Experimental Study of the Postfire Mechanical Properties of Grade 14.9 Superhigh-Tension Bolt

Abstract

Fire hazard has been one of the most critical factors to threaten the safety of steel structures. Steel structures may not collapse when suffering fire and cooling down, but the serviceability of building after the fire needs to be evaluated before reusing. Hence, the postfire mechanical properties of steel are necessary. With the high- and ultrahigh-strength steel applied in the construction industry, superhigh-tension bolts (SHTB) are thus encouraged to apply in engineering. However, the current studies on the postfire mechanical properties mainly focus on the Grade 8.8s and 10.9s high-strength bolts. There is limited research work conducted to study the postfire mechanical properties of SHTB. In this study, coupon tests of Grade 14.9 SHTB were conducted, and the postfire performance was evaluated. Failure mode, stress-strain curve, elastic modulus, yield stress, tensile strength, tensile strain, percentage elongation after fracture, and percentage reduction of area were tested and analyzed. The postfire mechanical properties of different types of bolts were compared. The test results indicated that temperatures beyond 750°C cause decrease in the ductility of Grade 14.9 SHTB, regardless of the cooling condition. Accordingly, predictive formulas with good precision are proposed for the postfire mechanical properties of Grade 14.9 SHTB cooled down by air and water, respectively.