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This paper aims to evaluate the modal frequencies, damping ratios and mode shapes of an asymmetric building, modeled as general torsionally coupled buildings using a modified random decrement method together with the Ibrahim time domain technique based only on few floor acceleration response records from earthquakes. It is not necessary to measure earthquake excitation input. The general relationship between the reduced random decrement signature and the true free vibration response is derived analytically. Because only partial floor response measurements are used, a mode shape interpolation technique is developed to estimate the mode shape values for the locations without measurement, such that all floor responses can be obtained. The results were obtained from simulation data from a five-story building under the 1940 El Centro earthquake and actual records from a seven-story RC school building in northeastern Taiwan, due to an earthquake near the building site. The results show that the proposed system identification technique is capable of identifying structural dominant modal parameters and responses accurately even with highly coupled modes and high levels of noise contamination.

In this study, a new design procedure was developed for reducing the dynamic responses of torsionally coupled buildings, particularly existing buildings, under bilateral earthquake excitations, by incorporating the vibration control effectiveness of passive tuned mass dampers (PTMDs). Some practical design issues such as the optimal location for installation, movement direction and numbers of PTMD are considered in this study. The optimal parameters of the PTMD system are determined by minimizing the mean square displacement response ratio of the controlled degree of freedom between the building with and without PTMDs. In addition, parametric studies of the PTMD planar position and the detuning effect are undertaken to determine their influence on the response control efficacy. The numerical results from two typical multistory torsionally coupled buildings under bidirectional ground accelerations, recorded at the 1979 El Centro earthquake, verify that the proposed optimal PTMDs are more effective and more robust in reducing the building responses. Copyright (C) 2007 John Wiley & Sons, Ltd