Risk Management Series
Design Guide for Improving Hospital Safety in Earthquakes, Floods, and High 
Winds
FEMA 577 / June 2007

Appendix B - GLOSSARY OF TERMS

100-year flood. See “base flood.”

A
Astragal. The center member of a double door, which is attached to the fixed or 
inactive door panel.

B

Base flood. The flood having a 1 percent chance of being equaled or exceeded in 
any given year, commonly referred to as the “100-year flood.” The base flood is 
the national standard used by the NFIP and all Federal agencies for the purposes 
of requiring the purchase of flood insurance and regulating new development. 

Base flood elevation (BFE). The height of the base (1 percent or 100-year) flood 
in relation to a specified datum, usually the National Geodetic Vertical Datum 
of 1929, or the North American Vertical Datum of 1988. 

Base isolation. Also called seismic isolation.  A design concept that reduces 
the earthquake motions in the building superstructure by isolating the building 
from ground motions.

Basic wind speed. A 3-second gust speed at 33 feet above the ground in Exposure 
C. (Exposure C is flat open terrain with scattered obstructions having heights 
generally less than 30 feet.) Note: Since 1995, ASCE 7 has used a 3-second peak 
gust measuring time. A 3-second peak gust is the maximum instantaneous speed 
with a duration of approximately 3 seconds. A 3-second peak gust speed could be 
associated with a given windstorm (e.g., a particular storm could have a 40-mph 
peak gust speed), or a 3-second peak gust speed could be associated with a 
design-level event (e.g., the basic wind speed prescribed in ASCE 7).

Building configuration. Size, shape, and proportions of the building; size, 
shape, and location of structural elements; and the type, size, and location of 
nonstructural elements.

Building, enclosed. A building that does not comply with the requirements for 
open or partially enclosed buildings.

Building, open. A building having each wall at least 80 percent open. This 
condition is expressed by an equation in ASCE 7.

Building, partially enclosed. A building that complies with both of the 
following conditions:
1. The total area of openings in a wall that receives positive external pressure 
exceeds the sum of the areas of openings in the balance of the building envelope 
(walls and roof) by more than 10 percent.
2. The total area of openings in a wall that receives positive external pressure 
exceeds 4 square feet, or 1 percent of the area of that wall, whichever is 
smaller, and the percentage of openings in the balance of the building envelope 
does not exceed 20 percent.

These conditions are expressed by equations in ASCE 7.

Building, period. The rate at which a building will vibrate as a result of 
ground motion.

Building, regularly shaped. A building having no unusual geometrical 
irregularity in spatial form.

Building, simple diaphragm. An enclosed or partially enclosed building in which 
wind loads are transmitted through floor and roof diaphragms to the vertical 
main wind-force resisting system.

C

Coastal Flooding. The accumulation of water experienced along the Atlantic, 
Gulf, and Pacific coasts, and the Great Lakes due to storm surges, extratropical 
systems, tsunamis, and sometimes wind-driven waves. 

Components and cladding (C&C). Elements of the building envelope that do not 
qualify as part of the main wind-force resisting system.

Coping. The cover piece on top of a wall exposed to the weather, usually made of 
metal, masonry, or stone, and sloped to carry off water.

D

Damping. The rate at which natural vibration decays as a result of the 
absorption of energy. In buildings it is an inherent nature to resonate 
inefficiently to vibration depending on structural connections, kinds of 
materials, and nonstructural elements used. “Damping” design measures can reduce 
the magnitude of seismic forces. 

Design flood. The greater of the following two flood events:  (1) the base 
flood, affecting those areas identified as special flood hazard areas on a 
community’s Flood Insurance Rate Map (FIRM); or (2) the flood corresponding to 
the area designated as a flood hazard area on a community’s flood hazard map or 
otherwise legally designated.

Design flood elevation (DFE). The elevation of the design flood, including wave 
height, relative to the datum specified on a community’s flood hazard map.

Downburst. Also known as a microburst. A powerful downdraft associated with a 
thunderstorm. 

Down-slope wind. A wind blowing down the slope of mountains (frequently occurs 
in Alaska and Colorado).

Drift. The term used in seismic design to describe the horizontal deflection of 
structural members in response to seismic forces.

