RESISTANCE OF TALL BUILDINGS TO LARGE AIRCRAFT IMPACT AND FIRE

The demolition of the 420m high twin-towers of the World Trade Center on 11 Sept 2001 by terrorist attack has led structural and fire engineers to consider what might be done to reduce the consequences of terrorist attack against heavily-populated tall buildings by large, fuel-laden aircraft. The paper discusses the different structural behaviour of the two principal forms of structure used in tall buildings – the cantilever core structure and, as in the WTC towers, the framed-tube structure – and comments on the susceptibility of both to massive horizontal impact and progressive collapse. Some data are presented on the missiles and the targets in the WTC incident and some estimates of local impact forces are made, and the author speculates on the effect of these impacts on the structure. Structural features of some tall buildings in the UK, USA and Asia are given to illustrate differences in structural design. The hazard of progressive vertical collapse is considered in the context of what UK codes imposed on designers following the partial collapse of the Ronan Point block of flats in London in 1968. Comments are made on the vulnerability of steel and concrete to impact and fire caused by impact of aircraft, and indications are given on how vulnerability can be reduced. Finally some options are considered, ranging from the general to the specific, for protecting people in tall buildings. The paper was prepared before the Federal Emergency Management Agency published on the internet its first report ‘World Trade Center Building Performance Study’.