FEMA 453 Design Guidance for Shelters and Safe Rooms

Alternatively, an unreinforced masonry wall may be upgraded
with an application of shotcrete sprayed onto the wall with a
welded wire fabric. This method supplements the tensile capacity
of the existing wall and limits the extent of debris that might be
propelled into the protected space. Steel sections may also be installed
up against existing walls to reduce the span and provide an
alternate load transfer to the floor diaphragms. Load-bearing masonry
walls require additional redundancy to prevent the initiation
of a catastrophic progression of collapse. Therefore, the fragment
protection that may be provided by a spray-on elasto-polymer, a
fabric spall shield, or a metal panel must be supplemented with
structural supports that can sustain the gravity loads in the event
of excessive wall deformation. The design of stiffened steel-plate
wall systems to withstand the effects of explosive loading is one
way of achieving such redundancy and fragment protection.
These load-bearing wall retrofits require a more stringent design,
capable of resisting lateral loads and the transfer of axial forces.
Stiffened wall panels, consisting of steel plates to catch the debris
and welded tube sections spaced some 3 feet on center to supplement
the gravity load carrying capacity of the bearing walls, must
be connected to the existing floor and ceiling slabs by means of
base plates and anchor bolt connectors (see Figure 2-18).
A steel stud wall construction technique may also be used for
new buildings or the retrofit of existing structures requiring blast
resistance. Commercially available 18-gauge steel studs may be attached
web to web (back to back) and 16-gauge sheet metal may
be installed outboard of the steel studs behind the cladding (see
Figure 2-19). While the wall absorbs a considerable amount of
the blast energy through deformation, its connection to the surrounding
structure must develop the large tensile reaction forces.
In order to prevent a premature failure, these connections should
be able to develop the ultimate capacity of the stud in tension.
Ballistic testing of various building cladding materials requires a
nominal 4-inch thickness of stone, brick, masonry, or concrete.
Forced entry protection requires a ¼-inch thick layer of A36 steel
plate that is behind the building’s veneer and welded or screwed
to the steel stud framing in lieu of the 16-gauge sheet metal.
Structural deSign criteria
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