Large-scale testing and simulation of earthquake-induced ultra low cycle fatigue in bracing members subjected to cyclic inelastic buckling

Special Concentrically Braced Frames (SCBFs) are popular lateral load resisting frames due to their economy, structural efficiency and stiffness. Following the 1994 Northridge earthquake, braced-frames became increasingly common after brittle fractures were observed at beam-column connections in Moment Resistant Frames (MRFs). However, braced-frames are also susceptible to fracture at the middle plastic hinge of the brace, the brace to gusset plate connection, and the gusset plate to column or beam connection. This research primarily focuses on fracture at the middle plastic hinge, where the combined effect of global and local buckling during cyclic loading amplifies the plastic strain at the brace midpoint and initiates fracture. To develop a better understanding of the localized mechanisms affecting brace fracture, this work combines a large-scale experimental program with an intensive simulation study to investigate brace behavior across a wide-range of material types and geometries. The simulations employ continuum-based modeling techniques to accurately reproduce the stress and strain histories during cyclic loading while a novel micromechanical fracture model is evaluated as a means to predict the fracture initiation events. The fracture model operates at the continuum-level and captures the fundamental mechanisms responsible for ductile fracture unique to Ultra Low Cycle Fatigue (ULCF) conditions which differ from the well-defined High and Low Cycle Fatigue (HCF and LCF) mechanisms. From the large-scale brace experiments, cross-section width-thickness and slenderness ratios are shown to influence the brace axial deformation fracture ductility, such that a larger width-thickness ratio and a smaller slenderness tend to reduce ductility. Furthermore, the experiments are used to evaluate the fracture model at the large-scale where small-scale calibration tests and a multi-scale modeling procedure is used to connect the steel behavior at the micromechanical level to the finite element simulation results. The fracture predictions are encouraging considering the high level of complexity in modeling buckling phenomena and imperfect constitutive model behavior. The model is used to expand the experimental test matrix through parametric simulation of square and rectangular bracing components which, along with a synthesis of experimental results over the last twenty years, informs a general relationship between brace ductility and geometry.