Experimental investigation on the tensile behaviour of welded RHS high strength steel X-joints

The newest version of prEN 1993-1-8 (2022) prescribes a material factor (C_f) to reduce the design resistance of welded joints made of high strength steel (HSS) mostly due to the lack of available experiments, given the less ductility of HSS compared to mild steel. Additionally, it is stated that the material design yield strength should not exceed 0.8 times the ultimate strength (f_u) for the chord punching shear failure and the tensile brace failure. The mechanical background behind C_f and the 0.8f_u restriction for different types of joints and loading conditions is vague. In this paper, the validity of C_f and the 0.8f_u restriction is investigated experimentally by considering 18 welded rectangular hollow section X-joints tested in tension. A bi-linear model, which is suitable for an elasto-plastic global analysis considering the post-yielding stiffness, is proposed to characterize the nonlinear behaviour of the joint. The predicted resistance and failure mode, with and without considering the C_f and/or the 0.8f_u restriction, are compared to the experimental results. In addition, the predicted resistance corresponding to the experimental failure mode is investigated. It is concluded, based on the tested joints in this paper and literature, that C_f and the 0.8f_u restriction are not necessary for the design according to prEN1993-1-8. However, the predicted brace failure resistance is unconservative for tested joints that failed by brace failure if C_f for S700 or the 0.8f_u restriction for all steel grades is not considered.