Stress concentration factors of bird-beak SHS X-joints under brace axial forces

Bird-beak joints are a new type of welded square hollow section (SHS) joints and their fatigue behaviors are of the most attentions recently. The stress concentration factors (SCFs) of both square and diamond bird-beak X-joints under brace axial forces were systematically investigated in this research by using experimental and numerical methods. Four specimens including two square bird-beak X-joints and two diamond bird-beak X-joints were tested. Elastic strain distributions within crown and saddle areas were measured, and the strain concentration factors (SNCFs) at specified hot spots were obtained by using the quadratic extrapolation approach. Refined finite element models, whose accuracies have been validated by experimental data, were developed to parametrically simulate the stress concentrations at weld toes of such innovative joints. The influences of three non-dimension parameters (i.e., brace/chord width ratio β, chord wall slenderness ratio 2γ, and brace/chord wall thickness ratio τ) on SCFs of bird-beak X-joints were revealed. Comparison also shows that the saddle areas commonly contain the highest SCFs within whole joint, and that square bird-beak X-joints provide lower stress concentrations than diamond ones in case of identical non-dimensional parameters. Based on numerous results from parametric analysis, design formulas were finally proposed to calculate the SCFs at typical hot spots of both square and diamond bird-beak X-joints subjected brace axial forces.