Prestress loss of twisted wire strands and parallel wire strands at elevated temperatures

High-strength prestressing cables exposed to high temperature result in the prestress loss, potentially leading to failure and even collapse of prestressing structures. This paper experimentally investigates thermal expansion of 1860 MPa steel wires and prestress loss of 1860 MPa twisted wire strands and parallel wire strands at elevated temperatures initially. Strands are localised heated in prestress loss tests. Different highest temperatures and initial prestress levels are considered. The test results show that EC2 underestimates the thermal strain of 1860 MPa steel wire at more than 450 degrees C. Prestress loss of the prestressing strand increases as the temperature increases. Cable types have little influence on the final prestress loss ratio. The strand with higher initial prestress level results in less final prestress loss ratio at less than 400 degrees C. A series fitting formulas of thermal strain, thermal expansion coefficient and final prestress loss ratio of prestressing strands are proposed. In addition, the finite element model of prestressing strand at elevated temperatures is established and validated. The numerical analysis shows that the temperature of the strand in non-heated region is significant, and the stress of the strand rises and then fall during heating stage, due to the balance between thermal expansion and degradation of mechanical properties. Finally, the analytical model of the prestress loss of the strands at elevated temperatures is proposed. The effect of thermal expansion, degradation of the mechanical properties and high-temperature creep of the strand in the heated region and non-heated region is considered respectively. Simplified method is obtained for more efficient computation. The analytical results have great agreement with the test results. The application of analytical method on practical engineering is discussed, and improved calculation equation is proposed for large span cables exposed to localised fire.