A Numerical Study on the Significance of Slab for the Response of a Prototype Structure Under Travelling Fires

Abstract “Travelling fires” discriminate a fire plume at the near-field and a hot smoke layer preheating the ceiling at the far-field, with the intent of ensuring the robustness of structural design for large compartments under realistic fires. Once the fire is “travelling”, the near-field has a leading edge representing the fire spread, and a trailing edge representing the burnout of the fuel. Despite the recognised effects of travelling fires, the mainstream of efforts into their effect on structural response has been limited to 2D models using the finite element method (FEM). This paper aims to identify the importance of slab inclusion with a 3D FEM structural model for steel-composite structures under travelling fires, assessed against the corresponding simplified 2D structural frame models (i.e., with and without effective slab in the 2D steel frame model). The first step is a comparative structural analysis of a prototype composite structure under various design fire scenarios, including standard fire, parametric fires and travelling fires. The role of the fire protection scheme for the simplified 2D models against the 3D model for the numerical predictions is also explored. It is found that the structural load path, and the potential structural failure mechanisms, could be fundamentally different between the 3D model and the simplified 2D models. Although the 2D frame model tends to predict larger deflections (i.e., more conservative) than the 3D model, it could also significantly underestimate the large internal forces from the beams, so that the connections' failure under travelling fires might be overlooked. Further, due to the simplification of the 2D models in omitting the significant stiffness contribution from the slab and the adjacent structural components, the effect of the fire protection is likely to be amplified. This may give misleading information on the performance-based structural fire design under different travelling fire scenarios. Hence, the 3D model can be considered as feasible but also necessary for structural fire analysis for travelling fires as a complement to the simplified 2D model approach.