Boundary layer wind characteristics and wind-induced dynamic responses of a super-tall building

Abstract

This dissertation contains two parts which focus on the boundary layer wind characteristics and the wind-induced structural responses of a super-tall building, respectively. Wind information provides the preliminary, and sometimes, the most important reference source for many engineering practices, such as airborne-pollution mitigation, fire prevention, site selection of wind power plant, structural design, etc. The situation is more critical for some coastal areas, e.g. Hong Kong, whose climates are dominated by strong winds. In the first part of the thesis, investigations of wind characteristics are carried out based on multi-instrument measurements of both surface weather records and remote sensing wind probes in Hong Kong. Climate and gust characteristics of surface wind and topographic effects on both mean and turbulent wind components are studied. The results show that wind characteristics in Hong Kong are remarkably affected by local topographic features, and demonstrate distinct differences with those under open flat terrains. Thus, standardization of raw datasets is needed to remove the exposure influence on the wind speed observations for further usage. However, current empirical model based standardization scheme is mainly suitable for flat terrains which may become inappropriate under complex terrain conditions. Therefore, a data-driven standardization scheme is proposed on the basis of field measurements. The presented scheme has the merits of less computational cost, high efficiency, and good robustness. Its efficiency is further validated through wind tunnel tests. In particular, vertical profiles of tropical cyclones (TC) with an off-sea exposure within the whole atmospheric boundary layer are studied. It is found that the ensemble-mean speed profiles follow the logarithmic law well below 400 m with a low-level jet located around 500-700 m. Meanwhile, the mean wind direction is found to veer logarithmically with height in the lowest 3000 m beyond which it levels off and then turns back. A phenomenal model of wind profile is presented by analogy with wind turbulence spectrum. This model, covering the whole TC inflow layer, provides a much wider prediction range and is able to reproduce the speed-dependence of jet heights. Strong winds can result in severe wind-induced responses of civil structures. This issue is regarded as one of the most important aspects at the design stage of high rise buildings. Thus, the second part of this dissertation is concerned with the dynamic responses of a 492 m high building installed with an ATMD (active tuned mass damper) system. The effectiveness of this ATMD system and real performance of the actively controlled high-rise structure under typhoon condition are investigated to assess the benefits gained from the application of the vibration control techniques. Particularly, a synchronous and instantaneous analysis framework is presented through a comprehensive application of the linear-phase filtering and the time-frequency domain analysis techniques. Results via this framework analysis show that the instantaneous modal frequency (IMF) and the damping ratio of this building are positively correlated with each other. Based on this observation, a mechanism of damping due to the modulation of phase and amplitude is proposed. An empirical model is presented to formulate the variations of damping ratio and natural frequency of this tall building as a function of IMF. Meanwhile, an approach to calculate the instantaneous vibration direction (IVD) is put forward, based on the measurements from a bi-axial accelerometer at merely a single test position. IVD-based spatial mode identification techniques are presented and discussed with a demonstration of the case study of the SWFC building. Both the directionally swaying mode and the 1st torsional mode are recognized.

Date of Award	2 Oct 2013
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Qiusheng LI (Supervisor)