Expressions for the expected value of the first mode damping ratio are derived from 122 seismic responses from concrete buildings and 81 from steel. The results include dissipation at the soil-structure interface and are appropriate for the situations where this source of dissipation is not included in the model.Comparisons between models of different complexity indicate the appropriateness of using a single regressor, which is taken as the building height. It is shown that the Fisher Information on the damping increases with the number of response cycles and this result is used to define weights for the residuals of the regression.The effective damping in steel buildings, with the exception of very tall structures, is found larger than the 2% typically used in practice, while the 5% assigned to concrete proves to be close to the mean of the data set.
The merit of linear projections as a way to improve the resolution in damage detection under changing environmental conditions is examined. It is contended that if the data from the reference condition is balanced, in the sense that the number of feature vectors available for the various temperatures is similar, then projections, such as those in Principal Component Analysis and Factor Analysis, will not improve performance. Projections, however, help to control the false positive rate when the reference data set is not balanced. Analysis and simulation results suggest that previous claims on the merit of projection as a way to improve damage detection resolution under environmental variability may be too optimistic.
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