Modelling approaches for robustness assessment of multi-storey steel-composite buildings

This paper investigates the robustness of multi-storey steel composite buildings under sudden column loss scenarios using a multi-level framework developed at Imperial College London. The framework was previously applied at its lowest level using a grillage idealisation to obtain the floor system response by assuming a linear collapse mechanism, where further simplifying assumptions were made regarding the vertical connection between the different floors. In this paper, the nonlinear dynamic response of the floor system is established from detailed nonlinear static push-down analysis at different levels of structural idealisation, accounting also for the influence of inter storey continuity. At the highest level of idealisation, a detailed composite ribbed slab model is considered to represent more accurately membrane action and load distribution at large deflections. It is shown that the triangular dominant deformation mode, previously used at the individual beam level in grillage models, can affect the robustness prediction for the floor system. However, as the allowable vertical deflection increases with the availability of sufficient ductility supply, this assumption becomes realistic compared to the actual floor system deformed configuration. Furthermore, the effect of membrane action is found to increase structural resistance to column loss by 10% in comparison with the grillage model using EC4 effective width and tributary load distribution concepts. Overall, it is found that grillage models may still provide a good balance between accuracy and computational efficiency for practical robustness assessment, subject to incorporating essential modelling features as presented in this paper.