Kazuo Ohtake

Aerodynamic Characteristics of Tall Building Models with Various Unconventional Configurations

Tall buildings have been traditionally designed to be symmetric rectangular, triangular or circular in plan, in order to avoid excessive seismic-induced torsional vibrations due to eccentricity, especially in seismic prone regions like Japan. However, recent tall building design has been released from the spell of compulsory symmetric shape design, and free-style design is increasing. This is mainly due to architects’ and structural designers’ challenging demands for novel and unconventional expressions. Development of computer aided analytical techniques and of vibration control techniques using auxiliary devices has also contributed to this trend. Another important aspect is that rather complicated sectional shapes are basically good with regard to aerodynamic properties for crosswind responses, which is a key issue in tall-building wind-resistant design. A series of wind tunnel tests have been carried out to determine wind forces and wind pressures acting on 31 tall building models with various configurations: square plan, rectangular plan, elliptic plan, with corner cut, with corner chamfered, tilted, tapered, inverse tapered, with setbacks, helical, openings and so on. Dynamic wind-induced response analyses of these models have also been conducted. The results of these tests have led to comprehensive discussions on the aerodynamic characteristics of various tall building configurations, and studies on corresponding optimal structural systems.

Full-scale measurements of wind actions on Chiba Port Tower

Chiba Port Tower is 125 m high with a rhombus-shaped plan. On the top of the tower, a ‘Tuned Mass Damper’ (TMD, hereafter) was installed in order to reduce the vibration caused by strong winds and earthquakes. A lot of data during strong winds has been recorded since August 1987. From this data, it was found that the TMD works effectively when the response of the tower exceeds 0.5 cm/sec2 and that the TMD can reduce the wind-induced response of the tower by nearly 50 percent for all wind directions.

Chiba port tower: Full-scale measurement of wind actions Part 2. Basic properties of fluctuating wind pressures

This paper contains the study concerning the basic properties of pressure fluctuations which has been investigated on Chiba Port Tower starting about one year ago for the purpose of making clear the characteristics of wind pressure on claddings of building. The record of strong wind which we have selected as our observation object this time is taken only at the time of Typhoon whose wind speed at the top of the building is 20 m/s and over. Resultantly, the value of r.m.s. pressure coefficient depends largely on turbulence intensity of wind speed and the r.m.s. pressure coefficient is almost twice of turbulence intensity of wind speed on windward face, and about twice or three times on side face. According to power spectrum analysis, Karman vortex is recognized to occur periodically from this building, and Strouhal number is about 0.08∼0.11 based on the one side length of the building. Also the peak factor of pressure fluctuations on side and leeward faces, compared with the values on windward face, both mode and dispersion are large. It is cleared that the peak factor of pressure fluctuations on side and leeward faces inclines to be considerably larger than the peak factor of wind speed when the averaged period of peak value is under 0.5 second.

Study on Aerodynamic Vibration for Super High-rise Building

2.螺旋形状建築物のロッキング振動実験 筆者らは既報 1), 2), において,螺旋形状建築物が耐風設 計上,優れた空力特性を有することを示した。ここでは, アスペクト比 8 の螺旋形状建築物(以下,Helical180)と 正方形角柱(以下,Square)のロッキング振動実験を実施 し,振動性状を比較した結果を紹介する。 2.1 実験模型 想定建築物とロッキング振動模型の諸元を表1に示す。 模型縮尺は,質量比の制約から,既報 1), 2), の風力・風圧 実験での縮尺 1/1000 よりも大きい 1/694 と設定した。想 定建築物の質量,建築物基部での回転慣性および密度は 既報 と同じとした。ロッキング振動模型は,Helical180, Square モデルとも固有振動数を約 10Hz,減衰定数を 0.5% に調整した。想定建築物の固有振動数は 0.1Hz であり, 実現象と振動実験の時間比は約 1/100 となっている。 表 1 ロッキング振動模型の諸元 想定建築物 振動模型 縮尺 1/1 1/694 質量(kg) 1.78×10 0.532 回転慣性(kgm) 9.03×10 5.59×10 高さ(m) 400 0.576 代表長(m) 50 0.072 密度(kg/m) 178 ← 設計風速(m/s) 70.6 10.2 固有振動数(Hz) 0.1 10.1~10.3

Peak Wind Force Coefficients for Balcony Handrail

集合住宅のベランダ手摺や壁面から突出した広告板 (以下,突出広告板と呼ぶ)は,広く普及しているにも 拘わらず,風荷重設定のためのピーク風力係数が建築基 準法の告示 1)や荷重指針 2),3)に示されていない。ベランダ 手摺に関する既往の研究 4),5)によると,隅角部以外の一般 部においては,ベランダの外側と内側で平均的には等圧 となるため,ピーク風力係数は壁面よりかなり小さいこ とが示されているが,隅角部のピーク風力係数について は実験結果に差異が認められる。また,高さ 1m程度の 手摺に対して,ピーク風力係数の平均化時間を 1 秒とし ており,ピーク値を過小評価している可能性がある。一 方,突出広告板に関しては,建築物の形状や設置される 位置によってピーク風力係数は異なると考えられるが, 既往の研究事例は見当たらない。そこで,本編では,国 土交通省の建築基準整備促進事業の一環として,中層集 合住宅のベランダ手摺と突出広告板を対象に,風洞実験 を実施したので,その結果の一部を報告する。