The need for hierarchies of acceptance criteria for probabilistic risk assessments in fire engineering. Fire Technology.

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For most fires occurring in buildings with a concrete structural frame, the structural elements do not collapse during fire exposure, and further use of the building after fire may be possible. Fire can nevertheless result in a permanent loss of strength and thus a post-fire evaluation of the residual load bearing capacity has to be made to inform decisions on continued use and the need for structural repairs. This evaluation is however particularly difficult due to the many uncertainties associated with both the fire exposure and the characteristics of the structural elements. These uncertainties cannot be neglected when determining the residual capacity since adequate safety is a major societal concern as indicated by the predominance of safety in current design standards and guidance documents. In this paper a comprehensive methodology is presented for the assessment of the residual capacity of concrete structures after exposure to fire. The methodology is introduced through application to a real-life case study of an apartment fire with a focus on the end-span of the affected continuous concrete slab. It results in a reliability-based evaluation of the maximum allowable characteristic value for the imposed load on the slab. The presented methodology is useful to make informed decision about continued use of structures after a fire event.

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Asimplified full-probabilistic calculation tool is developed which is capable of calculating the bending moment capacity of simply supported beams exposed to fire. It is found that the uncertainty with respect to the reduction factors for the material properties at elevated temperatures and the uncertainty with respect to the concrete cover have a negative effect on the safety level corresponding to the design value of the bending moment capacity calculated according to the Eurocode. Furthermore, the model allows an objective comparison of different design alternatives with respect to the safety level.

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