# Numerical Analysis of a Moderate Fire inside a Segment of a Subway Station
## 1 Introduction
The obtained temperature field histories are subsequently used as the basis for thermomechanical analyses of the load-carrying behavior of the tested structure during the fire test. The specific challenges of these two types of simulations refer to:  
* the boundary conditions required for the analysis of the heat transfer problem and
* the material behavior of concrete, subjected to both mechanical loading and elevated temperatures. Both items involve significant uncertainties. This provides the motivation for corresponding sensitivity analyses. They are described in the following. Regarding the thermal boundary conditions, the first approach is based on prescribing one specific temperature history at the entire interior surface of the structure. The prescribed temperature evolution is set equal to the average of the readings of two temperature sensors, which were positioned at a distance of 2 mm from the heated surface. One sensor was located inside the top slab and the other one inside the right wall. In the second approach, the heated inner surface of the structure is subdivided into three sub-regions. Each of them is subjected to a uniform distribution of a specific temperature history. The three required temperature histories are selected, in the context of model updating, such that satisfactory agreement between simulation results and temperature measurements is obtained. The material behavior of concrete is simulated as either temperature-independent and linear-elastic or temperature-dependent and elasto-plastic. For the latter simulation, the Concrete Damaged Plasticity (CDP) model of Abaqus FEA is used [42]. Thereby, the nonlinear constitutive behavior of concrete and steel, including their temperature-induced degradation, agree with the recommendations by the Eurocode 2 [43] and the fib Model Code [44]. Results obtained from the described structural sensitivity analyses will allow for an assessment of the relative importance of thermal boundary conditions and the material behavior of concrete. This is important for structural engineers whose research efforts are devoted to dealing with large uncertainties that still exist in this research area. In addition, the simulation results will allow for providing recommendations regarding the inspection of RC structures after non-catastrophic fire events. The present paper is organized as follows. In Section 2, experimental data from the scaled fire test of an underground reinforced concrete structure are presented. In Section 3, transient simulations of the non-stationary heat transfer in the reinforced concrete structure are described. This includes sensitivity analyses with two different types of thermal boundary conditions. In Section 4, the load-carrying behavior of the structure during the fire test is analyzed, based on thermomechanical numerical simulations. This includes sensitivity analyses with two different types of material models for concrete. In addition, the two temperature field histories of Section 3 serve as input. Thus, altogether, four structural simulations are described. The comparison of simulation results and experimental measurements focuses on two time instants: (i) the time instant right before the fire test (at that time, the structure was already subjected to mechanical loading) and (ii) 30 min after the start of the fire test. Finally, Section 5 contains a summary and conclusions.