INTERFERENCE EFFECTS OF TWO BUILDINGS ON PEAK WIND PRESSURES

Transcription

1 The Seventh Asia-Pacific Conference on Wind Engineering, November 8-, 9, Taipei, Taiwan INTERFERENCE EFFECTS OF TWO BUILDINGS ON PEAK WIND PRESSURES Wonsul Kim, Yukio Tamura and Akihito Yoshida 3 Ph.D. candidate, Wind Engineering Research Center Tokyo Polytechnic University Kanagawa, Japan, kim@arch.t-kogue.ac.jp Professor, Wind Engineering Research Center Tokyo Polytechnic University Kanagawa, Japan, yukio@arch.t-kougei.ac.jp 3 Associate Professor, Wind Engineering Research Center Tokyo Polytechnic University Kanagawa, Japan, yoshida@arch.t-kougei.ac.jp ABSTRACT Most standards or codes for wind loads have been derived for isolated buildings. However, in real environments, wind loads on tall buildings in a group can be quite different from those on the isolated tall buildings. Also, surrounding tall buildings can significantly increase or decrease local wind loads affecting cladding, as well as overall wind load. In this paper, local peak wind pressure coefficients on two tall buildings were studied using wind tunnel experiments for various locations of an interfering building and several wind directions for cladding design. Measured wind pressure coefficients were compared with those on the walls of an isolated building. Also, to investigate the interference effects for smallest C p,min for several wind angles, interference effects are presented in detail. The results show that the local peak wind pressure coefficient on the walls of a principal building largely depended upon the location of an interfering building in the walk region and upon the wind angles. KEYWORDS: INTERFERENCE EFFECTS, WIND LOADS, CLADDING DESIGN, LOCAL PEAK WIND PRESSURES, HIGH-RISE BUILDINGS Introduction Most wind load standards have been derived for isolated buildings. However, in real environments, wind loads on tall buildings surrounded by other tall buildings can be quite different from those on isolated tall buildings. Surrounding tall buildings can significantly either increase or decrease local wind loads acting on the cladding of a building, as well as the overall wind load on a building. Unfortunately, few codes or standards have referred to windinduced interference effects on wind loads on buildings, but only briefly accommodate and warn about them [AS 7. (989), AIJ (4) and ASCE7-5 (5)]. This is due to complex natural problems and large variables such as building geometries, relative locations of adjacent building(s), wind directions, upstream terrain conditions and so on. Interference effects have been studied by many researchers over the past several decades [Bailey and Kwok (985), Khanduri et al. () and Chen and Lin (5)]. Most previous researches have studied interference effects such as along- and across-wind responses of buildings depending on the locations of adjacent building(s) using wind tunnel experiments. Some researchers have quantified wind loads to determine the interference mechanisms among three or more buildings [Gu et al. (5) and Zhao and Lam (8)]. In particular, Xie and Gu (7) reported on the interference effects due to relative spacing between groups of two and three buildings through an extensive number of wind tunnel experiments. They proposed

2 The Seventh Asia-Pacific Conference on Wind Engineering, November 8-, 9, Taipei, Taiwan regression equations that reflect the inherent complex relationship to simplify the expressions of interference effects, as well as how to use their results in design of real tall buildings. Unfortunately, however, most past studies have focused mainly on wind loads on the principal building for structural design. This paper focuses on local wind pressures on walls of an identical pair of high-rise buildings using wind tunnel experiments to quantify cladding wind loads. It also investigates interference effects for smallest minimum peak wind pressure for several wind angles. Experimental set-up The wind tunnel experiments were carried out in a boundary layer wind tunnel at the Wind Engineering Research Center at Tokyo Polytechnic University. Using the spireroughness technique, the wind characteristic was simulated by the exponents of a power law with a mean speed profile of.33 according to the Korean Building Code [(KBC 5)]. The wind direction was varied in 5 o intervals in the range from o to 45 o. The measured longitudinal mean wind velocity and the turbulence intensity profiles, and the turbulence spectrum of wind simulation, are shown in Figure and Figure, respectively, where U g is the mean wind speed at gradient height Z g. The gradient height for exposure category A (α =.33) was 5m. I u (%) Nomalized height, Z/Z g.4. Reference height Zg = 5m mean wind speed longitudinal turbulence intensity α = Nomalized velocity, U/ Ug Figure : Mean wind velocity profile and turbulence intensity profile. Measured velocity spectrum at top of building nsu(n)/σu - - Von Karman spectrum: nsu( n) 4 nlu/ Uh = σ u nl u + 7 Uh - - nl u /U h Figure : Spectrum of fluctuating wind velocity at top of a building. 5/6 with L u = 7m in full scale

