Target safety levels for structural fire resistance based on lifetime cost optimisation

Deflection criteria are sometimes used to assess the fire resistance of large structural floorplates for which the accurate modelling and prediction of all possible modes of fire-induced failure is not yet possible. The safety level obtained by applying deflection criteria during design and analysis is, however, unclear. To ensure adequate safety and allow for the derivation of a simplified deflection-based structural fire design format, target safety levels (i.e. target reliability indices) are derived herein using the concepts of Lifetime Cost Optimization. The example case of a simply supported concrete slab is evaluated in detail, since for this case both deflection limit states and strength (bending) limit states can be calculated relatively easily, and the obtained optimum design solutions and target safety levels compared. By introducing a ‘fire-damage parameter’ (which incorporates the fire damage cost, fire occurrence rate, and discount rate), a differentiation in target safety levels is obtained. Although exploratory, the results in the current paper suggest the feasibility of deriving target safety levels on this basis to be used in conjunction with deflection criteria, to ensure adequate structural fire resistance.