Representative strain-based fatigue and fracture evaluation of I-shaped steel bracing members using the fiber model

To study the fatigue and fracture behavior of I-shaped steel bracing members, numerical simulations based on OpenSees for 18 Q235 and 21 ST12 welded steel braces under constant-amplitude cyclic axial displacement were conducted. As the numerical results are consistent with experimental data, a representative strain, which is modified by correlative parameters (including slenderness ratio, width-to-thickness ratio, height-to-thickness ratio and yield stress) of the brace and calculated by corresponding equations, has been proposed as a new control parameter to predict the fracture life of bracing members under low-cycle fatigue loads. According to the good logarithmic—linear relationship between the representative strain and the fracture life, the predicted fracture lives of all specimens were almost always located within a safe scatter band of 1.5. Moreover, the miner method was used for calculating the cumulative damage of 39 bracing members. An allowable damage factor was discussed to assess the fatigue damage and to predict the fracture life under a variable-amplitude cyclic load. Then, a simulation of 5 single diagonal and 8 inverted-V braced frame tests was conducted. It was found that the predicted fracture lives of all these braced frames are very close to their true lives shown in tests and are mostly located within a safe scatter band of 1.5, indicating that the simulation can provide a relatively accurate evaluation on the fatigue performance. In addition, the damage distribution and damage development of the brace section in simulations are in good agreement with the failure mode in the physical tests, which can generally correspond to the cracking process and the complete fracture behavior of the bracing members.