Uncertainty in Structural Fire Design

Structural fire safety requirements implicitly balance up-front investments in materials (protection or element sizing) with improved performance (loss reductions) in the unlikely event of a fire. For traditional prescriptive fire safety recommendations, the underlying target safety levels are not clear to the designer, nor is the associated balancing of risk and investment costs. While easy to apply, guidance / code-based approaches to the specification of fire protection / resistance have the severe disadvantage that the level of safety investment is not tailored to the specifics of the case, resulting in large overinvestments in some cases, and possibly insufficient structural fire safety in others. This observation is a major driver for the use of performance-based design (PBD) methodologies, where the fire safety design is tailored to the needs of the building. However, it is posited in this chapter that traditional PBD in a structural fire context is deterministic, with the safety foundation premised upon the collective experience of the profession. It is separately noted that building forms are increasingly uncommon in nature, due to material choice, height, failure consequences, etc., and, as such, collective experience is increasingly a weak safety foundation. This is where probabilistic methods add value and provide a quantified / explicit basis for demonstrating the adequacy of a design. This chapter, thus, focusses on uncertainties and uncertainty quantification in the context of structural performance in the event of fire, introducing reliability and risk concepts, with supporting data and applications.