Viscous Damping in Elastic Seismic Response


Abstract eng:
Classical damping constitutes an infinitely small subset of all possible dissipation distributions and, as such, it is never the best fit to the actual mechanism. The model is, however, widely adopted because the information needed to specify a nonclassical distribution is seldom available in the design phase. From a practical perspective the question is not, therefore, whether the damping is classical or not, but whether errors attributable to adopting the classical model may be important. A result that has been known for many years but which is not widely recognized is the fact that if the poles of the dominant modes are well separated the off-diagonal terms of the damping in modal coordinates have a small effect in the response. For closely spaced frequencies, however, the specifics of the distribution are important and response predictions using the classical model can be in significant error. Needless to say, the matter takes special relevance when the ratio of effective motion duration to fundamental period is large since damping is most important in these cases. Other than recognize that uncertainties arising from the damping model are particularly large when frequencies are close, there is usually little that can be done about this matter (within practical constraints) at the design stage. This paper includes analytical work clarifying the role of the frequency gap and exemplifies the analytical observation in numerical simulations. Another issue sometimes raised regarding the suitability of the damping model pertains to the discrepancy between the (essentially) frequency independent dissipation observed in tests and the linear dependence associated with viscosity. Could this matter be such that the identified apparent damping shows dependence on the spectral characteristics of the excitation? Although the strict answer is yes, and the paper shows this to be so in an academic example, analytical examination and numerical results show that the identified damping for realistic input motions is the one that matches the rate of dissipation at resonance. The paper also takes a look at the question of the fictitiously high damping that is sometimes attributed to the mass proportional term in the Rayleigh model when base isolation is used and shows that this “undesirable feature” does not arise if the added terms in the augmented terms of the damping are adequately formulated. The paper includes a summary from a study where first mode damping ratios were identified for a large number of steel, concrete, masonry and wood structures and used to obtain predictive equations. It is shown that in steel and concrete the regressor with the most predictive ability is building height and it is contended that this is so because this parameter is a surrogate for the ratio of building volume to footprint, and thus to energy loss at the soil-structure interface. In masonry and wood structure the optimal regressor is shown to be the 5% damped pseudo-acceleration spectral ordinate. The limits that information theory impose on the variance with which damping can be identified from earthquake response is discussed and it is shown that for typical conditions the lower bound of the coefficient of variation is 25 to 50 times larger than for natural frequency.

Conference Title:
Conference Title:
16th World Conference on Earthquake Engineering
Conference Venue:
Santiago (CL)
Conference Dates:
2017-01-09 / 2017-01-13
Rights:
Text je chráněný podle autorského zákona č. 121/2000 Sb.



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 Record created 2017-01-18, last modified 2017-01-18


Original version of the author's contribution as presented on USB, paper 4986.:
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