AbstractIn this study, the behavior of rectangular hollow-section (RHS) X-joints in compression is investigated, with a focus on the failure mode of chord sidewall failure, which is more likely to occur when the brace-to-chord width ratio is close to 1.0. For joints with a width ratio of 1.0 (or full-width joints), the test-backed numerical analysis in this study clearly indicated that sidesway instability could readily occur even if sufficient out-of-plane restraint is provided. The sidesway instability was found to considerably impair the ultimate strength of full-width joints. A new borderline width ratio is proposed, beyond which the chord face plastification and chord sidewall failure limit states start to interact and should be considered simultaneously in the joint design. This investigation was motivated by a previous experimental observation, wherein the failure mode of the tested joints with a borderline width ratio of 0.85 as per current design standards was essentially the less ductile chord sidewall buckling observed in full-width joints. Based on simple mechanics, an analytical and more rational expression for the borderline width ratio is derived and validated in this study. After existing formulas for the chord sidewall strength were thoroughly reviewed and all the findings and developments were collated, an improved design recommendation that can cover high-strength steels whose yield stress is as high as 700 MPa is suggested for RHS X-joints under brace axial compression.