Nonlinear finite element analysis of transient thermal field for planar reinforced concrete frame in fire

A nonlinear finite element program is proposed for determining the three-dimensional (3D) transient thermal fields in reinforced concrete (RC) frame subjected to fire. Physical nonlinearity, such as nonlinear variation of material properties with temperature, is considered in this program, but spalling of high-strength concrete in fire is not included. The validation of the program is examined using experimental and calculation results provided by other researchers. The 3D transient temperature fields in a single-span one-storey RC frame subjected to fire with ISO834 standard heating process and those in a two-span three-storey RC frame subjected to fire with ISO834 standard heating process and with different fire locations are, respectively, determined using the proposed program. Simulation results indicate that along the longitudinal directions of structural members, little change in the thermal fields occurs except for the zones close to the joints. In this way, the 2D model is accurate enough to predict the thermal fields in the most mid-portion of the structural members, but the 3D model should be adopted to predict the thermal fields at joints and in the zones around the joints. For convenience, a simple but reliable analysis method is put forward for determining the 3D transient thermal fields close to joints in RC frame.