Franchini, Andrea; (2024) Consequence-oriented fire intensity optimisation: methodology development and application to bridges. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Current methodologies for the performance-based fire safety design of structures adhere to typical engineering workflows, which commence by defining hazard scenarios and resulting structural loading conditions. Subsequently, structural response analysis is conducted to assess hazard consequences in terms of desired damage or loss metrics. These metrics are then evaluated to determine whether the performance objectives are met or design adjustments are required. However, the described workflow is sub-optimal for fire safety design. Indeed, the strong interaction between fire phenomena and the surrounding (structural) environment characteristics enables the selection of design variables that influence fire dynamics and combustion processes to ultimately reduce fire intensity. Additionally, the fire-structure coupling effect highlights limitations when using conventional risk-based approaches for design purposes. Based on these considerations, this thesis advances the current performance-based fire design practice by developing a methodology that: i) treats fire scenarios as design variables, exploiting fire-structure coupling effects to optimise the balance between increasing structural member capacity and diminishing fire intensity ("fire-intensity optimisation"); ii) quantifies risk in terms of maximum fire consequences ("consequence-oriented") and identifies the fire scenarios yielding them; iii) uses uncertainty propagation methods to assess the effects of selected random parameters on the estimated consequences. The thesis also demonstrates how the proposed methodology, called the Consequence-oriented Fire intensity Optimisation (CFO) approach, can optimise the trade-off between implementing passive fire protection and enhancing a structure's "inherent fire capacity," defined as its ability to retain integrity/functionality without additional fire safety measures. The CFO approach is demonstrated in the context of fire safety design of bridges—a field that still receives limited attention despite the substantial direct/indirect economic losses resulting from these events and their frequent occurrence. Overall, this thesis highlights that treating fire scenarios as output design variables to optimise facilitates safer, more cost-effective and more sustainable structures.

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