Effects of wind incidence angle on wind pressure distribution on square plan tall buildings

Transcription

1 J. Acad. Indus. Res. Vol. 1(12) May RESEARCH ARTICLE ISSN: Effects of wind incidence angle on wind pressure distribution on square plan tall buildings S.K. Verma 1, A.K. Ahuja 2* and A.D. Pandey 3 1 Dept. of Civil Engg., PEC University of Technology, Chandigarh, India 2 *Dept. of Civil Engg., 3 Dept. of Earthquake Engg., Indian Institute of Technology Roorkee, Roorkee, India ahujafce@iitr.ernet.in * ; Abstract Experimental study was carried out on rigid model of a square plan tall building made of Perspex sheet in a closed circuit wind tunnel under boundary layer flow. Wind pressure coefficients are calculated from wind pressure values measured at many pressure points on all 4-wall surfaces of the model. Purpose of the study is to generate large amount of data for the designers to be able to design structural frame as well as wall claddings safely under wind loads. The results of the experimental study are presented in the form of pressure contours. Keywords: Square plan tall building, Perspex sheet, wind pressure coefficients, pressure contours. Introduction Wind is one of the important loads to be considered while designing tall buildings. The designers refer to relevant wind loading codes for evaluation of wind loads. Information regarding wind pressure coefficients, both internal and external, on buildings with flat and sloping roofs are available in code of practices of various countries dealing with wind loads [AS/NZS: , ASCE: , IS: 875-(Part-3)-1987, NBC-(Part-4)-1995]. However, the information available in such codes regarding wind pressure or force coefficients even for a simple plan shape such as square one is for limited wind directions only. Critical values of wind pressures at skew angles, may be useful for designing specially the claddings, are generally not available to the designers. Although many researchers (Isyumov and Poole, 1983; Stathopoulos, 1985; Balendra and Nathan, 1988; Kwok, 1988; Mir, 1988; Ahuja and Jain, 1990; Jamieson et al. 1992; Kawai, 1998; Gomes et al. 2005; Amin, 2008; Dalui, 2008) have carried out experimental studies on the models of tall buildings, yet information available is not comprehensive enough to do wind load analysis on wall claddings correctly by the designers. An experimental study is thus carried out on the model of a square plan tall building to measure wind pressure distribution on its all wall surfaces under varying wind incidence angles. Materials and methods Wind tunnel and flow characteristics: The experimental study is conducted in the closed circuit, closed test section wind tunnel available at the Dept. of Civil Engineering, IIT Roorkee, Roorkee, India. It has a test section of 1.30 m (width) x 0.85 m (height) and the length of test section is 8.25 m. A manually controlled turntable is installed at 6 m distance from the upstream edge of the test section. Model under study is installed at the centre of the turntable, which is rotated to adjust the angle of wind incidence. The model is tested for 7 wind incidence angles namely 0, 15, 30, 45, 60, 75 and 90. Under normal condition, the flow inside the wind tunnel is uniform. However, model of the tall building is tested in an artificially generated gradient velocity field, which is generated by placing the grid of horizontal bars at upstream end of the tunnel. The grid is made of 21 number aluminium pipes of external diameter 9.5 mm. The bars are closely spaced near the floor and their spacing is increased as one move away from the floor. The velocity of flow at different heights of the wind tunnel from the floor is measured with the help of Pitot tube. The velocity profile in the wind tunnel at the centre of the turntable has power law index of 0.15 which corresponds to terrain category 2 as per IS: 875 (part-3)-1987 and the maximum value of turbulence intensity near the floor is about 2.5%. The free stream wind velocity at 0.5 m above the floor is maintained as 15 m/sec in the present study. Model description: The model of the square plan tall building is made of 5 mm thick Perspex sheet. The prototype is assumed to have plan dimensions of 30 m x 30 m and height of 180 m. The model is fabricated at a length scale of 1:600 resulting in model dimensions of 50 mm x 50 mm in plan and 300 mm in height (Fig. 1). Stainless steel pressure tubing of 1.0 mm internal diameter and mm length are placed flush with the model surface as pressure tapping. One end of each PVC tube is connected to stainless steel pressure tubing and another end is connected to Baratron Pressure Transducer. The building model is provided with in all 105 pressure points as shown in Fig. 2 with 21 points each on faces-a to C and 42 points on face-d.

2 J. Acad. Indus. Res. Vol. 1(12) May Fig. 1. Dimensions of the tall building model and wind directions (All dimensions are in mm).. Where, Pmean = Local mean pressure for a given averaging time, Po = Static (ambient, atmospheric) reference pressure, = Air density, V = Free stream wind velocity. Results and discussion At 0 wind incidence angle, wind strikes at right angle to face-a and hence the entire face experiences pressure (Table 1) with face average value of Cp(mean) as 0.82 which is quite similar to the values suggested by wind loading codes of Australia, India and USA (Table 2). This value goes on decreasing with increase in wind incidence angle. It becomes suction at 60 and remains negative till wind incidence angle is 90 (Table 3). Apart from face average values of mean wind pressure coefficients, Table 3 lists maximum and minimum values of mean wind pressure coefficients also. The peak suction value is observed at 75 wind incidence angle on face-a. Values of mean wind pressure coefficients in Table 3 indicate that there is significant effect of wind incidence angle on face average, maximum and minimum values of mean wind pressure coefficients for face-a. Fig. 2. Positions of pressure tapings on various faces of the building model. Table 1. Mean wind pressure coefficients on face-a at 0 wind incidence angle. Pressure Point No. Cp(mean) Processing of results: The wind pressures on the surfaces of the model are measured at each pressure point using pressure transducer and are expressed in the form of non-dimensional pressure coefficients defined as follows: Mean wind pressure coefficient = Cp(mean) = Pmean P 1/ 2 V o 2 Table 2. Comparison of face average Cp (mean) values at 0 wind incidence angle for a square plan building. Wind loading code Experimental value IS ASCE AS/NZS

