Monitoring load levels in multi-wire steel strands is crucial to ensuring the proper structural performance of post-tensioned concrete structures, suspension bridges and cable-stayed bridges. The post-tensioned box-girder structural scheme is widely used in several bridges, including 90% of the California inventory. In this structural typology, prestressing tendons are the main load-carrying components. Therefore loss of prestress as well as the presence of structural defects (e.g. corrosion and broken wires) affecting these elements are critical for the performance of the entire structure and may conduct to catastrophic failures. Unfortunately, at present there is no well-established methodology for the monitoring of prestressing (PS) tendons able to provide simultaneous and continuous information about the presence of defects as well as prestress levels. In this paper the authors develop a methodology to assess the level of load applied to the strands through the use of ultrasonic nonlinearity. Since an axial load on a multi-wire strand generates proportional contact stresses between adjacent wires, ultrasonic nonlinearity from the inter-wire contact must be related to the level of axial load. The work presented shows that the higher-harmonic generation of ultrasonic guided waves propagating in individual wires of the strand varies monotonically with the applied load, with smaller higher-harmonic amplitudes with increasing load levels. This trend is consistent with previous studies on higher-harmonic generation from ultrasonic plane waves incident on a contact interface under a changing contact pressure. The paper presents the results of experimental researches on free strands and embedded strands, and numerical studies (nonlinear Finite Element Analysis) on free strands.© (2011) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.