An Improved Tension Evaluation Approach for Smart Strands

Monitoring strand tension is crucial for bridge safety assessment and reinforcement design. Smart strands, composed of a straight core wire embedded with a fiber grating and six helical wires, have been developed for tension monitoring. In this study, an improved tension evaluation method based on smart strands was developed. First, the external load-induced strain of the core wire was obtained by applying a developed complete temperature model to temperature and wavelength measurements of the grating, which can fully deduce the temperature-induced strain. Subsequently, an analytical iterative model, accounting for the off-axis effect of the helical wires, was derived to convert the core wire strain into the corresponding strain in the helical wires. Finally, the total tension of the smart strand was calculated by summing the contributions from the core and helical wires based on their respective strains. The proposed method was validated using laboratory experiments, demonstrating that the complete temperature compensation model exhibits universality across statically determinate and indeterminate structures, achieving an error reduction exceeding 55.6% in statically indeterminate systems. Furthermore, the proposed tension evaluation method achieves estimation errors below 2.5% and reduces maximum tension errors by 56.6% compared with conventional methods that directly use the core wire strain as the average strain. The results indicate that traditional temperature compensation models for optical fiber sensors cannot fully eliminate temperature effects, mainly because of the neglect of temperature field effects rather than sensor limitations.