Displacement measurement under dynamic excitations enables the evaluation of bridge performance and serviceability. However, measuring the dynamic displacement has been a challenging task, since conventional sensors are unfeasible. Recently, the emerging non-contact sensing techniques such as global positioning system (GPS), robotic total station (RTS), laser Doppler vibrometer (LDV) and vision-based system provide potential alternatives in the dynamic displacement monitoring of bridges.
VES members undertook a dynamic displacement monitoring test on the Tamar Bridge, using remote sensors. Tamar Bridge is a suspension bridge in southwest England, which spans the River Tamar between the City of Plymouth on the east bank, and the town of Saltash on the west bank.
This test investigated the feasibility and accuracy of three kinds of remote sensors, including GPS, robotic total station and the vision-based system. Back in July, VES members performed a similar test on Humber Bridge, and the information can be found in the following link. (https://veswordpresscom.wordpress.com/2015/09/23/mid-span-deflection-monitoring-of-humber-bridge-using-cameras-2/)
Compared with Humber Bridge, Tamar Bridge is shorter in span, and undergoes less movement, so it is more challenging to track movement with conventional sensors such as GPS and total station.
During the test at Tamar Bridge, a prefabricated target frame with a pattern of concentric circles was mounted on the hand rails at mid-span, together with GPS rover receiver and a circular prism. About 380 metres away, near the Tamar Bridge Office, a camera tracked the two dimensional movement of the target frame, and a robotic total station recorded the three dimensional coordinates of the prism reflector. The bridge was subjected to ambient excitations such as wind, vehicular loading and thermal effects.
The following two figures show 70 second signals of displacement in the temporal and frequency domains. Camera and GPS sensors obtained similar displacement time histories under traffic loads, but the camera measurement captured more dynamic components of the bridge motion.