Seismic response of steel moment connections to built-up box columns with varied diaphragm designs

This study investigates the seismic behavior of steel moment connections between I-shaped beams and built-up box columns incorporating internal, through, and external diaphragm configurations. These connection types were evaluated through full-scale cyclic tests and nonlinear finite element analysis (FEA). The experimental program assessed cumulative ductility, energy dissipation, and failure mechanisms, revealing that diaphragm placement critically influences plastic hinge formation and damage concentration. Specimens with external diaphragms demonstrated superior ductility and energy dissipation, while internal and through diaphragm configurations exhibited brittle fracture near the column face. Fractographic and metallographic analyses confirmed that failure was driven by geometric detailing rather than weld defects or material inconsistencies. A unified multilinear envelope curve was proposed to model the force-deformation behavior of these connections, addressing gaps in current design standards. The findings offer practical recommendations for diaphragm detailing and nonlinear modeling of box-column connections under seismic demands.