Failure Behavior of Axially Loaded Tubular Y-Joints under Fire

The failure behavior of axially loaded circular hollow section (CHS) tubular Y-joints under fire is investigated. As the plastic yielding at the intersection of the chord and the brace is the most possible failure mode, the yield line theory is adopted to evaluate the joint ultimate capacity. A commercial finite element (FE) package ABAQUS is used to simulate the joint behavior. A sequentially coupled thermal-stress analysis is activated to analyze the stresses of the joint at higher temperatures. The FE model of the joint has been verified against the experimental results of the CHS tubular T-joints under fire. The numerical analysis showed that the joint failed at the location of the intersection between the chord and the brace. The type of failure modes is dependent upon the geometric parameters (theta, beta and gamma) and the load ratios (n) of the joint. The critical temperature and the fire resistance limit of the joint under various brace loads are also investigated. It is found that the axial reaction of the chord due to thermal expansion of steel material is greatly larger than that caused by applied load, which seriously affects the fire resistance of the joint.