Owing to their high strength, efficient material utilization, and lightweight, prestressing steel cables have been extensively used as tension members in bridges and building structures. Their continued and wide application has necessitated introduction of new design concepts of beam string, suspen-dome, tension membrane, cable-stayed bridges, prestressed concrete structures, etc.
Fire is one of the most extreme conditions likely to be experienced through the service life of such structures. The effects of creep, degraded material properties, and thermal expansion in steel cables at high temperatures lead to stress relaxation and tension loss, thereby affecting the strength and stiffness of cable-supported structures to even causing collapse. In-service prestressing forces in steel cables reach up to 55% of the ultimate tensile strength. Therefore, its important to come up with a systematic and detailed data on temperature-dependent mechanical properties of steel cables to effectively assess the load-bearing capacity of long-span cable-supported structures likely to be exposed to fire thereby ensuring adequate structure fire design.
Cold-drawn prestressing cables experience greater reduction in their mechanical properties at high temperatures when compared to hot-rolled steel. This is because the beneficial effect of cold-working process on strength is eroded at high temperatures. Unfortunately, most previous studies on the effects of elevated temperatures on mechanical properties of prestressing steel have mainly focused on twisted wire strands and individual wires. Studies focusing on parallel wire strands applied extensively in cable-stayed and suspension bridges and long-span building structures are rare. Also, the data given in the current design codes is mainly based on test results of experiments conducted on steel wires and twisted wire strands, and their applicability to parallel wires strands applied in long-span steel structures still needs further investigation.
Researchers Yong Du, Hong-hui Qi, Professor Jian Jiang, Professor J.Y. Richard Liew, and Professor Guo-Qiang Li from China University of Mining and Technology in China, experimentally investigated the mechanical properties of 7-wire parallel wire strands at elevated temperatures. The study was performed in two series of tests: steady-state tensile tests and thermal elongation tests. Their work is currently published in the journal, Construction and Building Materials.
The research team conducted the steady-state and thermal elongation tests on 39 strand specimens at different temperatures. They then obtained rapture strand and stress-strain curves using charge-coupled device camera systems. The authors obtained thermal-dependent thermal strain, proportional limit, Young’s modulus, ultimate strength, effective yield strength, rapture strain, and ultimate strain and compared to ACI 216, EC2, and other existing data. They also conducted a comprehensive comparison to existing design codes and test data of single wires, twisted strands, reinforcing bars, and high-strength structural steel.
From the developed strain-stress curves of parallel wire strands, the authors observed an obvious strain hardening effect before 300 °C while ductility improved significantly after 400 °C. They reported a relatively slow reduction in mechanical properties of parallel strands before the first target temperature and a rapid reduction once the target temperature was increased. They defined the effective yield strength as stress at 2% total strain instead of 1.25% as defined for the 1860 MPa twisted strands.
Currently, the EC2 prestressing steel calculation methods provide conservative estimates for proportional limit, yield strength, and ultimate strain of parallel wire strands. The EC2 estimates do not take into account the full range of stress-strain curves of parallel strands as they ignore the strain hardening effects and underestimates rapture strain. On the other hand, ACI 216 provides unconservative predictions for reduction factors of ultimate strength of parallel steel strands. These findings uncover the need to improve the current design codes to make them usable for the design of parallel wire strands.
The authors observed that parallel wire strands had higher rapture strain, higher ultimate strain, higher thermal strain, lower reduction factors of proportional limit than twisted and single wire strands. However, parallel, single and twisted wire strands had similar reduction factors of ultimate and yield strength. The authors however noticed larger deviations in reduction factors of Young’s modulus for single and twisted wire strands.
The researchers proposed a mathematical model to define the complete stress-strain curves of parallel wire strands by dividing the curve into elastic, plastic, necking, and rapture stages. They also proposed calculation methods to find thermal elongation coefficient, thermal strain, and reduction factors of Young’s modulus, ultimate strength, proportional limit, and effective yield strength. The proposed new methods agree well with the test results recording significantly low deviations.
Yong Du, Hong-hui Qi, Jian Jiang, J.Y. Richard Liew, and Guo-Qiang Li. Mechanical properties of 1670 MPa parallel wire strands at elevated temperatures. Construction and Building Materials, issue 263 (2020), 120582.