taken to reduce or permanently eliminate the longterm
risk to life and property from natural hazards.
N
National Geodetic Vertical Datum (NGVD) – Datum established in 1929
and used as a basis for measuring flood, ground, and structural elevations;
was previously referred to as Sea Level Datum or Mean Sea Level. The Base
Flood Elevations shown on most of the Flood Insurance Rate Maps issued by
the Federal Emergency Management Agency are referenced to NGVD or,
more recently, to the North American Vertical Datum.
Near-source-generated tsunami – Tsunami generated by a source located
near the site such that it arrives within a 30-minute timeframe, and the effects
of the triggering event are felt at the site.
National Flood Insurance Program (NFIP) – The federal program created
by Congress in 1968 that makes flood insurance available in communities
that enact and enforce satisfactory floodplain management regulations.
Nonstructural wall – A wall that does not support vertical loads other than
its own weight.
North American Vertical Datum (NAVD) – Datum used as a basis for
measuring flood, ground, and structural elevations. NAVD, rather than the
National Geodetic Vertical Datum, has been used in many recent flood
insurance studies.
P
Pier foundation – Foundation consisting of isolated masonry or cast-inplace
concrete structural elements extending into firm materials. Piers are
relatively wide in comparison to their length, and derive their load-carrying
capacity through skin friction, end bearing, or a combination of both.
Pile foundation – Foundation consisting of concrete, wood, or steel
structural elements driven or jetted into the ground, or cast in place. Piles are
relatively slender in comparison to their length, and derive their load-
FEMA P646 Glossary 141
carrying capacity through skin friction, end bearing, or a combination of
both.
Plain concrete – Structural concrete with no reinforcement or with less
reinforcement than the minimum amount specified for reinforced concrete.
Plunging Breaker – A type of breaking wave when the wave front curls
over, forming a tube; it usually happens on beaches where the slope is
moderately steep.
Post foundation – Foundation consisting of vertical support members,
usually made of wood, set in holes and backfilled with compacted material.
Precast concrete – Concrete, usually a discrete structural member, that is
formed, placed, and cured at one location, and subsequently moved and
assembled into a final location in a structure.
Probabilistic maps – Maps of predicted tsunami effects including for
inundation zone, flood depths, and flow velocities, based on a method
involving probability and uncertainty.
Progressive collapse – ASCE/SEI Standard 7-02 defines progressive
collapse as “the spread of an initial local failure from element to element
resulting eventually, in the collapse of an entire structure or a
disproportionately large part of it.”
R
Rapid drawdown – A sudden reduction in water level immediately prior to
the first tsunami wave, or between tsunami waves.
Reinforced concrete – Structural concrete reinforced with steel.
Retrofit – Any change made to an existing structure to reduce or eliminate
potential damage to that structure from flooding, erosion, high winds,
earthquakes, or other hazards.
S
Scour – Removal of soil or fill material by the flow of floodwaters,
frequently used to describe storm-induced, localized conical erosion around
pilings and other foundation supports where the obstruction of flow increases
turbulence.
Sea wall – Solid barricade built at the water’s edge to protect the shore and
to prevent inland flooding.
142 Glossary FEMA P646
SEI – Structural Engineering Institute of ASCE.
Shearwall – Load-bearing or non-load-bearing wall that transfers in-plane
forces from lateral loads acting on a structure to its foundation.
Special Flood Hazard Area (SFHA) – Under the National Flood Insurance
Program, an area having special flood, mudslide (i.e., mudflow), and/or
flood-related erosion hazards, and shown on a Flood Hazard Boundary Map
or Flood Insurance Rate Map as Zone A, AO, A1-A30, AE, A99, AH, V, V1-
V30, VE, M, or E.
Storm surge – Rise in the water surface above normal water level on an
open coast due to the action of wind stress and atmospheric pressure on the
water surface.
Stillwater elevation – Projected elevation that floodwaters would assume,
referenced to the National Geodetic Vertical Datum, the North American
Vertical Datum, or some other datum, in the absence of waves resulting from
wind or seismic effects.
Structural fill – Fill compacted to a specified density to provide structural
support or protection to a structure.
T
Topography – Configuration of a terrain, including its relief and the position
of its natural and man-made features.
Tsunami – A naturally occurring series of ocean waves resulting from a
rapid, large-scale disturbance in a body of water, caused by earthquakes,
landslides, volcanic eruptions, and meteorite impacts.
Tsunami inundation zone – The region flooded by tsunami penetration
inland.
Tsunami inundation elevation – The elevation, measured from sea level, at
the location of the maximum tsunami penetration
Tsunami runup – Rush of tsunami waves up a slope, terrain, or structure.
Tsunami runup height – The difference between the elevation of maximum
tsunami penetration and the elevation of the shoreline at the time of tsunami
attack.
Tsunami water level – The difference between the elevation of the highest
local water level and the elevation of the shoreline at the time of tsunami
attack.
FEMA P646 Glossary 143
U
Undermining – Process whereby erosion or scour exceeds the depth of the
base of a building foundation, or the level below which the bearing strength
of the foundation is compromised.
Uplift – Vertical hydrostatic pressure caused by the volume of displaced
water under a building.
