Structural behaviour and design of chord plastification in high strength steel CHS X-joints

Abstract

This paper aims to investigate the structural behaviour and static strength of high strength steel circular hollow section (CHS) X-joints under axial compression in the braces. Extensive numerical simulations on the CHS X-joints using S460, S700, S900 and S1100 steel were carried out. The failure mode of the CHS X-joints investigated is chord plastification. Effects of heat affected zones on the initial stiffness and static strength of the CHS X-joints are found to be relatively insignificant. Suitability of the mean strength equation adopted by the CIDECT design guide for the CHS X-joints was evaluated against results obtained from the numerical simulations in this study and experimental tests in the literature. In general, the CIDECT mean strength prediction is slightly unconservative for CHS X-joints in S460 steel and becomes increasingly unconservative with increasing steel grade. This is because the improved yield stress of high strength steel generally could not be fully utilised in the CHS X-joints mainly due to the adopted indentation limit i.e. 3% of chord diameter. The recommended ranges of chord diameter to wall thickness ratio (2γ) are 2γ ≤ 40 for steel grades ranging from S460 to S700 and 2γ ≤ 30 for steel grades greater than S700 and up to S1100 to allow for more effective use of high strength steel. The suggested range of brace to chord diameter ratio (β) is 0.2 ≤ β ≤ 1.0 for steel grades ranging from S460 to S1100, which is the same as the current CIDECT validity range of β ratio. A mean strength equation was proposed for the CHS X-joints with 2γ and β ratios which are within the suggested ranges. The statistical analysis shows that the proposed mean strength equation can produce reasonably accurate and consistent strength prediction. The proposed mean strength equation was converted to a design strength equation for the design of high strength steel CHS X-joints.