On the out-of-plane buckling of RHS X-joints – Analytical and numerical approaches

Abstract

Out-of-plane buckling (OPB) is not widely known failure mechanism in tubular X-joints with equal brace and chord widths (β = b1/b0 = 1.0) but it can occur in the chord member of tubular X-joints axially compressed by the brace members. The OPB consists of distortional and torsional deformations in the chord member, but only the distortional mode is involved in elastic energy of the system. In this work, an analytical formulation for the critical OPB capacity is established by the deformed shape in a beam on elastic foundation (BEF). A theoretical model for the ideal-elastic buckling capacity is computed by considering the in-plane bending of the chord faces and elastic bending of the cross-section as the frame structure. The obtained critical buckling loads are compared with numerical results obtained using linear buckling analyses by finite element (FE) models. The theoretical model provided a reasonable agreement with the numerically obtained eigenvalues with a tendency to slightly underestimate load-carrying capacity. The FE results showed that the OPB of X-joint can be the critical ideal-elastic failure mode in the wide range of various joint geometries and thus be regarded as a competitive failure mechanism for the widely known chord sidewall buckling. However, the results were obtained for the ideal-elastic buckling modes and, eventually, the actual failure mode is highly depended on the existing geometrical imperfections. Nevertheless, experimental tests have also indicated an existence of the OPB mode, and structural designers and analysts of tubular connections should be aware of this potential failure mode.