Accurate cable force determination using ambient vibration measurements

Knowledge of the tensile forces in cables of bridges or in external tendons is required for regular inspection and safety assessments. Force determination by lift-off tests with hydraulic jacks can result in considerable expenditure as well as the danger of damage. However, alternatively conducted vibration measurements have shown that, due to improper identification of natural frequencies or the use of over-simple force - frequency relationships, accuracies achieved up to now are sometimes not achieved. For high cable forces and short cables, errors of up to +/- 10% could be realized. Fast and accurate methods for assessing cable forces are thus required. The main target of this paper is the presentation of a practical applicable procedure to determine cable forces in civil engineering constructions by means of vibration measurements. By taking into account the identified natural frequencies, the bending stiffness and the boundary conditions, accuracy in the range of +/- 1% may be achieved.