Probabilistic Models for Fatigue Resistance of Seven-Wire Prestressing Strands and Stay Cables

Abstract Parallel seven-wire steel prestressing strands are the dominant form of the main tension elements (MTE) used in stay cables. In this paper, probabilistic models for fatigue resistance of seven-wire prestressing strands that are not embedded in concrete and are subjected to axial stresses are developed. Available test data from seven-wire strand fatigue tests were collected and analyzed to develop probabilistic models and nonlinear S–N curves using survival analysis techniques. Results indicate that the fatigue resistance of classic stress-relieved strands produced before early 1980s is higher than the modern low-relaxation strands that have been manufactured since then. Neither the conventional classic nor the modern strands have a good chance of passing the fatigue qualification tests required by the latest design standards for use in stay cables. Only the cable-quality (CQ) strands can pass the latest fatigue qualification tests with a less than 2.5% probability of failure. Nonlinear S–N equations for all three strand types and stay cables made with CQ strands are proposed using a log-logistic parametric survival model.