Initial stiffness, ultimate capacity and failure mechanisms of tubular X-joints with external ring or plates at fire conditions

This research work evaluates the effect of the ring, the plate and the joint geometry on the initial stiffness, ultimate strength and failure mechanisms of tubular X-joints subjected to axially compressive load at different fire conditions. At the first step, a finite element model was generated and verified by 19 experimental tests and 8 numerical models carried out by the present first author and other researchers. At the next step, 1170 numerical models were conducted. Using the generated FE models, the structural behaviour under different fire conditions (200°C, 400°C, 600°C and 800°C) was evaluated and compared to the corresponding behaviour at normal temperature (20°C). Results showed that both of the external ring and plate can notably increase the initial stiffness and ultimate capacity at all different elevated temperatures. Also, the use of these methods significantly improves the failure mechanisms at fire conditions. Two theoretical design formulas with high accuracy were proposed, based on the yield volume model, to determine the ultimate capacity in ring reinforced X-joints and plate reinforced X-joints at elevated temperatures.