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    Title: 高層建築順風向等值靜態設計風載重之研究
    Other Titles: Development of an advanced alongwind equivalent static design wind loads for tall buildings.
    Authors: 蔡明樹;Tsai, Ming-shu
    Contributors: 淡江大學土木工程學系博士班
    鄭啟明;Cheng, Chii-ming
    Keywords: 高層建築;順風向;設計風載重;風洞實驗;Tall Building;Alongwind;design wind load;Wind tunnel test
    Date: 2008
    Issue Date: 2010-01-11 05:24:31 (UTC+8)
    Abstract: 高層建築所承受的風載重,可分為順風向、橫風向及扭轉向三組同時作用的風力。其中之順風向風力是由風速壓與逼近流的紊流特性所決定,基本上可透過準穩定定理與條狀理論合理的進行預測,因此各國風力規範中之相關條文大致接近。至於橫風向及扭轉向風力則是因為渦散分離及紊流尾跡等現象,產生橫向不平衡風壓及不對稱風壓所造成,因為其學理較為複雜,目前仍不易以合適之解析模式或半經驗模式合理預測。
    在目前台灣規範中,根據結構頻率與流場紊流特性計算而得之順風向陣風反應因子,在建築物高度上之分布為一固定值。然而對於柔性建築物之設計風載重而言,其風力之擾動部份佔整體風力之比重相當大,然而其在高度上之分配方式接近慣性力之分布,與平均風力及擾動風力之背景部份明顯不同。因此本文根據風工程界近年來之研究成果,針對高層建築順風向設計風力之計算式提出修正方案,使其能更正確的反應柔性建築結構的動力特性。同時,本文亦藉由一系列之風洞試驗,進行風載重修正方案所需風力參數之量測。
    根據風洞實驗觀測結果提出順風向設計風力修正模式,包含下列假設:(1)假設迎風面之平均風力及擾動風力在高度上分佈遵循條狀定理與準穩定理論;背風面風力則為均勻分布;(2)擾動風力之背景部份引入折減因子以適度考慮空間相關性的影響;(3)共振部份之設計風力依慣性力進行高度上的分配。
    本文參考目前國內風力規範對於橫風向及扭轉向風載重使用條件之幾何限制,設計一系列之風壓模型風洞試驗,以取得順風向設計風力修正模式中所需之氣動力參數。選取之模型斷面高寬比為3、4、5、6、7;深寬比為1/5、1/4、1/3、1/2、1/1、2/1、3/1、4/1、5/1,同時以現行規範定義之地況A、B、C為逼近流場,進行建築物表面風壓之物理模擬風洞試驗。
    文章最後以數值分析計算6棟不同幾何外型之建築物,在台灣規範定義之3種地況對應之逼近流場作用下,其順風向之等值設計風載重。其中選取之建築物斷面高寬比為3及6,深寬比為1/3、 1/1、 3/1,建築物高度分為90公尺及180公尺。評估風載重之方式包括本文推導之半經驗式及台灣現行風力規範,並以風洞實驗歷時風力資料進行之結構分析結果,做為二者分析結果之比較之標準。計算結果顯示,本文推導公式在地況A及B較目前風力規範更能有效反應風洞試驗之評估結果,而在地況C則有偏低之趨勢。該結果說明本文對於風載重之假設,以及據此推導所得之評估模式,能合理表達真實建築物之設計風載重分布情形與數值大小。
    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.
    Appears in Collections:[Graduate Institute & Department of Civil Engineering] Thesis

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