Vulnerability of simple braced steel building under extreme load

This paper studies the behaviour of steel building under extreme load. A 10-storey steel building with a variety of lateral bracing system for a column loss is first investigated using conventional alternate path approach. The numerical study shows that the simple braced steel frames are susceptible to progressive collapse due to a column loss, due to weak column-to-beam joints compared to moment resisting frames, which have higher redundancy to redistribute the damaged column load to all other floor level. The alternate path approach, which is a threat-independent methodology, is commonly used as a design guide for minimising the potential for progressive collapse. This method may not be always conservative in assessing the robustness of the structure, especially for building subjected to heavy blast loads and thus nonlinear direct blast analysis is often needed to investigate the building response under such extreme load. The same 10-storey steel frame response is investigated for a surface blast using the direct blast analysis. The analysis results show that a heavy blast load may wipe out a series of columns/beams at once instead of a single one. High blast pressure may also induce large lateral drift and leads to significant damage to structural elements spreading over several storeys of the building. Generally, such extensive damage cannot be captured using the alternate path approach. The present investigation recommends that the nonlinear direct blast analysis should be performed in order to capture the true behaviour of such buildings subjected to extreme blast loads.