Effects of material ductility and cooling methods on the bearing strength of steel bolted connections

This paper first investigates the material ductility effect on the bearing strength of bolted connections in mild steel (S275), high strength steel (G550) and ultra-high strength steel (S1150), the latter having never been tested in the literature. Importantly, it also investigates the effects of different cooling methods on the post-fire bearing strength of ultra-high strength steel specimens. The specimens were cooled in either air or water from temperatures as high as 1000 °C, resulting in a very wide range of material ductility. It was found that material ductility (and therefore the cooling methods for particular exposure temperatures) had a significant effect on the bearing strength, namely the bearing coefficient increased considerably with a particular combination of exposure temperature and cooling method. Either air or water cooling may lead to the higher material ductility, depending on the exposure temperature. An additional finding is that the elongation at fracture is a more appropriate parameter of material ductility than the ratio of tensile strength to yield stress, for bolted connections failing in bearing. A bolted connection in the steel with a greater elongation at fracture experiences more extensive shear yielding before the bearing fracture occurs, and therefore has a higher bearing coefficient. This situation is not necessarily true for a bolted connection in the steel with a higher ratio of tensile strength to yield stress. The paper also discusses an equation that is reasonably accurate for estimating the combined bearing and end-splitting strength of a serial bolted connection.