Post-Fire Response of Single-Shear Bolted Lap Joints with Threads Included and Excluded from Shear Plane

Abstract

This study investigates the behavior of single-shear bolted lap joints with threads included in the shear plane (N-bolts) and threads excluded from the shear plane (X-bolts) after exposure to fire temperatures. To examine the post-fire performance, the lap joints are subjected to a complete heating and cooling cycle and then loaded until failure. Sixteen single bolted-lap joints are heated up to various targeted temperatures ranging from 400°C to 900°C and then cooled back to 20°C then loaded till failure. Following heating and cooling stages, the specimens are subjected to a direct tension test to determine the reduction in the shear capacity and the slip resistance of the lap joints. Post-fire reduction factors are estimated from the axial load-displacement curves to determine the residual strength of N- and X-bolted lap joints after exposure to fire temperatures. These proposed reduction factors are added to the ANSI/AISC 360 [1] equation for design purposes of bolted connections in shear after exposure to fire temperatures. The results showed that the X- bolted lap joint resulted in larger reduction in bolt shear capacities compared to those with N-bolts. The strength of lap joints with N-bolts started to degrade after 500°C exposure and reached a maximum reduction in shear strength of 40% at 900°C. However, the strength of lap joints with X-bolts started to degrade after being exposed to temperatures beyond 400°C and reached a maximum reduction in shear strength of 42% at 750°C. Finite Element (FE) models were developed to validate the performed experimental program. The models studied the behavior, strength capacities and failure modes of bolted lap joints at post-elevated temperatures while considering the effect of the threads in the shear plane. The results show that FE models predict the force-axial displacement relationships as well as the failure mode of the lap-joints with a very high accuracy at ambient and post-elevated temperatures. This study provides preliminary data that can support the development of design guidelines of bolted connections during and after fire exposure while considering the effect of threads in the shear plane of bolted lap-joints.