Influence of Loading Rates on Single Shear-Bolted Lap Joints in Fire

Abstract

The objective of this research is to investigate the effect of loading rates or implicit creep on the strength and deformation capacities of single shear-bolted lap joints and shear tab connections at elevated temperatures. To address this issue, sixteen single shear-bolted lap joints were tested under two different loading rates for temperatures ranging from 400°C to 700°C. The rate- and the temperature-dependent retention factors for the bolt shear capacities were presented and compared with previous studies available in the literature. The effect of loading rate and temperature on the bolt pretension force were also examined. The results show that all tested bolted lap joints failed in bolt shear. Also, the results of the slow loading rate tests showed a larger reduction in the bolt shear capacities when compared to the fast tests for temperatures larger than 400°C. That is, the effect of loading rate on the bolt shear capacity ranges from 18% to 36% difference for temperatures ranging from 450°C to 700°C, respectively. Finally, a strength reduction coefficient was introduced in the bolt shear capacity equation to account for the loading rate effect when designing bolted connections in fire.

Finite Element (FE) models were developed and validated against the performed experimental program and others available in the literature. The models studied the behavior, strength capacities and failure modes of bolted lap joints and shear tab connections at elevated temperatures with and without the thermal creep effect. The results showed that the thermal creep effect changes the failure mode and decreases the bolt strength capacities of bolted connections at elevated temperatures.

Finally, a step-by-step fire design procedure and an example of an isolated shear tab connection were presented. The design includes the thermal creep effect by incorporating the developed strength reduction coefficient in the bolt shear capacity equation. The outcome showed that adding the effect of thermal creep decreases the capacity of the connection by 40% at 500°C.

This research shows that considering the effect of thermal creep when designing bolted connections during fire is essential for structural fire-engineering applications. Including this effect provides more precise prediction of their behavior.