SCF of bird-beak SHS X-joints under asymmetrical brace axial forces

This research investigated the stress concentration factor (SCF) superposition of bird-beak square hollow section (SHS) X-joints under asymmetrical axial brace forces. Three typical non-dimensional parameters (i.e., β = 0.6, 2γ = 20, τ = 0.5) were employed for tubular joints. Finite element models of conventional, square bird-beak, and diamond bird-beak SHS X-joints were constructed by accurately considering the geometries of welds, and the structural hot spot stresses at weld toes were obtained using the extrapolation method. Numerical results show that, in case of single-side brace axial force, the stresses on the unloaded brace side of bird-beak X-joints are obviously higher the values at the corresponding locations of conventional X-joints, and as a result the SCF errors induced by the existing superposition formula of conventional joints are significant for bird-beak joints. The mechanism was revealed to be the interactions between adjacent walls of the chord by further investigating the compatible deformations of these joints. A modified superposition formula, in which a new load case considering both single-side brace axial force and reversed chord in-plane bending has been employed, was introduced as the solution. The SCF variations with non-dimensional parameters under the new load case were also identified by conducting parametric analysis. The proposed superposition approach gives reasonable predictions of SCF of bird-beak SHS X-joints under asymmetrical brace axial forces.