Thermo-mechanical analysis of an underground car park structure exposed to fire

Underground concrete structures composed of flat slab-column connections are sensitive to punching failure at ambient temperatures. During fire, however, due to the restraint of thermal curvatures an increase of the punching load can occur, possibly leading to premature collapse. In this paper, this problem is studied numerically by the combination of CFD and mechanical finite element analysis, whereas further considerations are developed by the use of plasticity theory. The basis for the fire scenario used in the analysis is found in the literature as a real fire. The Belgium standard NBN S 21-208-2 for the design of the ventilation system in underground car parks with respect to car fires is used as well. The dimensions of the underground car park are based on a real fire accident that occurred in Gretzenbach, where the structure actually failed due to punching following a rather small fire. In the analysis, special attention is given to the increase of the axial load on the slab-column connection with temperature. For the mechanical properties and boundary conditions assumed, it is found that during fire the axial load may increase with a factor which, at least in the worst cases, can be close to the safety factor found from large scale fire tests.