Dry floodproofing. An adjustment, modification, or addition of a feature, or any 
combination thereof, that eliminates or reduces the potential for flood damage 
by sealing walls and closing openings to keep water from entering a building. 

Ductility. The characteristic of certain materials—steel in particular—to fail 
only after considerable distortion or deformation has occurred.

E

Escarpment. Also known as a scarp. With respect to topographic effects, a cliff 
or steep slope generally separating two levels or gently sloping areas. 

Equivalent Lateral Force (ELF) Procedure. A procedure used in building design 
that allows adjustments of the design force for varying site seismicities, 
alternative soil types, different structural and nonstructural systems and 
materials, different building heights, and occupancies of varying importance. 

Epicenter. The epicenter is the place on the surface of the earth under which an 
earthquake rupture originates, often given in degrees of latitude and longitude.

Exposure. The characteristics of the ground roughness and surface irregularities 
in the vicinity of a building. ASCE 7 defines three exposure categories—
Exposures B, C, and D. 

Extratropical storm. A cyclonic storm that forms outside of the tropical zone. 
Extratropical storms may be large, often 1,500 miles (2,400 kilometers) in 
diameter, and usually contain a cold front that extends toward the equator for 
hundreds of miles.

F

Fault. A fault is a fracture along which the blocks of earth’s crust on either 
side have moved relative to one another parallel to the fracture. 

Federal Emergency Management Agency (FEMA). The Federal Emergency Management 
Agency is the Federal agency which administers the National Flood Insurance 
Program (NFIP).

Flashing. Any piece of material, usually metal or plastic, installed to prevent 
water from penetrating a structure.

Flood Insurance Rate Map (FIRM). The official map of a community on which FEMA 
has delineated both the special hazard areas, and the risk premium zones 
applicable to the community.

Flood Insurance Study (FIS). An engineering study performed by FEMA to identify 
flood hazard areas, flood insurance risk zones, and other flood data in a 
community; used in the development of the FIRM.

Floodplain. Any land area, including the watercourse, that is susceptible to 
partial or complete inundation by water, from any source.

Floodplain management regulations. Zoning ordinances, subdivision regulations, 
building codes, or special-purpose ordinances that set flood-resistant standards 
for new construction, land use, and development.

Flood profile. A graph of computed flood elevations at points located along a 
riverine waterway. A flood profile typically is available for a waterway that 
has Base Flood Elevations (BFEs) shown on the Flood Insurance Rate Map (FIRM). 
Flood profiles are usually found in the Flood Insurance Study (FIS) report.

Floodway. The channel and that portion of the floodplain that is to be reserved 
to convey the base flood, without cumulatively increasing the water surface 
elevation more than a designated height.

Floodway fringe. The area of the floodplain outside of the floodway, where 
floodwaters may be shallower and slower. 

Freeboard. A factor of safety, usually expressed in feet above a flood level, 
for purposes of flood­plain management. Freeboard also compensates for the many 
unknown factors that could contribute to flood heights greater than the height 
calculated for a selected size flood and floodway conditions, such as wave 
action, constricting bridge openings, and the hydrological effect of 
urbanization of the watershed. A freeboard of from 1 to 3 feet is often applied 
to critical facilities.

G

Glazing. Glass or a transparent or translucent plastic sheet used in windows, 
doors, and skylights.

Glazing, impact-resistant. Glazing that has been shown, by an approved test 
method, to withstand the impact of wind-borne missiles likely to be generated in 
wind-borne debris regions during design winds.

Ground motion. The movement of the earth’s surface from earthquakes or 
explosions. Ground motion is produced by waves that are generated by sudden slip 
on a fault or sudden pressure at the explosive source, and travel through the 
earth and along its surface.

H

Hurricane-prone regions. Areas vulnerable to hurricanes; in the United States 
and its territories de­fined as:
1. The U.S. Atlantic Ocean and Gulf of Mexico coasts, where the basic wind speed 
is greater than 90 miles per hour.
2. Hawaii, Puerto Rico, Guam, U.S. Virgin Islands, and American Samoa.

Human intervention. The presence and active involvement of people necessary to 
enact or implement floodproofing measures prior to the onset of flooding.

