17.1 Introduction

The case history of the 41-storey building at 140 William Street, Melbourne, Australia, detailed in Thomas et al. [66] vividly illustrates the potential benefits of risk-based fire safety design. There, compliance with the existing fire code would have required an extremely costly replacement of the inter-floor insulation. The problem was put in the hands of a team from BHP (Broken Hill Pty Ltd) Research Laboratories. They used a pioneering risk-to-life analysis based on stochastic modelling to compare the safety of a building complying with existing requirements and that of an alternative design based on improving the reliability of the sprinkler system. The latter turned out to provide greater safety at a vastly reduced cost and it was eventually adopted by the authorities.

In 1991, the existing building was a large freestanding building located centrally in the City of Melbourne. It had 41 levels, of which 37 could be occupied, and was used as an office building. See Figure 17.1 for an elevation of the building. All the columns and the bracing members were heavily encased in concrete and in the case of the external columns were further encapsulated with steel plate. The floors were supported on long-span castellated steel beams framing between primary beams and the perimeter of the building. A typical floor framing plan is shown in Figure 17.2. The floor slabs were of composite construction using lightweight concrete and steel decking. The soffit of the floor slabs and steel supporting beams were sprayed with asbestos-based...