FEMA 453 Design Guidance for Shelters and Safe Rooms

integrity and provide an additional measure of safety to occupants.
Incorporating continuity, redundancy, and ductility into
the design will allow a damaged building to bridge over a failed
element and redistribute loads through flexure or catenary action.
This will limit the extent of debris that might otherwise rain
down upon the hardened shelter. Where specific threats are defined,
the vulnerable structural components may be hardened to
withstand the intensity of explosive loading. The local hardening
of vulnerable components in addition to the indirect prescriptive
detailing of the structural system to bridge over damaged components
will provide the most protection to the building.
2.4.1 Structure
Both steel frame and reinforced concrete buildings may be designed
and detailed to resist the effects of an exterior vehicle
explosive threat and an interior satchel explosion. Although steel
construction may be more efficient for many types of loading,
both conventional and unconventional, cast-in-place reinforced
concrete construction provide an inherent continuity and mass
that makes it desirable for blast-resistant buildings.
Reinforced concrete is a composite material in which the concrete
provides the primary resistance to compression and shear and
the steel reinforcement provides the resistance to tension and
confines the concrete core. In addition to ductile detailing, which
allows the reinforced concrete members to sustain large deformations
and uncharacteristic reversals of curvature, the structural
elements are typically stockier and more massive than their steel
frame counterparts. The additional inertial resistance as well as
the continuity of cast-in-place construction facilitates designs that
are capable of sustaining the high intensity and short duration
effects of close-in explosions. Furthermore, reinforced concrete
buildings tend to crack and dissipate large amounts of energy
through internal damping. This limits the extent of rebound
forces and deformations.
2-20 Structural deSign criteria