Hydrodynamic load. Loads imposed by water flowing against and around an object 
or structure, including the impacts of debris and waves.

Hydrostatic load. Load (pressure) imposed on an object or structure by a 
standing mass of water; the deeper the water, the greater the load or pressure 
against the object or structure.

I

Impact-resistant covering. A covering designed to protect glazing, which has 
been shown by an approved test method to withstand the impact of wind-borne 
missiles likely to be generated in wind-borne debris regions during design 
winds.

Importance factor, I. A factor that accounts for the degree of hazard to human 
life and damage to property. Importance factors are given in ASCE 7.

L

Landslides. The slipping of soil and rock on sloping ground triggered by 
earthquake ground motion.

Liquefaction. The temporary change of loose granular soils and sand in the 
presence of water from a solid to a liquid state when subjected to ground 
shaking. 

Lowest floor. The lowest floor of the lowest enclosed area (including basement). 
An unfinished or flood-resistant enclosure, usable solely for parking of 
vehicles, building access, or storage, in an area other than a basement area, is 
not considered a building’s lowest floor, provided that the enclosure is 
compliant with flood-resistant requirements.

M

Magnitude. The magnitude is a number that characterizes the relative size of an 
earthquake. Magnitude is based on measurement of the maximum motion recorded by 
a seismograph. Best known scales are “Richter magnitude,” and “moment 
magnitude.” The moment magni­tude (Mw) scale, based on the concept of seismic 
moment, is uniformly applicable to all sizes of earthquakes.  

Main wind-force resisting system. An assemblage of structural elements assigned 
to provide sup­port and stability for the overall structure. The system 
generally receives wind loading from more than one surface.

Mean roof height, (h). The average of the roof eave height and the height to the 
highest point on the roof surface, except that, for roof angles of less than or 
equal to 10 degrees, the mean roof height shall be the roof eave height.

Missiles. Debris that could become propelled into the wind stream.  

Modified Mercalli Intensity (MMI) scale. Intensity scale used to measure the 
level of earthquake damage.  

Moment Frame. Frames in which structural members and joints resist lateral 
forces by bending. There are “ordinary,” “intermediate,” and “special” moment 
frames. The latter provide the most resistance.

N

National Flood Insurance Program (NFIP). A Federal program to identify flood-
prone areas nation­wide, and make flood insurance available for properties in 
communities that participate in the program. 

NEHRP. The Federal National Earthquake Hazard Reduction Program, enacted in 
1977, to reduce potential losses from earthquakes by funding research in 
earthquake prediction and hazards and to guide the implementation of earthquake 
loss-reduction programs. 

Nor’easter. Nor’easters are non-tropical storms that typically occur in the 
eastern United States, any time between October and April, when moisture and 
cold air are plentiful. They are known for dumping heavy amounts of rain and 
snow, producing hurricane-force winds, and creating high surfs that cause severe 
beach erosion and coastal flooding. A nor’easter is named for the winds that 
blow in from the northeast and drive the storm along the east coast and the Gulf 
Stream, a band of warm water that lies off the Atlantic Coast. 

O

Openings. Apertures or holes in the building envelope that allow air to flow 
through the building envelope. A door that is intended to be in the closed 
position during a windstorm would not be considered an opening. Glazed openings 
are also not typically considered openings. However, if the building is located 
in a wind-borne debris region and the glazing is not impact-resistant or 
protected with an impact-resistant covering, the glazing is considered an 
opening.

P

Passive Energy Dissipation. The reduction of earthquake forces in a building by 
the introduction of devices designed to dissipate the earthquake energy in a 
controlled manner using friction, hydraulics, or deformation of material 
specially placed for this purpose.

Peak Ground Acceleration. The largest acceleration that occurs during 
earthquake-induced ground motion.

R

Racking. Lateral deflection of a structure resulting from external forces, such 
as wind or lateral ground movement in an earthquake.

Resonance. The increase in vibrations and accelerations to a vibrating building 
from the transmission of ground motion.

Response spectrum. A characterization of ground motion (representing the suite 
of spectral or­dinates) measuring the extent of shaking different structures 
will experience based on their natural period of vibration.  

