UNCERTAINTY ABOUT ENVIRONMENTAL VIBRATION TESTS: A CASE STUDY

摘要

Environmental vibration tests are useful tools for the evaluation of real structures, since they can be carried out during the structure operating conditions. As drawback, the low intensity level of the vibrations makes it difficult to identify precisely the modal parameters of the structure. The objective of the work is to study the uncertainty related to the identification of modal parameters of masonry structures subjected to environmental vibrations. The work deals with experimental records derived from a masonry barrel vault that is conceived as a sequence of arches according to its cross sectional behavior. Two different structural configurations are considered: the vault in its original configuration and the vault in its retrofitted configuration according to current design concepts. Two symmetric arches for each vault configuration are analyzed and the relevant dynamic response monitored. The modal parameters (frequencies, mode shapes and damping ratios) are estimated using operational modal analysis techniques. Four different records, for each structural configurations, are processed to investigate the stationary of the estimates. The results indicate that even though the modal parameters are quite different between the two configurations the related uncertainty are similar in contrast to the common assumptions that the retrofitted configuration should be less dispersive than the original one. As concerns the individual modal parameters it is confirmed that the frequencies present the lower uncertainties whereas the damping ratios are endowed with the higher uncertainties. In any case the uncertainty level does not hide the general pattern of the damping ratios. The imprecision in the mode shapes identification, measured through the MAC index, shows unexpectedly stable and accurate estimates.