Bolted and welded connections in ultra-high strength steel at elevated temperatures and post-fire condition

Abstract

The collapse of World Trade Center (WTC) building illustrated that the fracture of connections can lead to a progressive collapse of steel frames. It is necessary to estimate the strength of a connection made with welds and bolts in a variety of fire situations. This thesis presents an assessment of the applicability of the American, European and Australian/New Zealand design codes, which are primary for carbon mild structural members for high temperature behaviour of ultra-high strength steel having yield stress greater than 1150 MPa. It is found that the predictions given by those design specifications are too optimistic for the strength of ultra-high strength steel at elevated temperatures. The laboratory test results of ultra-high strength steel cooled from temperatures as high as 1000 ℃ are also presented and compared with the post-fire material models for mild/high strength steels reported in literature. It highlights that the ultra-high strength steels have more drastic and severe strength reduction compared to other steel grades. This thesis presents the laboratory test results of bolted connections in ultra-high strength sheet steel under different fire scenarios, in addition to ambient temperature. It shows that when ductility of the steel material is extremely limited, the specimens tend to be failed in the inclined tension fracture, block shear by simultaneous tensile and shear rupture, and material piling-up that have seldom been reported in the literature. It also shows that the material ductility has a much more significant effect on the bearing and block shear strengths, which are closely associated with the strain hardening in the shear zone. The catenary effect that increases the shear-out strength is identified in bolted connections tested at high temperatures in which the ductility increases significantly. Finite element analysis is used to investigate the phenomenon observed in the preceding paragraph. The tensile and shear damage criteria developed by Ahmed et al. (2019) are included to simulate fracture in bolted connections, thereby being better able to replicate the experimental responses including the material softening in the load-deflection curve than the conventional finite element models reported in the literature. This thesis demonstrates that elongation at fracture is the most relevant parameter of material ductility for a structural steel connection, in contrast of the belief of the literature that considers the ratio of the tensile strength to the yield stress. As elongation at fracture increases, the normal stresses over net section area become uniform in contrast to the assumption of Australian/New Zealand design code for the net section strength. At high temperature, the average shear stress can be achieved up to 0.8 F_u rather than 0.6 F_u or F_y used in the current design codes for the block shear strength. This thesis investigates the accuracy of equations specified in the American and European codes for determining the strength of fillet welded connections in ultra-high strength steels. It shows that the ultra-high strength steel specimens encountered material softening more severely than conventional steels, so that the fracture took place in different angles in weld metal from the theoretical assumption.