Richter magnitude scale. Developed in 1935 by Charles F. Richter of the 
California Institute of Technology. The magnitude of an earthquake is determined 
from the logarithm of the amplitude of waves recorded by seismographs. 
Adjustments are included for the variation in the distance between the various 
seismographs and the epicenter of the earthquakes. On the Richter Scale, 
magnitude is expressed in whole numbers and decimal fractions. For example, a 
magnitude 5.3 might be computed for a moderate earthquake, and a strong 
earthquake might be rated as magnitude 6.3. Because of the logarithmic basis of 
the scale, each whole number increase in magnitude represents a tenfold increase 
in measured amplitude; as an estimate of energy, each whole number step in the 
magnitude scale corresponds to the release of about 31 times more energy than 
the amount associated with the preceding whole number value. 

Ridge. With respect to topographic effects, an elongated crest of a hill 
characterized by strong relief in two directions.

Riverine Flooding. The accumulation of runoff from rainfall or snowmelt, such 
that the volume of flow exceeds the capacity of waterway channels and spreads 
out over the adjacent land.

S

Seiche. A wave that oscillates in lakes, bays, or gulfs from a few minutes to a 
few hours as a result of seismic or atmospheric disturbances.
Seismic ground motion. The movement of ground surfaces in every direction 
simultaneously, back and forth, side to side, and up and down during an 
earthquake.

Seismograph. Also known as seismometer, is an instrument used to detect and 
record earth­quakes. Generally, it consists of a mass attached to a fixed base. 
During an earthquake, the base moves and the mass does not. The motion of the 
base with respect to the mass is commonly transformed into an electrical 
voltage. The electrical voltage is recorded on paper, magnetic tape, or another 
recording medium. This record is proportional to the motion of the seismometer 
mass relative to the earth, but it can be mathematically converted to a record 
of the absolute motion of the ground. 

Shear. A force that causes parts of a material to slide past one another in 
opposite directions

Shear wall. Solid wall that resists shear forces, used in buildings constructed 
in earthquake and high-wind zones.

Sheetflow. Rainfall runoff that flows over relatively flat land without 
concentrating into streams or channels.

Spectral Acceleration. The acceleration to be experienced by structures of 
different periods.

Stiffness. Rigidity, or resistance to deflection or drift. A measure of 
deflection or of staying in alignment within a certain stress.

Stillwater elevation. The elevation that the surface of coastal flood waters 
would assume in the absence of waves, referenced to a datum.

Straight-line wind. A wind blowing in a straight line with wind speeds ranging 
from very low to very high (the most common wind occurring throughout United 
States and its territories). 

Substantial damage. Damage of any origin sustained by a structure, whereby the 
cost of restoring the structure to its predamage condition equals or exceeds 50 
percent of the market value of the structure before the damage occurred (or 
smaller percentage if established by the authority having jurisdiction). 
Structures that are determined to be substantially damaged are considered to be 
substantial improvements, regardless of the actual repair work performed. 

Substantial improvement. Any reconstruction, rehabilitation, addition, or other 
improvement of a structure, the cost of which equals or exceeds 50 percent of 
the market value of the structure (or smaller percentage if established by the 
authority having jurisdiction) before the start of the improvement. 

T

Torsion. Twisting around an axis. The center of the mass does not coincide with 
the center of re­sultant force of the resisting building elements causing 
rotation or twisting action in plans and stress concentrations. Symmetry in 
general reduces torsion. 

Tsunami. An unusually large sea wave produced by submarine earth movement or a 
volcanic eruption.

W

Wet floodproofing. Permanent or contingent measures and construction techniques, 
applied to a structure or its contents, that prevent or provide resistance to 
damage from flooding while allowing floodwaters to enter the structure. 
Generally, this includes properly anchoring the structure, using flood-resistant 
materials below the BFE, protection of mechanical and utility equipment, and the 
use of openings or breakaway walls. 

Wind-borne debris regions. Areas within hurricane-prone regions located:
1. Within 1 mile of the coastal mean high water line, where the basic wind speed 
is equal to or greater than 110 mph, and in Hawaii.
2. In areas where the basic wind speed is equal to or greater than 120 mph.