APPENDIX D (Continued)
General Building Construction. Knowing the charge weight, it is possible to develop
general guidelines based on the performance of average construction.
Reinforced-concrete buildings, steel-frame buildings, and reinforced-masonry
buildings will perform equal to, or better than, these average levels of performance.
Prefabricated steel buildings will perform about average. Wood, non-reinforced masonry, and
glass-faced buildings will perform worse than these averages.
The blast mechanism destroys a building by overloading elements, such as walls, that
receive the full load. Even very modest blast pressure loads are in excess of conventional wind
loads. Brittle elements, such as non-reinforced masonry, are easily ruptured and collapse. More
durable walls, such as those made of concrete, deflect and crack extensively, usually spalling
(material shattered by the blast load and projected into the surrounding area) off most of the
concrete cover over the reinforcement.
Voids (i.e., broken windows) will allow the blast pressure to leak into the building. If
the walls remain sufficiently intact, the blast load is then transferred into the supporting frame,
which collects the load from all the tributary areas and is itself overloaded. Typically, columns
near the blast side of the building, lacking sufficient shear resistance at the end connections, are
totally dislodged and blown inward. This results in a progressive collapse mechanism.
Progressive collapse is defined as the sequential failure of components of a building, leading to a
total structural failure. For example, a column failure leads to support-beam failures, which then
cause other floors and columns above them to fail in sequence.
Progressive collapse is a critical factor in structures lacking redundant elements and
load paths. Consider a building roof truss, which is a structural frame, usually based on the
rigidity of a triangle and composed of straight members subject to longitudinal compression or
tension or both, functioning as a beam or cantilever. If a truss is damaged by failure of one of its
bottom tension members, it will deflect and sag and be unable to carry any load. The roof load
will be transferred to adjacent trusses. The load on these adjacent trusses will exceed the design
capacity, resulting in the failure of additional trusses and, ultimately, failure of the entire roof
system. Redundant lateral bracing helps limit this damage.
A similar process could occur with damage to one of the columns supporting roof
beams or trusses. Destruction of the supporting column results in failure of the member being
supported, which then redistributes the load to surrounding columns and places additional forces
on these members from the sagging structure.
Failure of connections is also a critical area for progression of collapse. Blast loading
may dislodge footings, which damages columns, which then affects the beams. Often, an
explosion may load a structure in a direction opposite to the way it was intended to carry loads,