The effects of heating and cooling of steel in fire are similar to tempering and annealing. The mechanical properties of steel after exposure to fire will, therefore, be different from the initial properties. High-strength bolts, which are widely used in steel structural connections, are more temperature-sensitive than hot-finished steel because the manufacturing process of high-strength bolts consists of cold drawing, annealing, cold forging and heat treatment. A number of experiments have been carried out on the mechanical properties of high-strength bolts at elevated temperatures, but very few studies have focused on the mechanical properties of high-strength bolts after fire. To obtain the mechanical properties of high-strength bolts after fire, including stress-strain curves, strength and elastic modulus, a series of experiments on 10 . 9S bolts and 8 . 8S bolts was carried out. In order to simulate real situations in the event of fire, both natural cooling and water cooling were employed in the experimental study. The results show that heating and cooling has a great effect on the mechanical properties of high-strength bolts after fire and that the effects of natural cooling are very different from those of water cooling.

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