Experimental study on stress relaxation properties of structural cables

Cable stress relaxation causes tension loss at high stress in high-strength structural cables, thereby influencing the stiffness and strength of cable-supported or tension structures. Previous studies have mainly focused on steel wires (SWs) and spiral strands; however, several new types of structural cable, such as semi-parallel wire strands (SPWS), Galfan spiral strands (GSSs), and steel tie rods, have emerged and have been widely applied in recent decades. The axial force during normal service state occasionally reaches 55% of the ultimate tensile strength. Relaxation data on structural cables are rare and are urgently required for structural design and lifetime safety evaluation. Therefore, experimental studies on five types of structural cable were conducted, in which the initial stress levels were 70%, 55%, and 40% of the ultimate tensile strength. Three different working ambient temperatures of 15 ± 2 °C, 20 ± 2 °C, and 25 ± 2 °C were considered. Special onsite anchoring methods and anchorage devices were established and proven effective. Results showed that logarithmic expression could well express the relationship between relaxation rate and time duration. The relaxation rates of SPWSs were larger than those of SWs. GSSs had considerably larger relaxation rates than other cable types due to their material properties and special twist characteristic. The relaxation rates of all types of cable increased with the initial stress and temperature. The recommended values for the 50-year relaxation rates for different structural cables were predicted and calculated.