Precise Evaluation of Structural Dynamic Properties of High-rise Buildings under Wind Excitations

風激勵下高層建築結構動力特性的準確評估研究

Student thesis: Doctoral Thesis

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Award date13 Nov 2020

Abstract

From the viewpoint of structural engineering, the wind effect is a major concern in the design of high-rise buildings, especially for those located in typhoon-prone regions. For the wind-resistant design of slender structures, accurate estimate of their structural dynamic properties or modal parameters (i.e., natural frequency, damping ratio, and mode shape) is of importance for evaluating their wind-induced responses. It is commonly accepted that the most reliable method to evaluate the structural dynamic properties of a high-rise building is by utilizing field measurements directly obtained from the prototype structure. However, several critical issues (e.g., beating phenomenon caused by closely located modes, effect of damper systems instrumented in high-rise buildings, and time-variant characteristics of modal parameters) are usually neglected in the estimation process utilizing field measurements, which may result in erroneous estimates of structural dynamic properties. To this end, this thesis presents a comprehensive study on the precise evaluation of structural dynamic properties of high-rise buildings by means of full-scale field measurements during typhoon events.

First, this thesis presents a detailed study of the beating effect on modal parameter identification by means of field measurement and numerical simulation. The beating phenomenon is clearly observed from the typhoon-induced acceleration responses of a 420-m-tall building, and the mechanism behind the phenomenon is revealed numerically by a two-degrees-of-freedom model. The effect of beating on modal identification is investigated accordingly, and the results show that the beating has a significant effect on damping estimation whereas it has a limited effect on frequency estimation. Furthermore, an effective modal-direction-based modal decomposition method is proposed to eliminate the adverse effect of beating on modal identification of high-rise buildings.

Second, this research investigates the effect of an active tuned mass damper system on structural dynamic properties of a monitored 600-m-tall building during a violent typhoon. By conducting a contrastive analysis between structural responses of the monitored building with and without the operation of the damper system, the performance of the damper system for suppressing the structural vibrations is investigated and the effect of the damper system on the structural dynamic properties is examined. The results show that the damper system can significantly boost the damping ratios of the fundamental sway modes of the monitored building, so as to mitigate the wind-induced vibrations.

Third, this thesis focuses on the time-variant characteristics of modal parameters of high-rise buildings during typhoons. An effective method for identifying the time-variant modal frequency (TVMF) of high-rise buildings is proposed, and then it is applied to identify the TVMF of a 392.5-m-tall high-rise building during Super Typhoon Mangkhut. The estimation errors in damping evaluation of high-rise buildings by conventional modal identification methods (i.e., random decrement technique (RDT) and Bayesian spectral density approach) due to the TVMF are systematically investigated, and an effective correction method for eliminating such errors is proposed for accurate damping estimation.

Subsequently, this thesis proposes new prediction models for the structural dynamic properties of high-rise buildings over 300 m in height. Given that existing prediction models are generally based on field measurements obtained from lower buildings, their applicability to super-tall buildings should be validated. More importantly, the above-mentioned critical issues such as the beating effect and TVMF that affect the modal identification of high-rise buildings were often neglected in the establishment of existing models, which may thereby compromise the accuracy of these models. Based on the field measurement study of several high-rise buildings with considering these critical issues, this study proposes an empirical model for natural frequency and suggests a damping model for the design of high-rise buildings.

Furthermore, this thesis investigates the reliability of a widely used modal identification method (i.e., RDT) for damping estimation of high-rise buildings. By adopting a bootstrap-based reliability analysis scheme, the reliability of damping results estimated by the RDT using four commonly used triggering conditions is systemically investigated through numerical simulation. In addition, based on the field measurements obtained from the monitored 600-m-tall building, this reliability analysis scheme is proved to be effective for evaluating the reliability of damping results of high-rise buildings based on field measurements.

To sum up, this dissertation focuses primarily on the precise evaluation of structural dynamic properties of high-rise buildings by means of full-scale field measurements during typhoons. The objective of this study is to enrich the knowledge and further the understanding of structural dynamic properties of high-rise buildings, and the outcomes are expected to provide valuable information for the wind-resistant design of future skyscrapers.