3 The Seventh Asia-Pacific Conference on Wind Engineering, November 8-, 9, Taipei, Taiwan In paper, the experimental model is referred to as the principal building and the other model is referred to as the interfering building (adjacent building). The aspect ratio of the principal building was 4, and the model scale was /4. 5 pressure taps were installed on the walls of the principal building, as shown Figure 3(a). The interfering model building which the size was same as that of principal building was placed at different locations upstream and downstream of the principal building, which was fixed at (x, y) = (, ). Both models were orientated with one face normal to the wind direction and the center-to-center distance between them was varied by X longitudinally and Y laterally, as shown in Figure 3(b). The pressure data were obtained by sampling at 78 Hz for a period. The time histories of wind pressures were filtered by means of a moving average filter and each test case was sampled times. The maximum peak wind pressure coefficient, C p,max, and minimum peak wind pressure coefficients, C p,min, were defined as C,max = pˆ / q and C = p q, where p ˆi and q p,min i / H H H p i H p i are maximum and minimum wind pressure at point i and = /ρu is the velocity pressure at the reference height. To simplify the complexity for notation of pressure coefficients, largest maximum and smallest minimum values of the 5 pressure taps were expressed as largest C p,max and smallest C p,min, respectively. H = 8mm (8m in full scale) Wind D = 7mm (8m in fulscale) B = 7mm (8m in full scale) (a) Experimental model (b) Coordinate grid for relative locations of interfering model Figure 3: Expermental model and experimental set-up with an interfering building at various locations on X-Y coordinattion Experimental Results and Analysis Mean Along-wind Force Coefficients The interference factor was initially introduced by Saunders and Melbourne (979). The majority of interference effects in later researches have been represented using nondimensional interference factor terms such as the proportion of fluctuation, along- and acrosswind responses and so on from wind loads on a principal building affected by a nearby interfering building, and compared to those for an isolated building. In this paper, the interference factors (IFs) for along wind force coefficients between two identical buildings were compared with those in past studies [English (993) and Xie and Gu (4)] as shown in Figure 4, where S x is the center-to-center spacing between two tall buildings and B is the principal building width. Curve fitting to predict the along-wind

4 The Seventh Asia-Pacific Conference on Wind Engineering, November 8-, 9, Taipei, Taiwan interference factor was proposed by English (993). The formula is given in polynomial form as: IF x x x 3 = () where x log [ Sh ( b) / hb] = +, S is the clear spacing between the two buildings, b and h are the breadth and the height of the principal buildings. Triangular and square shapes were studied by Xie and Gu (4) for aspect ratio of 6 for square buildings. Exposure categories B and D according to the Chinese Load code corresponded to exponents of a power law with a mean wind speed profile of and.3, respectively. In Figure 4, the present test data were in very good agreement with those of past studies. It can be seen that the mean along-wind force coefficients are reduced due to shielding effect, as shown in Figure 4. The shielding effect is a special case of interference in which wind loads on a building are actually reduced due to obstruction of wind flow by the upwind building, where the windward wall of the principal building is immersed in the wake English Xie & Gu (Category B: GB59-) Xie & Gu (Category D: GB59-) Present test (Category A: KBC-5) Interference factor (IF) B y D S x x WIND S x /B Figure 4: Interference effects on along wind force for wind direction o - Left-side Windward Right-side Leeward Left-side Windward Right-side Leeward (a) Isoalted building (b) Interfering building at (S x, S y ) = (.5B, B) Figure 5: Distribution of mean wind pressure coefficients on walls of principal building with and without an interfering building

