Experimental and Numerical Studies of Reinforced Concrete Columns Confined by Circular Steel Tubes Exposed to Fire

Reinforced concrete columns confined by steel tubes, also known as steel tube–confined reinforced concrete (STCRC) columns, are a kind of composite column in which the outer steel tube acts predominantly as hoop reinforcement. This is achieved by the provision of breaks to the longitudinal continuity of the steel tube. The compressive behavior and seismic performance of STCRC columns have been extensively studied in the last few decades. However, knowledge of the fire behavior of STCRC columns is very limited. Hence, experimental and numerical studies to investigate the response of STCRC columns under combined thermal (fire) and structural loading are presented herein. Four full-scale STCRC columns and one concrete-filled steel tubular (CFST) column were first axially loaded and then subjected to fire until failure. The measured furnace temperatures, specimen temperatures, axial displacement versus time curves, and fire resistance of the columns are presented and discussed. A nonlinear finite-element model employing a sequentially coupled thermal-stress analysis was then developed and validated against recent fire tests on STCRC and CFST columns reported in the literature. Following extensive parametric studies, a simplified method is proposed for predicting the temperatures of the steel tube, reinforcing bars, and concrete. Design rules are then proposed for predicting the load-bearing capacity of STCRC columns exposed to fire, which are consistent with the design method for STCRC columns at ambient temperature.