The knowledge of tensile forces in cables of cable-stayed bridges or in external tendons is required for regular inspection and safety assessment of these structures. Force determination by lift-off tests with hydraulic jacks is connected with considerable expenditures as well as the danger of damages. But also vibration measurements conducted up to now have shown that the accuracies achieved sometimes were not satisfying. Above all for high cable forces and short cables, errors of up to ± 10% could be determined. Therefore fast, accurate and non-destructive methods for assessing cable forces are required. Main target of this paper is the presentation of a recently developed, accurate and practically applicable procedure to determine cable forces for civil engineering constructions by means of vibration measurements. By taking into account the identified natural frequencies, the bending stiffness as well as the boundary conditions an accuracy in the range of ± 1% may be achieved. The application of the described method is shown on one selected cable stayed bridge crossing the Danube-Channel in Vienna.
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