The fire vulnerability of flat slabs is a well-recognised issue, extensively documented through both numerical and experimental studies. In particular, the risk of punching shear failure in flat slabs under fire conditions requires careful consideration. A notable limitation of much of the existing experimental and numerical research is the use of slab specimens or FE models with boundary conditions that do not accurately represent those in real structures. While the effects of material degradation at high temperatures are widely examined, a clear gap exists in research concerning the fundamental influence of boundary conditions (BCs) and thermally induced deformations on the thermo-mechanical behaviour of flat slabs. This paper addresses this gap through a thermo-mechanical finite element analysis of the Gretzenbach car park collapse using Abaqus, modelling an axisymmetric slab-column segment that captures smeared reinforcement effects. The heat-transfer analysis, based on a passenger car fire with 5 MW peak and 5000 MJ load, drives rapid heating at the soffit, with temperatures up to 850°C and 358°C in the reinforcement, and steep gradients that induce significant compression near the column-slab connection. Preliminary results up to 1000s of fire reveal 4 MPa top tensile cracking and 35 MPa bottom compression crushing, reducing the effective depth by 27% and resulting in 39-46% punching shear capacity utilisation, with 25% column load amplification, highlighting the significance of thermal deformations and BCs in punching shear failure.