ESTIMATING NATURAL VIBRATION CHARACTERISTICS OF BRIDGE CABLES BASED ON N4SID SUBSPACE METHOD

Abstract

Cable maintenance is important for cable structures such as cable-stayed and Nielsen-Lohse bridges. In the current maintenance practice, cable tension is estimated from the lower mode natural frequencies of the cable. It is theoretically possible to estimate the bending stiffness of the cable and the parameters of the dampers installed on the cable if the higher mode natural frequencies, damping factors, and mode shapes are available. Conventionally, the natural vibration characteristics of the cables are estimated manually from the acceleration Fourier spectrum. The estimation accuracy of the lower mode natural frequencies is high. However, the estimation accuracy of the higher mode natural frequencies is not high, and the estimation accuracy of damping factors and mode shapes is low irrespective of modal order. In this paper, the N4SID, one of the subspace methods, is adopted to estimate the natural vibration characteristics of the cables. The N4SID is applied to numerical and experimental results of bridge cables to investigate the estimation accuracy. The numerical investigation found that the accuracy of the natural frequencies and damping factors is high, the accuracy of mode shape is high in the cable without a damper, and the accuracy of mode shape is only high for the limited low modes and low for the other mode in the cable with a damper. The experimental investigation found that the N4SID has better accuracy than the conventional estimation method.