Mechanical properties of high tensile steel cables at elevated temperatures

This study is motivated by increasingly prevalent use of cable-tensioned spatial steel structures and suspension bridges. Fire is one of the extreme conditions that need to be taken into consideration in the design of such structures. In this paper, steady-state tests have been conducted on steel cables with tensile strength of 1860 MPa, which consist of a group of 7-wire twisted strands, to study their full range of stress strain relationships at elevated temperature. Thermal elongation test of steel cables has also been conducted. A charge-coupled device camera (CCDC) system is used to capture the full range of stress-strain relationship of high tensile strength steel cables till rapture at elevated temperature. The reduction factors of proportional limit, elastic modules, effective yield strength and rupture strength at different temperature were obtained from the steady state tests and compared with that proposed by EN 1992-1-2. The experimental work discovered that EN 1992-1-2 overestimated effective strain up to 2% and ignored the stress-hardening phase for high tensile strength cables within the full temperature range. The effective yield strength with 1.25% strain and a full range of stress-stain model considering stress-hardening phase are proposed. Finally, several sets of reduction factors and thermal elongation coefficient as a function of temperature have been proposed by fitting the test results. The reduction factors of pre-stressing strands proposed by EN 1992-1-2 for pre-stressing concrete is found not suitable for steel cables which are widely used for pre-tensioned steel structures. The reduction factors proposed in the present paper are found to be reasonable for steel cables. The experimental work also shows that the mechanical properties of steel cables at elevated temperature depends on whether the cable is made from straight wires or twisted wires.