Evaluation of Damping Ratios for the Seismic Analysis of Tall Buildings

This paper examines damping ratios in tall buildings in California inferred from motions recorded in instrumented buildings during earthquake motions. A total of 41 measured seismic responses from 14 buildings with 20 or more stories are analyzed using a parametric least-squares system identification technique in the time domain. The reliability of the inferred damping ratios is first examined based on the sensitivity of the structural response to variations in modal damping ratios. Using only inferred damping ratios deemed reliable, it is found that the damping ratios of the fundamental mode decrease with increasing building height, reaching values lower than 2.5%-the damping value that is commonly recommended for seismic design in the United States for buildings taller than 150 m. The variation of damping of higher modes with frequency is then examined by studying the results of those buildings where damping ratios of at least the first three modes are reliably inferred. It is shown that the variation of damping with frequency follows an approximately linear trend. No evidence was found to suggest that a mass-proportional model could be appropriate. The amplitude dependency of damping is then analyzed for buildings having four or more recorded earthquakes, finding no statistical correlation between the peak roof drift ratio and the identified fundamental damping ratio. (C) 2016 American Society of Civil Engineers.