Applicability of 14.9 grade high strength bolt: Performance of material and connection

The newly developed Grade 14.9 ultra-high-strength bolt (F14T) exhibits a tensile strength approximately 200 MPa higher than that of conventional Grade 12.9 bolts. To apply this bolt to joints in bridges and steel structures, a comprehensive evaluation of its mechanical properties and joint behavior is essential. In this study, the mechanical performance of Grade 14.9 bolts was experimentally investigated through tensile tests, shear tests, wedge tensile tests, and nut proof load tests to verify their applicability to real structural systems. In addition, torque coefficient tests were conducted to confirm the uniform introduction of bolt pretension based on the linear relationship between applied torque and clamping force. Slip-critical tensile tests were performed to examine the load-displacement response of bolted joints with inorganic zinc-rich coatings. The results revealed a three-stage behavior consisting of friction-dominated resistance, slip initiation accompanied by stiffness degradation, and subsequent load recovery attributed to bolt-hole bearing. Based on the experimental results, a nonlinear stress-strain model for Grade 14.9 high-strength bolts was proposed, and key design parameters including the shear strength ratio and slip factor were derived. This study addresses a significant gap in current design provisions, as mechanical property requirements for Grade 14.9 bolts have not been established in major international standards such as EN and KDS, and the derived parameters are intended to serve as a reference for the future development of standardized provisions for ultra-high-strength bolted connections.