Longitudinal shear performance of lightweight steel-UHPC composite connections based on large-diameter high strength friction-grip bolts

In order to improve the construction efficiency and reduce the self-weight of composite bridges, prefabricated thin UHPC (ultra-high performance concrete) decks can be connected to steel girders by high-strength, large diameter friction grip through-bolts. This paper presents a pilot experimental study into the shear performances of such composite connections. Thirty push-out tests were carried out to observe the influences of bolt diameter, slab thickness, bolt strength, pretension force and random asymmetric clearance of the bolt within the slab and joist. The observed failure modes include bolt fracture, pronounced permanent flexure of the bolts due to extensive yield, and crushing/splitting of the UHPC. 36% and 25% increases in diameter (from 22 to 30 mm) and tensile resistance (from 795 to 995 MPa) in 55 mm thick slabs led to 43% and 11% improvement respectively in connection shear resistance. Moreover, a bolt pretension force increase from 150 to 200 kN and a slab decrease in thickness from 55 to 35 mm respectively changed the failure mode from bolt bending to bolt fracture and reduced the shear resistance by 15%. Random asymmetric clearances of the bolts between 0.17 and 5.43 mm led to 88%-100% reduction of connection stiffness at the first slip stage and decreased shear resistance by up to 11%. Regression analysis shows that connection shear strength, stiffness and ductility are mixed polynomial and exponential functions of the key connection variables.