Robustness of steel framed buildings

Since the catastrophic failures of the twin towers at the World Trade Centre, avoidance of progressive collapse has become a major concern of designers of multi-storey buildings. Following the partial collapse of a residential apartment block in the UK in 1968, the Building Regulations were amended and required designers to build in measures to reduce the likelihood of damage to a small part of a building resulting in collapse of a disproportionately large part of the structure. All parts of a building are required to be tied together to ensure that they cannot be dislodged easily and, in the event of structural failure of a member, alternative load paths may be mobilised. In the years since 2001, UK practice has often been cited as good practice. Although the UK approach has appeared to work well, there has been little investigation into whether the design recommendations adequately protect a damaged structure from progressive or total collapse. For this reason, a series of studies was carried out on a typical steel-framed building designed according to the guidelines given in the UK code for structural use of steel in building (BS5950). The analysis used LS-DYNA, a non-linear explicit/implicit finite element code capable of modelling the dynamic behaviour of structures. This investigation examined the structural performance of the buildings, such as the resisting mechanism or if collapse occurs, the failure mechanism during progressive collapse, when key structural members were removed. Most current guidelines for designing against disproportional collapse are based on a static analysis of a damaged structure or an assumed alternative load path which is in turn assured by compliance with design.