Net section tension strength of bolted connections in ultra-high strength sheet steel during and after fire

This paper investigates the net section tension strength of bolted connections in ultra-high strength sheet steel (yield stress greater than 1000 MPa) under various fire scenarios through laboratory testing. The specimens are tested at room (ambient) temperature and elevated temperatures up to 700 °C, and are subjected to air or water cooling from temperatures as high as 1000 °C. The degradations of the tensile strength at elevated temperatures are much more severe than design code predictions. Substantial recovery of the tensile strength is feasible using water cooling if the steel has been exposed to 1000 °C, by almost 90%. Based on finite element analyses incorporating tensile fracture simulation, the paper determines that elongation at fracture is the most relevant parameter of material ductility for a structural steel connection. Tension fractures in very low ductility materials tend to be inclined to the loading direction of the bolted connection. This paper also examines the alternative formulae for determining the net section tension strength of a stagger bolted connection. Their accuracy is verified through the use of a certain geometry, which eliminates any potential errors due to the use of incorrect material tensile strength in the calculations. Cochrane's original formula is confirmed to be significantly more accurate than the simplified version used in the literature and some design codes, for the tested specimens.