ISO 834 standard fire test and mechanism analysis of square tubed-reinforced-concrete columns

The tubed-reinforced-concrete (TRC) column is an innovative steel-concrete composite column and its steel tube is terminated at beam-to-column connections to mainly work as hoop reinforcements without sustaining axial load directly. Fire performance of TRC columns differs from that of concrete-filled steel tubular (CFST) columns since the axial deformation behaviour of the TRC columns would mainly depend on the inner reinforced concrete and local buckling of steel tube is minimised. However, no research has been reported on the behaviour of square TRC columns under fire exposure. Five slender square TRC columns subjected to standard fire and axial loading were tested in this study and the effects of load ratio and load eccentricity were investigated. Failure mode of the test specimens was dominated by global flexural buckling, whereas tube local buckling was also observed. The experimental results show that load ratio has a significant influence on the fire resistance of test specimens while the influence of load eccentricity is marginal. A sequentially-coupled thermo-mechanical finite element analysis (FEA) model was developed using ABAQUS. This FEA model was validated well against the test results when using the measured column end rotations as realistic boundary conditions. Different from the case of a CFST column, the axial load applied to a TRC column in fire is mainly sustained by the concrete and reinforcing bars and the high-temperature capacity contribution of steel tube is neglectable. With the increase of exposure time, the applied load gradually transfers from concrete to reinforcements until the yielding of re-bars.