Critical Temperature of Axially Compressed Square CFST Column Exposed to Standard Fire

Nowadays, concrete-filled steel tubular (CFST) with square cross section has been widely used in high-rise buildings. Numerous studies have been carried out on the excellent performance of square CFST columns under fire, both experimentally and numerically. Current codes of practice provided fire resistance calculation methods for structural steel members are based on the load bearing capacity and the critical temperature. However, there are only equations to predict the fire resistance of CFST members using methods based on load bearing capacity. For the convenience of application, it is necessary to figure out a simplified solution for the fire safety design of square CFST columns under axial compression according to temperature information. To investigate the temperature of square CFST columns under fire, experimental data of 131 CFST specimens under axial compression were collected. The width of cross sections varied from 140mm to 630mm; the width-thickness ratio of steel tube varied from 16 to 66; the yield strength of steel tube varied from 235MPa to 785MPa; the compressive strength of concrete cylinder varied from 15MPa to 173MPa. Most specimens were exposed under ISO-834 or ASTM-E119 standard fire, while some were heated at a constant rate. A finite element (FE) model was developed in ABAQUS/Standard to analyse the fire resistance of square CFST columns under axial compression. The model was consisted of a thermal analysis module and a mechanical analysis module. The FE model was validated by experimental results in the test database. The mechanism and the development of temperature field of square CFST columns under fire was investigated. Discussions were made on representative temperatures, and the critical temperature was determined for square CFST columns exposed to standard fire. The parametric study was conducted to clarify the effects of different variables on the critical temperature, including the axial load ratio, the width and the width-to-thickness ratio of steel tube, the material strength and the slenderness ratio. The equation of critical temperature was then proposed according to FE results. The critical temperatures by the simplified equation were compared with those by FE models and experiments. The results indicated that the proposed equation was capable of predicting critical temperatures of square CFST columns under axial compression with reasonable accuracy.