Load-deflection behaviour of flat slab exposed to fire: parametric analysis on the spring stiffness influence

Flat slabs are widely used in underground structures, such as car parks, due to their large surface area, static efficiency, and large span-depth ratios. This paper provides a parametric analysis on spring stiffness influence into the load-deflection behaviour of flat slabs exposed to fire. To understand the behaviour of flat slabs under fire conditions, computer modelling tools were applied, based on NonLinear Finite Element Analysis (NLFEA). Calibration was performed to obtain the load–displacement curve closest to the curve and the punching shear failure mode obtained in the experimental test. As boundary conditions, considering the slab as unrestrained (allowed to expand, and free to rotate), the slab rested on steel rods with spring criteria applied to the middle of bottom surface. The analysed parameter was the vertical stiffness values. The experimental results from the literature were compared with FE results, changing the vertical stiffness values. The difference between the experimental model and FE stiffness predictions is caused by the supporting reaction frame stiffness, which was not perfectly rigid. The discrepancy illustrates the importance of the restraint conditions for FE modelling.