5 The Seventh Asia-Pacific Conference on Wind Engineering, November 8-, 9, Taipei, Taiwan caused by the interfering building as shown in Figure 5 (b). Figure 5 shows distribution of C p,mean on walls of principal building with and without the interfering buliding. For close separation, (S x, S y ) = (.5B, ), the principal building may experience severe suction, that is, a pull towards the interfering building, as occurred on the windward wall surface in Figure 5(b). Local Peak Wind Pressure Coefficients Figure 6 shows the variations of largest maximum and smallest minimum wind pressure coefficients with wind angles and configuration of an interfering building. The results for the isolated building were compared with those for various configurations of the interfering building. At wind angle o, When the adjacent building is located at (S x, S y ) = (.5B, ), both of the along-wind load and largest maximum wind pressure coefficient are less than those of the isolated building, but, for the other arrangements, largest C p,max are similar to that on isolated building, while the along-wind load dramatically decreases due to the shielding effects as shown Figure 4 and Figure 6 (a), i.e., the configuration of the interfering building and wind angles caused almost no interference effects on the largest C p,max on the principal building. For the smallest C p,min, when the interfering building was located in tandem, the overall smallest C p,min increased to -4.7 at wind angle o, but it dramatically decreased to -. because of obstruction of wind flow by the upwind interfering building as the wind angle increases, as shown Figure 6(a). Largest Cp,max 4 3 (Sx,Sy) = (-4B, ) (Sx,Sy) = (-B, ) (Sx,Sy) = (-.5B, ) (Sx,Sy) = (.5B, ) (Sx,Sy) = (B, ) (Sx,Sy) = (3B, ) (Sx,Sy) = (4B, ) (Sx,Sy) = (6B, ) Smallest Cp,min (a) Tandem configuration (Sx,Sy) = (-4B, ) (Sx,Sy) = (-B, ) (Sx,Sy) = (-.5B, ) (Sx,Sy) = (.5B, ) (Sx,Sy) = (B, ) (Sx,Sy) = (3B, ) (Sx,Sy) = (4B, ) (Sx,Sy) = (6B, ) Largest Cp,max 4 3 (Sx,Sy) = (,.5B) (Sx,Sy) = (, B) (Sx,Sy) = (, 3B) (Sx,Sy) = (, 4B) Smallest Cp,min (Sx,Sy) = (,.5B) (Sx,Sy) = (, B) (Sx,Sy) = (, 3B) (Sx,Sy) = (, 4B) (b) Side-by-side configuration Figure 6: Variations of largest maximum and smallest minimum wind pressure coefficient with wind angle and configuration of an interfering building

