pecially the change in the fourth mode is remarkable, as it split in two modes during the lowering of the column, with one lower and one higher frequency than before. The frequency changes in the fifth mode cannot be evaluated, as its uncertainty is very high compared to the other modes. Also, the change in the frequencies when cutting the tendons (between G and K) is not significant. Only after the uplifting of the column and before cutting the first tendon (between G and H), some of the frequencies are dropping, probably due to the settling of the structural system after the uplifting. However, no significant change in the frequencies can be observed afterwards. 5 Conclusion In this paper, confidence intervals on modal parameters during a progressive damage test were successfully computed with an improved algorithm. Using an automated monitoring procedure, the modal parameters of the S101 Bridge during this damage test were obtained completely automatically together with their confidence bounds. The artificially introduced damage scenarios “lowering of a column” could be clearly linked to changes in the natural frequencies, while “cutting the tendons” did not have a significant influence on the frequencies. It was shown that the confidence bounds on the modal parameters are essential when evaluating the changes in the modal parameters of the structure due to the introduced damages. System identification results obtained by covariance and data driven SSI are very similar, with slightly lower confidence bounds for results obtained from data driven SSI with UPC. Acknowledgments This work was partially supported by the European project FP7-NMP CP-IP 2139682 IRIS. We also thank VCE for providing the data from S101 Bridge. References 1. Basseville, M., Benveniste, A., Goursat, M., Hermans, L., Mevel, L., van der Auweraer, H.: Output-only subspace-based structural identification: from theory to industrial testing practice. Journal of Dynamic Systems, Measurement, and Control 123(4), 668–676 (2001) 2. Benveniste, A., Fuchs, J.J.: Single sample modal identification of a non-stationary stochastic process. IEEE Transactions on Automatic Control AC-30(1), 66–74 (1985) 3. Benveniste, A., Mevel, L.: Non-stationary consistency of subspace methods. IEEE Transactions on Automatic Control AC-52(6), 974–984 (2007) 4. Do¨hler, M., Lam, X.B., Mevel, L.: Confidence intervals on modal parameters in stochastic subspace identification. In: Proceedings of the 34th IABSE Symposium. Venice, Italy (2010) Confidence Intervals of Modal Parameters during Progressive Damage Test 249
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