Creep behavior of Grade 10.9 high-strength bolts under and after fire

Abstract

• Experiments were conducted on creep behaviour of Grade 10.9 high-strength bolts under and after fire. • The Field & Field model is calibrated for simulating creep behavior of high-strength bolts at elevated temperatures. • The temperature, load ratio and loading duration are key factors affecting the creep behavior of T -stub connections. • High-strength bolts have more obvious creep deformation at high temperatures than high-strength steels. • A reduction factor is recommended to consider the creep effect on determining strength of T -stub connections in Eurocode 3. High-strength bolts are widely used in beam-column connections of high-rise steel framed buildings. At the event of a fire, high-strength bolts may experience significant creep in a state of both high stress and high temperature, leading to fracture of connections and even progressive collapse of structures . This paper experimentally and numerically investigates the creep behavior of Grade 10.9 high-strength bolts under and after fire. The creep strain–time curves at different temperatures and stress ratios are obtained using DIC systems. The micromorphology and failure mechanism of high-strength bolts is examined by SEM. The Field & Field creep model is calibrated against test results, and parametric studies are conducted on creep behavior of T -stub connections. The experimental results show that the creep deformation of high-strength bolts is significant at high temperatures, and the creep strain is positively correlated with temperatures and stress ratios. While the creep effect of high-strength bolts after fire can be ignored. The creep-induced fracture of high-strength bolts under fire is in a ductile manner, and that after fire is in a brittle manner. High-strength bolts have more obvious creep phenomenon at high temperatures than high-strength steels, and it is unsafe to directly apply creep data of high-strength steels to high-strength bolts. The numerical results show that the Field & Field model is suitable for characterizing the creep behavior of high-strength bolts under fire with an applicable temperature range of 20–600 °C and stress ratio range of 0.4–0.8. The temperature, load ratio and loading duration significantly affect the creep behavior of T -stub connections under fire conditions. A reduction factor of 0.8 for 500 °C and 0.6 for 600 °C is recommended to consider the effect of creep on the strength of bolts in determining bearing capacity of T -stub connections in Eurocode 3. It is necessary to consider the effect of creep for predicting the fire behavior of high-strength bolts and steel connections, ensuring a safe design.