FEMA 453 Design Guidance for Shelters and Safe Rooms

progressive collapse resulting from extreme loading may therefore
best be achieved through the localized hardening of vulnerable
columns. These columns need only be upgraded to a level of
resistance that balances the capacities of all adjacent structural
elements. At greater blast intensities, the resulting damage would
be extensive and create global collapse rather than progressive
collapse. Attempts to upgrade the building to conform to the
alternate path approach would be invasive and potentially counterproductive.
2.3.2 loads and connections
Because the shelter will likely suffer significant damage in response
to extreme loading conditions, the shelter must be able
to withstand both the direct loading associated with the natural
or manmade hazard and the debris associated with the damaged
building within which it is housed.
Structural systems that provide a continuous load path that
supports all vertical and lateral loads acting on a building are
preferred. A continuous load path ties all structural components
together and the fasteners used in the connections must be capable
of developing the full capacity of the members. In order to
provide comprehensive protection, the capacity of each component
must be balanced with the capacity of all other components
and the connection details that tie them together. Because all
applied loads must eventually be transferred to the foundations,
the load path must be continuous from the uppermost structural
component to the ground.
After the appropriate loads are calculated for the shelter, they
should be applied to the exterior wall and roof surfaces of the
shelter to determine the design forces for the structural and
nonstructural elements. The continuous load path carries the
loads acting on a building’s exterior façade and roof through the
floor diaphragms to the gravity load-bearing system and lateral
load-bearing system. The individual components of the façade and
roof must be able to develop these extraordinary forces, though
2- 2 Structural deSign criteria