Vibration of prestressed beams: Experimental and finite-element analysis of post–tensioned thin-walled box-girders

In published works, very different explanations are given for the effect of prestressing on the beam dynamics, especially on the relationship between the vibrational effects caused by an external compressive force and those due to a prestressing force. To solve this conflict, free transverse vibrations of a post–tensioned thin-walled steel box-girder were investigated. Its fundamental frequencies were then measured for different values of prestressing. Subsequently, the experimental data were compared with a formula considering shear effects. The experimental data were also compared with two high-fidelity finite-element models assuming geometric nonlinearities. According to the obtained findings, and comparisons with tests reported in literature, the beam dynamics strongly depends on the contact of the cables with the surrounding section. If the tendons are not in contact with the surrounding cross-section, the beam dynamics due to an external compressive force is practically coincident to that caused by a prestressing force for low values of prestressing. In this case, the dynamics of prestressed beams are initially ruled by the compression-softening effect. For high values of prestressing, the beam would behave as a shallow arc with horizontal elastic supports. In this case, an increase of fundamental frequency is observed, but the yield stress would be exceeded. Conversely, in thin-walled steel girder-bridges with tendons in contact with the cross-section or draped with a deviator, the fundamental frequency is practically unaffected by prestressing. Thus, the fundamental frequency cannot be used as indicator for identifying long-term phenomena of prestressing losses, such as anchorage slips, tendon friction and relaxation.