Transamerica Building, that occupy special places in the public consciousness. Entertainment complexes might also be targets, and though coordinated attacks on a few day-care centers would not cause serious economic damage, the emotional toll would be enormous (NRC, 1988, 1995, 1999).

Representative Vulnerabilities

Major and monumental buildings, like most others, are vulnerable to structural failure induced by various combinations of impact, explosion, and fire. In addition, the occupants may be threatened by toxins. Scenarios suggested by recent events include the impact of commercial airliners, business jets, and small private planes. The few incidents involving piston-engine impacts with tall buildings, including the 1945 collision of a B-25 with the Empire State Building and the 2002 crash of a Cessna 172 light plane into a building in Tampa, suggest that these aircraft had insufficient energy and fuel to cause great general damage or to precipitate collapse. Intermediate-size jet aircraft of the kind used by large businesses for their executives, on the other hand, might pose a threat. Impact by commercial airliners is unambiguously catastrophic, as recently witnessed.

Prior to September 11, 2001, bombs were considered to be the principal threats to buildings. Information about such bombs may be found in the FBI’s Bomb Data Registry and the Bureau of Alcohol, Tobacco and Firearms (ATF) histogram of actual events. The magnitudes of such attacks on U.S. targets have so far been limited by the size and capacity of trucks permitted to park or circulate in the immediate vicinity of the target buildings.

Impacts and explosions, as illustrated in Oklahoma City’s Murrah Building and the 1993 and 2001 attacks on New York’s World Trade Center (WTC), can destroy key structural elements, allowing gravity to destroy much or all of the building (ASCE, 1996; Corley, 1998). Some structures (such as those designed for minimum weight) could be seriously jeopardized by the loss of just a few columns. Temperatures of 500°C reduce the strength of common structural steels by 50 percent, and 1000°C reduces the strength to near zero. Columns, floor diaphragms, and connections between the columns and floor joists are the vulnerable members (ASTM, 1998).

In reinforced concrete members, the fire resistance is integral because a thickness of concrete covers the embedded steel reinforcement, protecting the steel from the fire temperatures. With steel members, resistance is presumably achieved, by code, with a layer of fireproofing. But this superficial coating may not be applied properly, or sections of it may be removed from the structure over the course of time, thus compromising the level of protection. The forces from a major impact or explosion also may strip fireproofing from structural elements and assemblies, destroy detection and alarm circuits, break pipes and deplete the available water supply for fire protection, and render smoke control and alarm systems ineffective.



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