Comparison of laser Doppler vibrometer with contact sensors for monitoring bridge deflection and vibration

This paper compares results from dynamic live load tests using the non-contact laser Doppler vibrometer (LDV) system with those from two types of contact sensors. Bridge girder deflections and vibrations are simultaneously measured using a linear variable differential transducer (LVDT)-cable system (deflection) and geophone sensors (velocity), both attached to the girders, and the LDV, equipped with displacement and velocity signal decoders. Live load tests are performed on a 3-span continuous unit of the 9-span Doremus Avenue Bridge Replacement Project using 5-axle trucks of known weight and configuration. The Doremus Avenue Bridge is a composite steel slab-on-girder construction. Bridge response is compared for two of the 10 girders. Overall, the LDV measurements of deflection and velocity compare very well with those recorded by the contact sensors and may be used as an alternative to the two systems. Other advantages and disadvantages are also highlighted.