Dynamic load monitoring of the Tsing Ma Bridge using a high-speed FBG sensor system

Sensors for strain measurement are indispensable for structure monitoring. Fiber optic sensors based on Fiber Bragg Grating (FBG) technology are found to be suitable for strain sensing and have a number of advantages compared to conventional strain gauges. Recently, we carried out a field trial with a FBG sensor array system on the world’s longest span suspension bridge carrying both trains and regular road traffic (The Tsing Ma Bridge in Hong Kong). The experiment was carried out with a high-speed dense-channel demultiplexing/interrogation system for FBG sensor arrays (DC to ~20 kHz for all channels simultaneously). This system is originally developed for the detection of high-speed events (vibration, impact location and acoustic emission), but in this field experiment the high readout frequency has been used for noise reduction by averaging to achieve sub-pm resolution. More than 20 FBG sensors divided into 3 arrays were installed on different parts of the bridge (suspension cable, rocker and frame section). The goal of this field trial is to monitor the strain of the different parts of the bridge under railway load and highway load. Various measurements were performed including an overnight measurement of about 20 hours with a sampling frequency of about 500 Hz. The measurement results reveal the presence of significant higher frequency components in the FBG sensor signal during train passages. The results of the FBG sensor also compared with existing strain gauges. Although the sensors are not located at exactly the same location, great resemblance has been found.