A fully generalised approach to modelling fire response of steel-RC composite structures

A three-step model for the performance-based numerical simulations of the fire response of steel-RC two-layered beam-like composite structures is presented and validated. The first step consists of the determination of the evolution of temperatures in the structure׳s surroundings. Moisture and the heat transfer through the RC layer and the conduction of heat over the steel layer are obtained in the second step. In concrete, the transfer of water vapour, dry air, and free water is discussed as well as the evaporation and liquefaction phenomena and the dehydration of concrete and its thermal and mechanical degradation. Within the framework of the third step, a geometrically and materially non-linear mechanical response of the structure is proposed accounting for interlayer slips and uplifts as well as for various material-related phenomena such as the material hardening/softening and creep. The governing equations are solved numerically. An efficient, novel strain-based finite element formulation is introduced for the mechanical analysis. Due to its generality and consideration of several different possible non-linear material, geometrical, and interlayer contact phenomena and their couplings the model can be of a use to a broader fire science community for exploring the impact of different physical parameters on the results of the addressed numerical simulations, thereby providing directions for further research. In the paper a case of such a study is also demonstrated exploring the contribution of the steel sheet and the flexibility of the interlayer connection of a standard trapezoidal steel-RC slab to its ultimate fire resistance. A reasonable contribution of the sheet is proved if the stiffness ratio between the integrated and the external tensile reinforcement of the RC plate is low provided that the contact connection is sufficiently stiff.