Tall buildings are an important part of modern cities and it is important that developers have the confidence to continue to build high. Prior to 9/11 building designers tended to concentrate on gravity, wind and seismic loads; however, the ability to resist terrorist attack is now a serious consideration. The precise mode of attack by terrorists is impossible to predict. This is identical to automotive design, in which robustness is achieved by targeting failure to energy absorbing large flexural deformations, whilst attempting to limit the sudden losses of strength due to connection failures (known as crumple zone design). In contrast, the present limit state design approach does not target failure. This paper demonstrates that the loads transferable from typical composite beams can be much higher than those predicted from ultimate limit state design. Furthermore, industry standard nominally pinned beam to column connections are shown to lack ductility. The combined effects of over-strength beams and low ductility connections could result in sudden connection failures due to bolt rupture if beams are subjected to the large sagging deflections associated with blast or impact loading. In response to this problem a capacity design approach is advocated. This targets failure through flexural failure in the beams. This is achieved by determining the upper-bound load capacity of the flexural members, thereafter designing all components further down the load path to resist these upper bound loads, rather than the conventional ultimate limit state design loads. The result is frames with relatively strong connections and weak beams. This will improve the ability to absorb energy from blast or impact through beam flexure and will help prevent sudden connection failures that can lead to progressive collapses.
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