Prestress Force Effect on Vibration Frequency of Concrete Bridges

This study was an attempt to determine if vibration frequencies of prestressed concrete members can be used to determine prestress losses. An actual post-tensioned bridge and a post-tensioned laboratory specimen were subjected to free vibration. Because the prestresses in the bridge and the beam were known, it was possible to determine if the changes in the measured frequencies would correlate with the prestress forces. The theoretical prediction for homogenous members was that, as the prestress force decreases, the frequency would increase because a reduction in the axial compressive load should stiffen the element. In contrast, the measured frequencies for both the bridge and the beam showed that, as the prestress force decreased, more microcracks opened and the members softened. Based on the measured data for the laboratory specimen, an empirical equation was developed that accounts for the effect of axial force on the rigidity of the element. This expression was used for an actual bridge and led to a reasonable estimate of the fundamental frequency at different prestress forces.