FEMA 453 Design Guidance for Shelters and Safe Rooms

Regardless of the materials, framed buildings perform best when
column spacing is limited and the use of transfer girders is limited.
Bearing wall systems that rely on interior cross-walls will
benefit from periodically spaced longitudinal walls that enhance
stability and control the lateral progression of damage. Bearing
wall systems that rely on exterior walls will benefit from periodically
spaced perpendicular walls or substantial pilasters that limit
the extent of wall that is likely to be affected.
Free-standing columns do not have much surface area; therefore,
air-blast loads on columns are limited by clear-time effects
in which relief waves from the free edges attenuate the reflected
intensity of the blast loads. Where the exterior façade inhibits
clear-time effects prior to façade failure, the columns will receive
the full intensity of the reflected blast pressures. Large stand-off
explosive threats may produce large inelastic flexural deformations
that could initiate P- induced instabilities. Short stand-off
explosive threats may cause shear, base plate, or column splice
failures. Near contact threats may cause brisance, which is the
shattering of reinforced concrete sections. Confinement of reinforced
concrete members by means of spiral reinforcement, steel
jackets, or carbon fiber wraps may improve their resistance. Encasement
of steel sections will inhibit local flange and web plate
deformations that could precipitate a section failure. Exterior
column splices should be located as high above grade level as
practical and match the capacity of the column section.
Load-bearing walls do not benefit from clear-time effects as columns
do and therefore collect the full intensity of the reflected
blast pressure pulse. Nevertheless, reinforced concrete loadbearing
walls are particularly effective if adequately reinforced.
Fully grouted masonry walls, on the other hand, are more brittle
and seismic levels of reinforcement greatly increase the ductility
and performance of masonry walls. Continuous reinforced bond
beams, with a minimum of one #4 bar or equivalent, are required
in the wall at the top and bottom of each floor and roof level. Interior
horizontal ties are required in the floors perpendicular to
the wall. The ties are equivalent to a #4 bent bar at a maximum
2-2 Structural deSign criteria