Oklahoma blast forces unsettling design questions
The bomb that brought down a government building in Oklahoma City killed hundreds of people after it was detonated near the building`s key supports. The blast has reopened a long-simmering debate on safety by design. Structurally, it`s not practical to designing bomb-proof buildings. But it is possible to engineer a structure to deform rather than go through immediate progressive collapse. Delaying collapse gives occupants extra time to evacuate. And that could mean the difference between life and death. The construction material of choice really {open_quotes}depends on the height of the building and the lateral load resisting system,{close_quotes} says Charles H. Thornton of Thornton-Tomasetti/Engineers, New York City. But whether in steel or reinforced concrete, moment-resisting frames, which are inherently redundant, give horizontal components the ability to take reversals of stress common in explosions, he says. Simple frames do not. {open_quotes}They go down like a house of cards,{close_quotes} says Thornton. In reinforced concrete moment frames, beam reinforcing steel is continuous. In simple reinforced concrete frames, beam rebar only penetrates the column for a determined number of inches based on the length of the span. And it is absent from the mid-span of the beam. In a blast from below, the beam, its top usually in compression and bottom in tension, deflects up, throwing the beam top into tension. With no rebar, it loses structural integrity, and falls apart. With rebar, it has a chance of surviving. If a building is not designed for blasts, a steel frame might be better under a reasonably small bomb because steel has equal capacity in tension and compression, and concrete has capacity only in compression, says Thornton.
- OSTI ID:
- 160028
- Journal Information:
- ENR, Vol. 234, Issue 17; Other Information: PBD: 1 May 1995
- Country of Publication:
- United States
- Language:
- English
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