FEMA 453 Design Guidance for Shelters and Safe Rooms

Steelwork is generally better suited to resist relatively low intensity,
but long duration effects of large stand-off explosions. Steel is
an inherently ductile material that is capable of sustaining large
deformations; however, the very efficient thin-flanged sections
make the conventional frame construction vulnerable to localized
damage. Complex stress combinations and concentrations may
occur that cause localized distress and prevent the section from
developing its ultimate strength. Steel buildings may experience
significant rebound and must therefore be designed to support
significant reversals of loading. Concrete filled tube sections or
concrete encased flanged sections may be used to protect the thinflanged
sections and supplement the inertial resistance. Concrete
encasement should extend a minimum of 4 inches beyond the
width and depth of the steel flanges and reinforcing bars may be
detailed to tie into the concrete slabs.
To allow the concrete encasement to be tied into the floor slabs,
the typical metal pan with concrete deck construction will require
special detailing. Metal deck construction provides a spall shield
to the underside of the slabs, which provides additional protection
to a near contact satchel situated on a floor. However, the internal
explosive threat will also load the ceiling slabs from beneath and
the beams must contain an ample amount of studs, which far exceeds
the requirements for conventional gravity design, to transfer
the slab reactions to the steel supports without pulling out. If the
slabs are adequately connected to the steel-framing members,
these beams will be subjected to abnormal reversals of curvature.
These reversals will subject the mid-span bottom flanges to transient
compressive stress and may induce a localized buckling.
Because the blast loads are transient, the dominant gravity loads
will eventually restore the mid-span bottom flange to tension;
however, unless it is adequately braced, the transient buckling will
produce localized damage.
The concrete encasement of the steel beams will provide torsional
resistance to the cross-section and minimize the need for intermediate
bracing. If the depth of the composite section is to be
minimized by embedding the steel section into the thickness of
2-22 Structural deSign criteria