One widespread problem in damping estimation of high-rise buildings is the neglect of structural modal directions, which may induce beating in measured dynamic responses along building geometric axes and thereby induce errors in damping estimations to some extent. Based on a proposed two degrees of freedom (2-DOF) simulation model, the effects of neglecting the modal directions on damping estimate are systematically investigated. The results show that the angular differences between the modal directions and the building geometric axes, as well as the frequency difference between the involved modes, both have significant effects on the damping estimate of high-rise buildings. This paper proposes a spectral method to determine the modal directions of highrise buildings and further validate this method by an analysis of full-scale measurements from four skyscrapers. The damping ratios estimated based on the responses along the identified modal directions are more accurate than those based on those measured along the building geometric axes. Furthermore, an empirical prediction model for damping ratio of high-rise buildings with heights over 200 m is proposed based on the field measured damping results of several buildings with consideration of the modal directions. The objective of this study is to improve the accuracy of damping estimation of high-rise buildings and therefore provide useful information for the structural design of future skyscrapers. K E Y W O R D Sdamping estimate, damping model, field measurement, high-rise building, modal direction

This paper proposes a method to identify the time-variant modal frequencies of high-rise buildings and applies the method to the field measurements from a 68-story high-rise building during Super Typhoon Mangkhut (September 16, 2018). The analyzed results show that the fundamental natural frequencies of the monitored building first gradually decreased by about 9% and then returned to 94% of the original values over time. Moreover, the effects of timevariant modal frequencies on damping estimation of high-rise buildings are investigated by a comprehensive parametric study. The results indicate that the change rate of frequency has significant effects on the damping estimation by the conventional modal identification methods such as the random decrement technique and Bayesian spectral density approach. Accordingly, an error model for describing the relationship between the estimation error of damping ratio and main parameters of time-variant modal frequencies is established. Then, an error model-based correction method is proposed for the conventional damping identification methods to eliminate the estimation errors of damping ratios due to the time-variant modal frequencies, which is proved to be effective through Monte Carlo simulation and is further applied to identify the time-variant damping ratios of the monitored high-rise building. This paper aims to further the understanding of structural dynamic properties of high-rise buildings during typhoons and provide accurate estimation of structural modal parameters.