FEMA P55 Coastal Construction Manual, Fourth Edition - Mad Cad

8 DETERMINING SITE-SPECIFIC LOADS Volume II
of practicality and cost-effectiveness, but this does not mean that solutions that provide life-safety protection
cannot be achieved while maintaining cost-effectiveness.
A more practical approach is to construct an interior room or space that is “hardened” to resist not only
tornado-force winds but also the impact of wind-borne missiles. FEMA guidance on safe rooms can be
found in FEMA 320, Taking Shelter from the Storm: Building a Safe Room for Your Home or Small Business
(FEMA 2008c), which provides prescriptive design solutions for safe rooms of up to 14 feet x 14 feet. These
solutions can be incorporated into a structure or constructed as a nearby stand-alone safe room to provide
occupants with a place of near-absolute protection. The designs in FEMA 320 are based on wind pressure
calculations that are described in FEMA 361, Design and Construction Guidance for Community Safe Rooms
(FEMA 2008a). FEMA 361 focuses on larger community safe rooms, but the process of design and many of
the variables are the same for smaller residential safe rooms.
An additional reference, ANSI/ICC 500-2008 complements the information in FEMA 320 and FEMA 361
and is referenced in the 2012 IBC and 2012 IRC.
Safe rooms can be designed to resist both tornado and hurricane hazards, and though many residents of
coastal areas are more concerned with hurricanes, tornadoes can be as prevalent in coastal areas as they are
in inland areas such as Oklahoma, Kansas, and Missouri. Constructing to minimum requirements of the
building code does not include the protection of life-safety or property of occupants from a direct hit of
large tornado events. Safe rooms are not recommended in flood hazard areas.
8.9 Seismic Loads
This Manual uses the seismic provisions of ASCE 7-10 to illustrate a method for calculating the seismic base
shear. To simplify design, the effect of dynamic seismic ground motion accelerations can be considered an
equivalent static lateral force applied to the building. The magnitude of dynamic motion, and therefore the
magnitude of the equivalent static design force, depends on the building characteristics, and the spectral
response acceleration parameter at the specific site location.
The structural configuration in Figure 8-21 is called an “inverted pendulum” or “cantilevered column”
system. This configuration occurs in elevated pile-supported buildings where almost all of the weight is at
the top of the piles. For a timber frame cantilever column system, ASCE 7-10 assigns a response modification
factor (R) equal to 1.5 (e.g., R = 1.5). For wood frame, wood structural panel shear walls, ASCE 7-10 assigns
an R factor equal to 6.5. The R factor of 1.5 can be conservatively used to determine shear for the design of
all elements and connections of the structure. An R factor of 1.5 is not permitted for use in Seismic Design
Categories E and F per ASCE 7-10.
ASCE 7-10 contains procedures for the seismic design of structures with different structural systems stacked
vertically within a single structure. Rules for vertical combinations can be applied to enable the base of the
structure to be designed for shear forces associated with R=1.5 and the upper wood frame, wood structural
panel shear wall structure to be designed for reduced shear forces associated with R=6.5.
ASCE 7-10 also provides R factors for cantilever column systems using steel and concrete columns. A small
reduction in shear forces for steel piles or concrete columns could be obtained by using what ASCE 7-10
calls a “steel special cantilever column system” or a “special reinforced concrete moment frame,” both of
which have an R = 2.5. However, these systems call for additional calculations, connection design, and
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