Evaluation of damping ratios inferred from the seismic response of buildings [electronic resource]

Estimation of the response of buildings to seismic loading requires knowledge of the damping in the building. Unlike mass and stiffness matrices, which can be determined from the geometry of the structure, the only reliable way to assess the damping in a building is through measurements of the structural response when subjected to dynamic loads. In the literature, and for many years now, different recommendations have been given for damp- ing ratios based on the measured response of buildings. However, most previous studies -- and especially those conducted prior to 1990 -- share common problems that can be roughly grouped in two categories: (1) The data employed consists almost exclusively of very low-amplitude motions, such as ambient vibrations of low-amplitude wind excitation, and there are many studies that have indicated that their results may not be directly applicable to earthquake analysis; and (2) many of the methods employed to estimate the damping ratios, particularly those of older studies, are now well recognized to not provide reliable results. This investigation focuses on solving these two issues, and is aimed at providing improved recommendations for modeling damping in buildings responding elastically when subjected to earthquake ground motions. In this dissertation, damping ratios inferred from 1335 seismic responses coming from 154 instru- mented buildings in California, are analyzed. These values were inferred using a parametric system identification technique in the time domain, and subjected to a series of reliability tests to ensure that only high-quality data was employed. A series of linear mixed-effects statistical models are evaluated in order to select the regressors that best explain the variance in the data. The influence of the building's fundamental period, height, aspect ratio, lateral-load resistant system, and material are examined, and a prediction equation for estimating the damping ratio of the fundamental mode is proposed. Damping in higher modes are also examined. The validity of the Rayleigh damping model, commonly used in seismic analysis of buildings, is evaluated and a prediction equation for damping in higher modes is proposed. The effects of soil-structure interaction on the overall damping ratios of buildings subjected to ground motions are also examined. A thorough mathematical derivation of explicit expressions for the absolute acceleration transfer function of a multi-story building with a circular, rigid foundation, sitting on an elastic half-space is presented. An optimization procedure is then developed to obtain the effective modal properties of a replacement fixed-base structure capable of approximating the transfer functions of a structure on a flexible base. A parametric study is conducted to analyze the variation of the effective damping ratio of the fundamental and higher modes with the non- dimensional wave parameter, the building height, aspect ratio, and the effective modal frequency. A series of numerical simulations are then conducted in order to assess the parameters that primarily govern the interaction effects. These purely analytical results are finally compared with the empirical results for the fundamental and higher modes.

Advisor: Miranda Eduardo (Miranda Mijares) primary advisor. Thesis advisor
Deierlein Gregory G. (Gregory Gerard)
Law K. H. (Kincho H.) advisor. Thesis advisor