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*  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 FEMAs 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 higherand 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 Matsutomis 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 Matsutomis 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 Matsutomis 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, Matsutomis 
full-scale data were obtained by the impact study in air, and Haehnel and 
Dalys 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 Dalys 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 Matsutomis empirical Equation D-2, which was 
derived based on a very large amount of experimental data. Considering that 
Matsutomis 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 Meyers (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 V1V30. 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 waters 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 V1V30 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, 
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ASCE/SEI Standard 7-05, American Society of Civil Engineers, 
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ASCE, 2006c, Seismic Rehabilitation of Existing Buildings, ASCE/SEI 
Standard 41-06, American Society of Civil Engineers, Reston, 
Virginia. 
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Bourgeois, J., Petroff, C., Yeh, H., Titov, V., Synolakis, C.E., Benson, B., 
Kuroiwa, J., Lander, J., and Norabuena, E., 1999, Geologic Setting, 
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Pure and Applied Geophysics, Vol. 144, Nos.3/4, pp. 569-593. 
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Carrier, G.F. and Yeh, H. 2005, Tsunami Propagation from a Finite 
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Chowdhury, S., Geist, E., Gonzlez, F., MacArthur, R., and Synolakis, C., 
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Curtis, G., 2001, A Multi-Sensor Research Program to Improve Tsunami 
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tsunami.disaster/, Cable News Network. 
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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