Experimental study on axial compression properties of cast steel spherical joints at elevated temperatures

Cast steel spherical joints address the challenges inherent to welded hollow spherical joints at multi-member intersections, such as welding difficulties, high residual stresses and stress concentrations. The failure of the joints during a fire could precipitate a progressive collapse of the entire structure. To evaluate their fire resistance, this study conducted axial compression tests on cast steel spherical joints under elevated temperatures. Failure modes and load-displacement curves were recorded to analyze the yield strength, ultimate strength, ductility coefficient, and initial stiffness of the joints. A parametric analysis was performed to assess how temperature, steel grade, geometric parameters of joints and tubes, and ribs influence the joints' ultimate bearing capacity and initial stiffness. Predictive formulas were derived for calculating the ultimate bearing capacity and initial stiffness of cast steel spherical joints under elevated temperatures. Experimental results indicate that as the temperature increases from 20 °C to 500 °C, the ultimate bearing capacities of G20Mn5N and G20Mn5QT joints decrease by 36.6% and 39.3%, respectively, while their initial stiffnesses are reduced by 48.7% and 44.0%. Based on thin-shell theory, the equation for initial stiffness of joints was revised.