Seismic performance and working mechanism of innovate bottom-flange-bolted type through-diaphragm connections

Abstract The traditional welded-flange through-diaphragm CFST column connections may develop fracture failures at bottom flanges. The bottom-flange-bolted upper-flange-welded (BFB-UFW) connection adopts a bolted connection between the bottom flange and the bottom diaphragm to reduce the early fracture risk. However, the asymmetric connection design would cause different moment bearing mechanisms and then influence the seismic performance of the connection. In this study, cyclic tests were performed on two BFB-UFW connections and one traditional welded-flange connection to investigate detailed working mechanisms and the influence of the bottom bolts on seismic performances of the connection. The BFB-UFW displayed good deformation abilities with the maximum drift ratio being higher than 5%. The bolted bottom flange connection could alleviate the deformation demand on beam flanges through relative plate sliding behaviors. Bolt slips in the bottom flange connection led to pinched hysteretic loops. The BFB-UFW connection with a strong bottom flange connection exhibited higher negative strengths than the positive bending strengths due to the Poisson effect and the asymmetric supporting effect from the bottom diaphragm. The BFB-UFW connection belongs to the semi-rigid full-strength type. Therefore, this connection can be applied to a moment resisting frame, but the stiffness, strength and ductility of the connection must be considered in structural design.