A videogrammetric technique for measuring the vibration displacement of stay cables

Stay cables in cable-stayed bridges are prone to large amplitude oscillations under external excitations. The vibration of the cables is predominantly measured by using accelerometers to measure the acceleration. The dynamic displacement is then usually obtained indirectly from the double integration of the acceleration data. This paper reports an experimental method of measuring the displacement of stayed cables using a digital video camera. With the newly developed videogrammetric technique, the video clips are transferred into image frames, from which the shape and location of the target are identified. The displacement time history is then captured. The technique is applied to a cable-stayed bridge to measure the dynamic displacement of stay cables. The displacement is compared with the acceleration data in the frequency and time domains. The results show that the displacement measured by the digital video camera is comparable to the counterparts integrated from the acceleration data. The vibration frequencies identified from the acceleration are finally used to estimate the tension forces of the cables. The results show that the tension forces have insignificant changes after one year's operation.