# Numerical Analysis of a Moderate Fire inside a Segment of a Subway Station
## 5 Conclusions
* The prescription of three different piecewise uniform temperature histories showed that, provided the temperature history at a specific surface is known and used as the local thermal boundary condition, the ingress of heat at that position of the structure is predicted accurately. * The surface temperature histories were only measured at two positions of the tested structure. Thus, assumptions concerning the thermal boundary conditions were indispensable for the numerical simulations. They are anyway questionable, because the unsolved problem of the dynamics of flames remains an important source of uncertainty; see, e.g., the study by [PERSON] et al. [34]. * The simulation with the improved thermal boundary conditions underlined that the temperature increase at the surface of the columns is expected to be larger than 280 \({}^{\circ}\)C. Thus, thermal degradation of the mechanical properties of concrete and steel occurs. This was the motivation to consider the temperature-dependent and elasto-plastic material behavior of concrete. The following conclusions are drawn from the four different types of thermomechanical simulations of the load-carrying behavior of the analyzed structure:  
* The load-carrying behavior of the structure, when subjected to mechanical loads only, was governed by linear-elastic material behavior. Although tensile cracking took place at several positions of the structure, inelastic material behavior did not play an important role. * The numerical simulations referring to the mechanical loading reproduced strain measurements that were only reliable at the right wall, whereas the absolute values of the strains at midspan of the left cell of the top slab were overestimated significantly. This has revealed two additional sources of uncertainty, namely the reliability of measurements and the initial state of the structure. It is likely that the highly statically indeterminate structure was damaged already before the test, by restrained shrinkage of concrete and by the transport of the structure from the production site to the test furnace. * The numerical simulations referring to the mechanical and the thermal loading reproduced strain measurements that were only reliable at the inner reinforcement at the midspan of the left cell of the top slab, whereas the absolute values of the strains at the other two positions at which strain measurements were available were underestimated significantly. The best results were obtained from the most realistic simulation, based on the prescription of three different piecewise uniform surface temperature histories, as well as on temperature-dependent elasto-plastic behavior of concrete. The Finite Element simulations provide insight into the structural behavior of the tested segment of a subway station. From the numerical results obtained with the nonlinear material model for concrete, the following conclusions are drawn:  
* Already moderate fires threaten the integrity of RC structures. As for the analyzed structure, the Finite Element simulations indicate localized tensile cracking at the outer surface of the structure, in regions where the top and the bottom slabs are connected to the walls and the columns; see Figures 21-26. In more detail, the Finite Element simulations indicate that the corners, i.e., the connections between the walls and the slabs, were damaged first (Figures 21 and 22), followed by a localized damage of the slabs, in the immediate vicinity of the connections to the columns (Figures 23-26). * Tensile cracking of concrete and the associated redistribution of stresses within the RC structure take place in a quasi-instantaneous fashion. This renders numerical stabilization approaches indispensable in order to achieve convergence in nonlinear Finite Element simulations. * Careful inspection of RC structures is strongly recommended also after moderate fire events (see, e.g., the methods proposed by [PERSON] [63]), even if the accessible interior surface of an underground structure appears to be undamaged.