V
V-Zone – See Coastal High-Hazard Area.
VE-Zone – Coastal High-Hazard Areas where the Base Flood Elevations
have been determined through a detailed study.
Vertical Evacuation Refuge from Tsunamis – A building or earthen
mound that has sufficient height to elevate evacuees above the tsunami
inundation depth, and is designed and constructed with the strength required
to resist the forces generated by tsunami waves.
W
Waterborne debris – Any object transported by tsunami waves (e.g.,
driftwood, small boats, shipping containers, automobiles).
Wave crest – The point of highest elevation in a wave profile.
Wave height – Vertical distance between the successive local maximum and
minimum elevations in a wave profile.
Wave zone – Area that coincides with V, VE, or V1–V30 Zones or Coastal
High-Hazard Areas.
FEMA P646 References 145
References
Abe, S., Sugaya, C., Tanaka et al., 2005, “Guideline for Tsunami Evacuation
Buildings,” Tsunami Evacuation Building Guideline Committee,
http://www.bousai.go.jp/oshirase/h17/tsunami_hinan.html (translated
from Japanese).
Abednego, L.G., 2005, “The Contribution of Indonesian Engineers
Association to Aceh Province After Earthquake and Tsunami,”
Proceedings of the Scientific Forum on Tsunami, Its Impact and
Recovery, AIT Conference Center, Bangkok, Thailand.
ACI, 2005, Building Code Requirements for Structural Concrete (ACI 318-
05) and Commentary (ACI 318R-05), American Concrete Institute,
Farmington Hills, Michigan.
ARC, 2002, Standards for Hurricane Evacuation Shelter Selection,
Publication No. 4496, http://www.tallytown.com/redcross/library/
StandardsForHurricaneEvacuationShelterSelection.pdf, American
Red Cross, Tallahassee Florida.
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Standing Coastal Structure, Ph.D. dissertation, University of
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FEMA P646 Project Participants 157
Project Participants
ATC Management and Oversight
Christopher Rojahn (Project Executive)
Applied Technology Council
201 Redwood Shores Parkway, Suite 240
Redwood City, CA 94065
Jon A Heintz (Project Manager)
Applied Technology Council
201 Redwood Shores Parkway, Suite 240
Redwood City, CA 94065
William T. Holmes (Project Tech. Monitor)
Rutherford & Chekene
55 Second Street, Suite 600
San Francisco, CA 94105
FEMA Project Officer
Michael Mahoney (Project Officer)
Federal Emergency Management Agency
500 C Street, SW, Room 416
Washington, DC 20472
FEMA Technical Monitor
Robert D. Hanson (Technical Consultant)
(Federal Emergency Management Agency)
2926 Saklan Indian Drive
Walnut Creek, CA 94595
Project Management Committee
Steven Baldridge (Project Technical Director)
BASE Research & Development, LLC
1164 Bishop Street, Suite 605
Honolulu, HI 96813
Frank Gonzalez
National Ocean & Atmospheric Administration
Pacific Marine Environmental Laboratory
7600 Sand Point Way NE, Building 3
Seattle, WA 98115-0070
John Hooper
Magnusson Klemencic Associates
1301 Fifth Avenue, Suite 3200
Seattle, WA 98101
Ian N. Robertson
University of Hawaii at Manoa
Dept. of Civil and Environmental Engineering
2540 Dole Street, Holmes Hall 383
Honolulu, HI 96822
Timothy J. Walsh
Dept. of Natural Resources, Geology & Earth
Resources
1111 Washington Street SE, P.O. Box 47007
Olympia, WA 98504-7007
Harry Yeh
Oregon State University
School of Civil & Construction Engineering
220 Owen Hall,
Corvallis, OR 97331-3212
158 Project Participants FEMA P646
Project Review Panel
Christopher P. Jones* (Chair)
5525 Jomali Drive
Durham, NC 27705
John Aho
CH2M Hill
301 West Northern Lights Blvd., Suite 601
Anchorage, AK 99503-2662
George Crawford
Washington State Military Dept.
Emergency Management Division
Camp Murray, WA 98430-5122
Richard Eisner
Governor's Office of Emergency Services
1300 Clay Street, Suite 400
Oakland, California 94612
Lesley Ewing
California Coastal Commission
45 Fremont Street, Suite 2000
San Francisco, CA 94105
Michael Hornick
DHS/FEMA, Region IX
1111 Broadway, Suite 1200
Oakland, CA 94607
Chris Jonientz-Trisler
Federal Emergency Management Agency Region X
130 228th Street SW
Bothell, WA 98021-9796
Marc L. Levitan
LSU Hurricane Center
Suite 3221 CEBA Building
Louisiana State University
Baton Rouge, LA 70803
George R. Priest
Oregon Dept. of Geology and Mineral Industries
Newport Coastal Field Office
P.O. Box 1033
Newport, OR 97365
Charles W. Roeder
University of Washington
Structural Eng.& Mechanics
233B More Hall, Box 352700
Seattle, WA 98195-2700
Jay Wilson
Clackamas County Department of Emergency
Management
2200 Kaen Road
Oregon City OR 97045
*ATC Board Representative