Mechanical Properties and Stress–Strain Relationship of Grade 14.9 Superhigh-Tension Bolt (SHTB) Under Fire

Grade 14.9 superhigh-strength bolts (SHTBs) are a type of high-strength steel bolt with a nominal tensile strength of 1400 MPa, which is significantly higher than the commonly used Grade 10.9 high-strength bolt (HSB), which has a nominal tensile strength of 1000 MPa. The use of an SHTB can reduce the number of bolts required in connections or joints, leading to material savings and improved construction efficiency. However, like HSB, the mechanical properties of an SHTB can be significantly degraded at high temperatures, though the extent of this reduction may differ. In this study, the authors designed and conducted experiments on SHTBs under elevated temperatures including both vibration and tensile coupon tests. Based on the test data, the stress–strain curves and key mechanical properties such as the Young’s modulus, yield stress, ultimate stress, ultimate strain, percentage elongation, cross-sectional area reduction, and failure strain were obtained and analyzed for various high-temperature conditions. Furthermore, a new three-stage model was proposed to describe the stress–strain relationship of SHTBs under fire conditions. Additionally, empirical formulae were developed to predict the mechanical properties of SHTBs under elevated temperatures, providing valuable insights for engineering applications and fire safety design.