# Numerical Analysis of a Moderate Fire inside a Segment of a Subway Station
## 5 Conclusions  
Because moderate fires happen much more frequently than fire disasters [41], the first 30 min of a fire test were analyzed. Sensitivity analyses by means of three-dimensional Finite Element simulations were used to address two specific challenges related to the prediction of the structural performance of a reinforced concrete segment of a subway station. These challenges are:  
1. the estimation of the spatial and temporal development of the temperature within the reinforced concrete structure and  
Figure 26: Plastic strains, developed on the outer surface of the top slab at its connection to the column right after the onset of cracking, as obtained from a thermomechanical simulation based on temperature-dependent and elasto-plastic material behavior, as well as on the prescription of one uniform surface temperature history; the results refer to the time instant 25 min and 1.2 s after the beginning of the fire test. Figure 25: Plastic strains, developed on the outer surface of the top slab at its connection to the column right before the onset of cracking, as obtained from a thermomechanical simulation based on the temperature-dependent and elasto-plastic material behavior, as well as on the prescription of one uniform surface temperature history; the results refer to the time instant 25 min and 1.0 s after the beginning of the fire test. the choice of a suitable material model for concrete subjected to mechanical loads and elevated temperatures. Experimental data from a scaled fire test of an underground substructure by [PERSON] et al. [40] were used as the reference for comparison with the numerical results. From these results, the following conclusions and remarks may be extracted:  
* Even under controlled laboratory conditions, different temperatures were measured at equal depths, but at different positions of the tested structure. This underlines significant uncertainties related to the interaction of the hot gas and the surface of the exposed structure. As emphasized by [PERSON] et al. [62], these uncertainties can hardly be reduced. * The temperature within the concrete and the steel elements at all positions at which thermocouples were placed, both in the slabs and the walls, remained below 100 \({}^{\circ}\)C during the first 30 min of the fire test. Thus, the material properties of concrete and steel remained approximately constant at these positions. This was the motivation to consider the temperature-independent and linear-elastic material behavior of concrete. * The numerical simulations clarified that the monitored column was exposed to significantly higher temperatures than the slabs and the walls. This was probably a result of the fact that the columns were positioned above the two heat sources of the furnace. Because of the elevated temperatures, a considerable thermal degradation of the material properties of concrete and steel took place in the columns. * The strain measurements clarified that strains resulting from the thermal loading dominate the total strains soon after the beginning of the fire. This underlines that even moderate fires do represent a considerable threat to the integrity and durability of RC structures. The challenge of the three-dimensional simulation was met in two parts. At first, the non-stationary heat transfer inside the reinforced concrete structure was simulated. The obtained temperature field histories were used as input for subsequent simulations of the load-carrying behavior of the reinforced concrete structure. Two types of thermal boundary conditions were combined with two types of material models for concrete. The following conclusions are drawn from the two different types of heat transfer simulations:  
* The prescription of one uniform temperature history at all exposed surfaces does not produce convincing results. Temperature measurements are particularly underestimated inside the right column and at the midspan of the left cell of the top slab.