A FBG pull-wire vertical displacement sensor for health monitoring of medium-small span bridges

With the increase of service life, a growing number of medium-small span bridges have suffered degradation and even serious damage, and the vertical displacement is one of the significant parameters for their safety condition and performance evaluation. In terms of medium-small span bridges, to solve the difficulty for long term vertical displacement measuring and the demand for low-cost monitoring, this paper proposes a fiber Bragg grating (FBG) pull-wire vertical displacement sensor, which owns advantages of low cost, high accuracy and convenient installation. Firstly, the calibration and temperature correction of the sensor were carried out. Secondly, the influences of the fixed angle and sag effect of the sensor on measuring results were investigated. Finally, the sensor was installed in an in-service hollow slab bridge and the static and dynamic load experiments were carried out. Results show that the linearity of the proposed sensor is 0.998, the sensitivity is 7.87 pm/mm, the hysteresis error is 1.54 %, the temperature correction coefficient is 18.08 pm/°C, the relative combined uncertainties are within 4 %; the fixed angle of this sensor is recommended to choose in the range of 15°-30° in order to obtain higher measuring accuracy; when θ  = 15° and the radius of the external wire is 0.3 mm, the limit of the external wire length can be set to 19.5 m to avoid the influence of the wire sag effect on measuring results; the sensor is feasible in vertical displacement measurement of in-service medium-small span bridge, and the relative measuring errors in both static and dynamic load experiments are less than 5 %.