Terrorism Risks and Economic Assessment of Infrastructure Protection against Progressive Collapse

Abstract

Existing infrastructure has proven to be resilient and robust against car and truck bombings. The impacts of terrorist attacks are often overstated, and the tragic events of 9/11 appear to be more of an aberration than a harbinger of worse to come. A case study describes fatality risks from progressive collapse caused by a large truck bomb, and then assesses probabilistic costs and benefits of design measures mandated by the United States to mitigate against progressive collapse for new federal government buildings. It was found that disproportionate collapse is a rare event, and there have been no fatalities from disproportionate collapse or other structural damage in Western countries in the 20 years since the tragic events of 9/11. Clearly, a specific threat involving a massive and larger than expected vehicle-borne improvised explosive device (VBIED) attack can lead to blast-resistant strengthening for buildings to be justified and cost-effective. However, the likelihood of nonspecific threats will need to exceed 2.5×10−4 per building per year for structural strengthening against progressive collapse to be cost-effective. This is equivalent to an average of six VBIED threats against large federal buildings per year, and that the threat involves a VBIED large enough to potentially cause progressive collapse. Policing and intelligence measures to warn or prevent terrorist attacks are likely to be a more cost-effective countermeasure.