MEASUREMENT OF PRESTRESS FORCE IN THE RODS OF STRESSED TIMBER BRIDGESUSING STRESS WAVES

The prestress levels in post tensioned steel rods are nondestructively evaluated using a stress wave technique. These prestressing rods (Thr ead-bars) are typically used in stressed timber bridges. Both laborato ry and field experimental measurements were conducted and the results indicate that the time of flight of the stress waves can be related to the prestress levels of the rods. An analysis was conducted by accoun ting for acoustoelasticity effect and the dispersion of waves to calcu late the prestress levels in the steel rods and to correlate the calcu lation with the experimental observation. This analysis provides a suc cessful description of the behavior of a longitudinal transient wave t raveling through a long, prestressed, circular rod. Results from field experiments of an in service stressed timber bridge are discussed, an d indicate that the proposed technique provides an efficient nondestru ctive measure of stress retention levels in the post tensioned steel r ods.