Laser Vibrometry for Bridge Post-Repair Investigation

Bridges are important elements of our traffic infrastructure. Structural failure of such elements may have devastating consequences. Therefore, monitoring the condition of such structures is of importance. Traditional monitoring (beyond visual inspection) requires the inspector or a measuring instrument to directly access the structural component to be evaluated. For locations difficult to access, laser-based vibration monitoring may now be of assistance. The authors have gathered field experience using a laser interferometer to determine the natural frequency of structural components, using ambient excitation only. Because the natural frequency is related to the stiffness of a component, some structural damages, as well as the effectiveness of structural repairs and strengthening, may be detectable. Further, repeat routine measurements at regular intervals (vibrational fingerprinting) may be able to flag structural changes which will prompt more detailed investigations by traditional means. Further, the authors have demonstrated that localised delamination of fibre composite patches on a concrete bridge can be detected using laser interferometry in combination with remote acoustic excitation. The authors are currently investigating whether this method can detect concrete delaminations, failing steel connections and other localised damages to structural elements. The paper also will address the potential of using laser vibrometry for post-repair structural system evaluation as a validation of the rehabilitation procedure. Results from several field tests are presented. It is observed that local frequency due to voided contacts seem to be promising for damage detection.