Seismic behavior of bottom-flange-bolted type through-diaphragm connection considering the slab effect

In composite beam connections, floor slabs are bonded to the upper flange of supporting beams, which leads to an upward shift of the neutral axis and an increased deformation demand at the bottom flange. Hence, premature fracture failures at the bottom flange were prevalent in past earthquake disasters. The bottom-flange-bolted upper-flange-welded (BFB-UFW) through-diaphragm CFST column connection employs the bolted bottom flange connection instead of the traditional welded flange design to reduce the risk of fracture failures. In this study, cyclic loading tests were performed on BFB-UFW connections with composite slabs to investigate the combined effect of asymmetric beam flange connections and composite slabs on seismic performance. The results indicated that the BFB-UFW connections provided pinched hysteretic relations that caused by plate sliding behaviors at the bottom flange connection. The bare beam connection displayed a higher negative moment strength compared to the positive one. The composite slab imposed a higher strengthening effect on the upward bending strength compared to the downward one. The two asymmetric strength developments complemented each other, thereby leading to similar levels of upward and downward bending strengths in the composite beam connections. Under large drift ratio rotations, buckling tendencies of upper beam flanges were restrained by the composite concrete slab. The bolted bottom flange connection allowed a large deformation capacity through the relative slips of the bottom flange, which then minimized the plastic strains and stress concentrations at the bottom flange. Therefore, composite beam BFB-UFW connections displayed good deformation ability with a steady strength increase before the 0.04 rad drift ratio rotations. The specimen with a weak bottom flange connection displayed the early initiation of plate sliding and increased rotation concentrations near the column.