Fracture prediction for square hollow section braces under extremely low cycle fatigue

This paper examines the extremely low cycle fatigue (ELCF) fracture of concentrically loaded square hollow section (SHS) braces subjected to cyclic loading. Numerical analyses are presented for both individual bracing members and bracing members integrated into concentrically braced frames (CBFs). The behaviour of the individual members was predicted using solid finite element (FE) simulations that employed a ductile fracture model and a nonlinear damage evolution rule. The solid FE model, which was validated using data from experiments, could adequately predict both the hysteretic response and the ELCF fracture cracking process. The coupled effects of instabilities (i.e. local and global buckling) and fracture on the ELCF performance of the braces were assessed, and the rotation capacity prior to fracture was quantified. This quantified rotation capacity was then incorporated into fibre-based FE models of CBFs as a member-level fracture criterion. The structure-level simulations were able to accurately capture the complex interactions between the frame components, i.e. the columns, beams, brace–gusset–plate​ connections and beam-to-column connections, and hence replicate the overall behaviour of CBFs, specifically, two-storey chevron braced frames. The influence of cross-section and member slenderness was evaluated and the importance of considering both in the development of cross-section slenderness limits was highlighted. The combined member- and structure-level simulation approach is proposed as an accurate and efficient means of assessing the seismic performance of CBFs.