Structural behaviour and design of high strength steel RHS X-joints

Abstract

This paper aims to investigate the structural behaviour and static strength of high strength steel rectangular hollow section (RHS) X-joints under axial compression in the braces through tests and numerical analysis. Eight RHS X-joints which were composed of fabricated steel tubes with a measured yield stress of 907 MPa were tested. Extensive numerical simulations on the fabricated RHS X-joints in S460, S690 and S960 steel were conducted using finite element (FE) analysis. The FE model was validated against the test results. The investigated failure modes are chord face plastification, chord side wall failure and a combination of these two failure modes. The effects of the heat affected zones (HAZ) and suitability of the strength equations adopted by the CIDECT design guide for the fabricated RHS X-joints were examined. The deformation capacity and ductility of test specimens which failed by chord face plastification could be considered as reasonably sufficient. The effects of material strength reduction in the HAZ on the joint initial stiffness are minor, but could significantly lower the joint strength. In general, the CIDECT strength prediction is increasingly unconservative with increasing steel grade for the RHS X-joints failing by chord face plastification. However, the CIDECT strength prediction is generally conservative for the combined failure modes, and becomes increasingly conservative with increasing chord side wall slenderness for chord side wall failure. The suggested ranges of brace to chord width ratio (β) and chord width to wall thickness ratio (2γ) are 0.4 ≤ β ≤ 0.85 and 2γ ≤ 60β − 1 for the RHS X-joints failing by chord face plastification to allow for more effective use of high strength steel, and corresponding strength equations were proposed. An analytical model of plate buckling was proposed and the deformation-based continuous strength method (CSM) originally developed for designing non-slender stainless steel cross-sections was adopted for the design of chord side wall failure in the RHS X-joints with β = 1.0 and 2γ up to 50. The proposed design method is also applicable for designing chord side wall failure in equal-width RHS X-joints using cold-formed and hot-finished carbon steel and cold-formed stainless steel. A linear interpolation approach using the proposed strength equations at β = 0.85 and β = 1.0 is suggested for the RHS X-joints with 0.85 < β < 1.0 and 2γ ≤ 50 which failed by the combined failure modes. The proposed strength equations can produce much more accurate and consistent strength prediction than the CIDECT design guide, and were converted to design strength equations for the design of high strength steel RHS X-joints.