SUMMARYThe theoretical and experimental investigation of a cable-stayed bridge after major repair is described in this paper. Strengthening mainly involved the suspension system (originally with prestressed concrete stays) which was retrofitted by means of external tendons. Full-scale tests were conducted to measure the dynamic response of the repaired system; the experimental program included both traffic-induced and free vibration measurements. A total of 16 vertical frequencies and mode shapes were identified in the frequency range of 0-10 Hz. In the theoretical study, vibration modes involving deck, towers and cables were determined by using finite element models which accounted for the strengthening effects. Two-and three-dimensional models were used so that the importance of three-dimensional modes was estimated as well. The experimental results were compared to natural frequencies and mode shapes computed using theoretical models. For most modes the measured and predicted modal parameters compare well, especially for the vertical modes involving in-phase motion of the stays.

The dynamic behaviour of two curved cable-stayed bridges, recently constructed in northern Italy, has been investigated by full-scale testing and theoretical models.\ud Two different excitation techniques were employed in the dynamic tests: traffic-induced ambient vibrations and free vibrations. Since the modal behaviour identified from the two types of test are very well correlated and a greater number of normal modes was detected during ambient vibration tests, the validity of ambient vibration survey is assessed in view of future monitoring.\ud For both bridges, 11 vibration modes were identified in the frequency range of 010 Hz, being a one-to-one correspondence between the observed modes of the two bridges. Successively, the information obtained from the field tests were used to validate and improve 3D finite element so that the dynamic performance of the two systems were assessed and compared basing on both the experimental results and the updated theoretical models

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The paper deals with the reliability assessment of P.C. cable-stayed bridges, but it is thought that the presented methodology is generally applicable. Due to several sources of uncertainties, the geometrical and mechanical properties which define the structural problem cannot be considered as deterministic quantities. In this work, such uncertainties are modelled by using a fuzzy criterion which considers the model parameters bounded between minimum and maximum suitable values. The reliability problem is formulated in terms of safety factor and the membership function over the failure interval is derived for several limit states by using a simulation technique. In particular, the strategic planning of the simulation is found by means of a genetic optimisation algorithm and the structural analyses are carried out by taking both material and geometrical non-linearity into account. An application to a cable-stayed bridge shows the effectiveness of the proposed procedure.

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