| Summary: | 博士 === 淡江大學 === 土木工程學系博士班 === 96 === The wind loading of a tall building can be divided into three components: alongwind, acrosswind and torsional wind loads. For the alongwind load, it is induced by the mean wind speed pressure and the turbulence characteristics of the approach flow. It is generally accepted that analytical model basing on quasi-steady theorem and strip theory can adequately predict the alongwind loading, and thus it was adopted by many building wind codes. As for the acrosswind and torsional wind loads, they are mainly induced by the wake flow. The mechanisms are complicated and can not be adequately modeled by analytical or semi-empirical models.
For the present Taiwan building wind code, the Gust Response Factor used in the alongwind design wind load is fixed-value calculated, based on the structure natural frequency and turbulence characteristics. Nevertheless, for a flexible tall building, the dynamic resonant part of the response plays a significant role in the design wind load. It is observed that the spatial distribution of the resonant part loading is different from the mean wind load and dynamic background part wind load. Hence, this project investigates the appropriateness of the current alongwind design wind load practice for a flexible tall building, and provides an alternative with a more precise procedure. This search also implemented a series of wind tunnel testing to measure the tall buildings’ wind loads in turbulent boundary layers designated by the current Taiwan building wind code.
A modified procedure for alongwind design wind load is proposed with the following conditions: (1)The mean and dynamic wind forces on the windward face follow the strip theory strictly; the wind forces on the leeward face assumed to be uniform; (2)The spatial correlation effect on the background part of equivalent static wind load is amended by a correlation reduction factor; (3)The resonant part is distributed based on the distribution of the inertia force.
In order to investigate a more clearer picture on wind load characteristics of rectangular shaped tall buildings, pressure models were established and tested in a boundary layer wind tunnel. Three turbulent boundary layer flows with power law index α=0.32, 0.25, 0.15, respectively, were created. The geometry variations of the pressure models in wind tunnel test are: aspect ratio 3, 4, 5, 6, 7; side ratios 1/5, 1/4, 1/3, 1/2, 1/1, 2/1, 3/1, 4/1, 5/1.
Numerical study is then performed on 6 different prototype buildings in 3 kinds of flow fields. The geometry variations of the buildings are: aspect ratio = 3, 6; side ratios D/B = 1/3, 1/1, 3/1 and the buildings’ heights are 90m and 180m. The equivalent static wind load based on the semi-empirical formulation is compared with the current Taiwan wind code and wind tunnel measurement. The outcome reflects that, in terrain A and B, the present design wind load model is closed to wind tunnel’s and is much more accurate than the present Taiwan wind code. But the present design wind load model is lower than wind tunnel results in terrain C. It’s shown that the assumptions of the wind load and the semi-empirical model using the assumptions are more precise procedures to evaluate alongwind design wind load of tall buildings.
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