3 J. Acad. Indus. Res. Vol. 1(12) May Table 3. Face average Cp(mean) values on different faces for various wind incidence angles. Building Face Wind angle Avg Max Min Avg Max Min Avg Max Min Avg Max Min Fig. 3. Contours of mean wind pressure coefficients at 0 wind incidence angle Distribution of wind pressure on entire face-a for 5 wind incidence angles namely 0, 15, 30, 45 and 60 are shown in Fig. 3 to 7. At 0 wind incidence angle, the pressure is found to be more at middle as compared to at edges. At 15, 30 and 45 wind incidence angles, it is observed that windward edge (edge close to face-b) experiences large values of pressure. With increase in wind incidence angle, suction is found to be more near leeward edge (edge close to face-d). At 60, the face is subjected to very less pressure or suction. At 75 and 90 wind incidence angles, entire face-a is subjected to suction. It is also observed from the contours of mean wind pressure coefficients that the pressure on the face along height is found to increase with height when there is positive pressure on the face. As the skewness of wind incidence angle increases, the pressure distribution on the face tends to be more uniformly distributed. For example at 45 wind incidence angle, this face experiences almost no variation along height. For all wind incidence angles, pressure near the top edge of the building is found to be smaller than lower region due to up-wash caused by separation of flow. Distribution of wind pressure on face-b for different wind incidence angles can be seen in Fig. 3 to 7. At 0 wind incidence angle face-b is side face and is parallel to wind. At 90 wind incidence angle, wind strikes at right angle to the face.

4 J. Acad. Indus. Res. Vol. 1(12) May Fig. 4. Contours of mean wind pressure coefficients at 15 wind incidence angle Fig. 5. Contours of mean wind pressure coefficients at 30 wind incidence angle Face A Face B Face C Face D

5 J. Acad. Indus. Res. Vol. 1(12) May Fig. 6. Contours of mean wind pressure coefficients at 45 wind incidence angle Face A Face B Face C Face D Fig. 7. Contours of mean wind pressure coefficients at 60 wind incidence angle

6 J. Acad. Indus. Res. Vol. 1(12) May As wind incidence angle increases from 0 to 90, positive pressure effect on this face goes on increasing. However, at 15 wind incidence angle the absolute value of face average Cp(mean) is found to be more than that at 0. Similarly at 75, positive pressure is found to be more than that at 90 as shown in Table 3. Strong suction at the top edge on windward side corner on face-b may be attributed to irregular formation of eddies particularly at this zone at 15. It is noticed from the values of Cp(mean) in Table 3 that there is significant effect of wind incidence angle on face average values of pressure coefficients for face-b i.e. side face, similar to the one observed for face-a as both the faces experience positive pressure at some angles and suction at some other angle. This face experiences negative pressure at all wind incidence angles. The face average Cp(mean) at 0 wind incidence angle (when face is leeward) is -0.76, which is (in terms of absolute value) maximum out of all 7 wind incidence angles. For leeward face there is large variation from the value specified in Indian standard on wind loads (Table 2). This discrepancy may be because of the fact that probably IS Code does not consider the effect of large turbulence at the back of the building model. For skew angles the face average Cp(mean) is almost same, i.e. for 15 to 75 the values of face average Cp(mean) are almost same as shown in Table 3. Absolute maximum value of face average Cp(mean) is observed as at 0 angle. This face experiences negative pressure at all wind incidence angles (Fig. 3 to 7). At 0 wind incidence angle (face is parallel to the wind), the face average Cp(mean) is maximum (absolute value) out of all angles and the value is as shown in Table 3. For skew angles, face average Cp(mean) is almost same (-0.60). At 90 wind incidence angle (when wind blows perpendicular to the face), face average Cp(mean) is which is close to the face average for face-c for 0. The values of face average for Cp(mean) is comparable to those obtained for face-c for analogous position. For 90 wind incidence angle, the face average is IS: 875 (part3)-1987 specifies value of for the corresponding situation. The maximum value of face average Cp(mean) for this face is at 0 and the value is Conclusion Following are the conclusive points derived at the end of the study: 1. The face of a tall building under direct impact of wind experiences positive pressure. 2. When there is positive pressure on the face, it is roughly parabolic distribution with higher value of pressure in the middle and decreasing towards the edges. 3. The pressure increases with height for most of the regions of the face experiencing positive pressure but it decreases near top of the face. 4. There is suction on side faces and on leeward face of the tall building. 5. Maximum suction on side face is at 15 and not at 0 wind incidence angle. 6. The experimental value of face average of Cp(mean) on leeward face differs significantly from those specified in most of the standards on wind loads. 7. The suction increases from windward edge to leeward edge at 0 wind incidence angle on side face of the building. 8. Face average Cp(mean) and pressure distribution on the face is significantly influenced with wind incidence angles with positive pressure on the face at some wind incidence angles and negative pressure on the face at some others. Acknowledgements The work presented in this paper is part of the work done by the first author under the supervision of remaining authors for his Ph.D. Degree at IIT Roorkee between 2005 and References 1. Ahuja, A.K. and Jain, M Wind loads on tall buildings. 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