Changes in the tensile strength of fasteners under thermal loading and cooling

This study addresses the limited understanding of how thermal exposure and subsequent cooling conditions jointly influence the residual tensile strength and deformation behavior of steel fasteners and welded joints, which is critical for reliable post-fire structural assessment. Current design provisions, including MSZ EN 1993-1-2:2024, do not explicitly incorporate the combined effects of peak temperature and cooling rate on residual strength, ductility, and brittle failure susceptibility under post-fire conditions. An experimental program was conducted in which threaded rods of different strength classes and welded joints were exposed to temperatures up to 1000 °C, followed by controlled slow (air) and rapid (water) cooling. Tensile strength, elongation, and failure modes were systematically evaluated. The results show that degradation is limited up to approximately 400–500 °C, while above 500–600 °C strength reductions become significant, reaching up to 50% at the highest temperatures tested. Rapid cooling generally increases residual tensile strength but leads to reduced ductility and more brittle failure behavior, whereas slow cooling results in lower residual strength but a more ductile response. Among the investigated configurations, V-weld welds demonstrated superior performance, while K-weld welds exhibited lower load-bearing capacity. These findings provide experimental evidence for improving post-fire assessment methodologies, contributing to safer structural evaluation, more accurate rehabilitation decisions, and enhanced resilience of fire-exposed steel connections.