A numerical investigation of 3D structural behavior for steel-composite structures under various travelling fire scenarios

In the recent development of the performance-based structural fire design, the “travelling fire” methodology has been gradually accepted as a necessary fire boundary condition. However, its application is still limited by uncertainties in the selection of different design travelling fire parameters, which result from the lack of relevant experimental data and corresponding validated structural finite element models which can be used in advanced travelling fire methodologies, e.g. the extended travelling fire methodology (ETFM) framework. This paper aims to fill this gap through modelling a prototype steel-composite floor structure
(regarded as a 'slice' of a large open-plan office), to investigate its true structural response under a wide range of travelling fire scenarios, with an emphasis on considering the effect of concrete slabs in a 3D finite element model, using LS-DYNA. To ensure the credibility of this numerical study, the model was validated against the experimental data from the Veselí Travelling Fire Test, and comparisons made with similar modelling efforts using Vulcan. In the parametric studies, eight cases were examined to investigate the structural response and the failure criteria, related to the selection of different key design parameters for the travelling fire within the ETFM framework, i.e. fire spread rate and inverse opening factor. It was found that solely satisfying the critical temperature (e.g. 550 oC for steel) and deflection criteria (e.g. beam span
over 20) for the structural members might not guarantee a safe structural design for travelling fire scenarios, and it is suggested that the steel yielding stress status during the cooling phase in tension should also be examined. Compared to the inverse opening factor, it appears that the selection of fire spread rates is likely to be more critical in identifying the worst travelling fire scenario for the structural response with fire protection.