Tension force and structural parameter identification of bridge cables

Vibration measurement is one of the most widely used methods for tension evaluation and condition assessment of stay cables in cable-stayed bridges. In the existing practice, the tension force of a cable is identified from the measured modal frequencies with the use of the taut string theory or empirical formulae, by assuming pre-determined structural parameters (geometric and material parameters) and boundary conditions of the cable. As a result, an inaccurate estimation of the cable tension may be obtained when there is an error in the pre-determined structural parameters and boundary conditions. Moreover, the commonly used empirical formulae are not applicable in the case when the cable is intermediately attached with dampers. In the present study, a method enabling simultaneous identification of cable tension and other structural parameters from the measured modal frequencies is developed. A precise finite element model (FEM) accounting for cable flexural rigidity, sag-extensibility, spatial variability of dynamic tension, boundary conditions, lumped masses and intermediate supports and/or dampers is first formulated as the reference model in parameter identification so that the modeling error is minimized. Then the measured multiple modal frequencies are used together with the FEM to figure out a nonlinear least-square optimization scheme which helps eliminate measurement error and allows for simultaneous identification of the cable tension and other structural parameters. Application of the proposed method to the Dongting Lake Bridge cables from in-situ ambient vibration measurements illustrates high identification accuracy and fidelity of the proposed method.