Numerical and experimental study on structural behavior of restrained CHS T-joints in transient fire tests

Experimental and numerical studies have been conducted to investigate the structural behavior of restrained circular hollow section (CHS) T-joints subject to fire. Four CHS T-joints having two different brace-to-chord diameter ratios, 0.47 and 0.69, were tested subject to two kinds of loading: axial brace compression and in-plane bending in fire condition following the iSO-834 fire curve. The two chord ends were connected to reaction A-frames incorporating the thermal restraint effect. The joint behavior was discussed based on maximum deformation, fire resistance and critical temperature. When the joint was under in-plane bending, local plasticization was dominant; the observed failure mode combined global bending and local punching shear when the joint was under brace axial compression. Finite element (FE) models were constructed by incorporating thermal restraints and temperature distributions in the joint regions from the tests. The verified FE models were then adopted to predict the critical temperature of other CHS T-joints where brace-to-chord diameter ratio, load ratio and thermal restraints were taken as variable parameters. Unlike T-joint at ambient temperature, a larger brace-to-chord ratio resulted in a greater critical temperature. Moreover, greater load ratio and thermal restraint caused decrease in fire resistance and critical temperature of CHS T-joint subject to both axial compression and in plane bending.