Vertical Displacement Measurements for Highway Bridges using Optical Devices/Sensors: A Preliminary Study

In many bridges, the vertical displacement is one of the most relevant parameters to be monitored in both the short and long term. However, it is well known amongst bridge authorities all over the world that it is difficult to measure the vertical displacements of a bridge. Current methods are often tedious to use and require the intervention of specialised operators. Therefore it prompts the need to find a simple and practical method to measure vertical displacements. This paper presents two methods using either optical fiber (FBG) sensors or a charge-coupled-device (CCD) camera respectively for vertical displacement measurements of bridges. The FBG sensor method is based on the measurement of horizontal displacements together with the inclinations at various points along a bridge using FBG strain sensors and FBG tilt sensors. The CCD Camera method utilizes image processing techniques for edge detection. The image processing techniques include grayscale threshold consideration and brightness, contrast and gamma correction. The proposed methods were tested and compared in laboratory using a concrete beam and a steel beam separately. The comparisons include their installations, costs, degrees of accuracy, external factors affecting the measurement, etc. The theoretical background for both methods including the use of FBG sensors for structural health monitoring will be discussed in the paper. It is concluded that both methods could be used for vertical displacement measurement and they could be complementary to one another. With their implementations, a big advancement in the technology of Structural Health Monitoring is expected.