This numerical simulation shows a composite slab at high temperature exhibiting thermoplastic behaviour. The analysis is carried out by coupling thermal and mechanical FE analysis. The temperature field in the slab is computed by running a transient thermal analysis, where a thermal load according to the fire curve ISO 834 is applied. The heat strongly affects the mechanical properties of concrete and steel, reducing their elastic modulus and yield strength. As a result, the slab deflection increases with the temperature, as expected during fire exposure. The concrete behaviour is simulated with a Double Drucker-Prager constitutive model, which takes into account the material fracture energy. The steel beams are modeled according to Von Mises Yield criterion.