Behaviour of bird-beak square hollow section X-joints under in-plane bending

An experimental investigation was conducted on bird-beak square hollow section (SHS) X-joints under in-plane bending. A total of nine specimens including traditional, square bird-beak and diamond bird-beak SHS X-joints were tested. The test results of joint strengths, failure modes, axial load-vertical displacement curves, bending moment-local deformation curves, bending moment-rotation curves and load-strain distribution curves of all specimens were reported. The corresponding finite element analysis (FEA) was also performed and calibrated against the test results. Therefore, an extensive parametric study was carried out to evaluate the effects of geometric parameters (β, τ and 2γ) on the behaviour of bird-beak SHS X-joints under in-plane bending. It is shown from the comparison that the ultimate loads and initial flexural stiffness of all specimens under in-plane bending increased with the increase of the β ratio. Furthermore, the ultimate loads and initial flexural stiffness of traditional, square bird-beak and diamond bird-beak SHS X-joints under in-plane bending increased progressively with the same β ratio, whereas the corresponding ductility decreased progressively. On the other hand, the joint strengths obtained from the test and parametric study were compared with the design strengths calculated using the current Eurocode 3, IIW Specification and Christitsas's equation. It is shown from the comparison that the current Eurocode 3 is quite conservative for the traditional SHS X-joints under in-plane bending, whereas the current IIW Specification and Christitsas's equation are generally appropriate for the traditional and square bird-beak SHS X-joints under in-plane bending, respectively. The new design equations were proposed for the square bird-beak and diamond bird-beak SHS X-joints under in-plane bending, which were verified to be accurate.