6 The Seventh Asia-Pacific Conference on Wind Engineering, November 8-, 9, Taipei, Taiwan Largest Cp,max 4 3 (Sx,Sy) = (.5B,.5B) (Sx,Sy) = (B, B) (Sx,Sy) = (.5B,.5B) (Sx,Sy) = (3B, 3B) (Sx,Sy) = (4B, 4B) Smallest Cp,min (c) Oblique configuration (Sx,Sy) = (.5B,.5B) (Sx,Sy) = (B, B) (Sx,Sy) = (.5B,.5B) (Sx,Sy) = (3B, 3B) (Sx,Sy) = (4B, 4B) Figure 6: Variations of largest maximum and smallest minimum wind pressure coefficient with wind angle and configuration of an interfering building (continued) For the side-by-side configuration, when the interfering building was located at (S x, S y ) = (,.5B), the smallest C p,min significantly increased to -4.9 at wind angles from θ = o to 5 o, as shown Figure 6(b). This can be explained by the fact that when the wind was channeled to flow through the space between the two buildings, high peak C p,min is induced on the wall of the principal building due to the channeling effects. On the other hand, the smallest C p,min for other cases was similar to those of the isolated building as the interfering building was located far from the principal building. For the oblique configuration, the smallest C p,min significantly increased for wind angles from 5 o to 4 o, as shown Figure 6(c). It should be noted that the interfering building located in an oblique configuration can cause more severe peak negative wind pressure than those due to tandem and side-by-side configurations. Interference Factors In order to investigate the interference effects for smallest C p,min in detail with various locations of the interfering building and several wind angles, the interference effects are presented in the form of non-dimensional interference factor (IF min ), which represents the smallest C p,min on the walls of the building with interference from an interfering building. The IF min can be expressed as: Smallest C p,min on walls with an interfering building IF min = () Smallest C p,min on walls without an interfering building In Figure 7, the increase in smallest C p,min is immediately apparent. From Figure 7(a), when the interfering building is located in tandem or side by side, IF min increases by about % which may be increased wind speed from the edges of the upwind interfering building and channeling effects between the two buildings. The interesting observation from Figure 7(b) is that the ridge of the contours is oriented at 45 o at wind angle 5 o. When the interfering building is located along this ridge, the largest IF min of.6 is obtained upwind at (S x, S y ) = (B, B), which indicates an increase of 6% over the isolated building s smallest C p,min of -3.5, and IF min is significantly increased. From Figure 7(c), the ridge of the contours is oriented in the direction of the incident wind and the contours exhibit a degree of symmetry about this ridge. And IF min increases to 5%, as shown in Figure 7(c).

7 The Seventh Asia-Pacific Conference on Wind Engineering, November 8-, 9, Taipei, Taiwan 4B 3B. Sy B B.9.. B -4B -3B -B -B B B 3B 4B 5B 6B S x Smallest C p,min () = -3.9 (a) Wind angle o. 4B Sy 3B B B B -4B -3B -B -B B B 3B 4B 5B 6B S x. Smallest C p,min () = -3.5 (b) Wind angle 5 o.9 4B.. 3B Sy B B B -4B -3B -B -B B B 3B 4B 5B 6B S x.9 Smallest C p,min () = -3. (c) Wind angle 45 o.9 Figure 7: Effects of wind angles on smallest minimum wind pressure coefficient Conclusions Interference effects for largest maximum and smallest minimum wind pressure coefficients on walls of principal buildings were studied for various locations of an adjacent building and several wind directions by wind tunnel experiments from viewpoint of cladding design. The results of this study have been presented with respect to the interference factor, IF min for smallest minimum wind pressure coefficient, and are summarized as follows. () The largest C p,max on the walls of a principal building are similar to that on an isolated building, although the along-wind load dramatically decreases due to the shielding

8 The Seventh Asia-Pacific Conference on Wind Engineering, November 8-, 9, Taipei, Taiwan effects. There are almost no interference effects on largest C p,max on a principal building due to locations of the interfering building and wind angles. () At wind angle 5 o, when the interfering building is located along ridge of the principle building, a largest IF min of.6 is obtained upwind at (S x, S y ) = (B, B), which indicates an increase of 6% over the isolated building s smallest C p,min of The interesting observation is that an interfering building located in oblique configuration can cause more severe peak negative wind pressure than one located in tandem or side-by side configurations. It is expected that the results of this study will be useful in estimating approximate cladding load on a building under interference for preliminary design purposes. Acknowledgement This study was funded by the Japan Society for Promotion of Science (JSPS), and the Ministry of Education, Culture, Sports, Science and Technology, Japan, through the Global Center of Excellence Program, 8-. The authors also gratefully acknowledge their support. References Architectural Institute of Japan (AIJ 4), Chapter 6: Wind Loads, Recommendations for Loads on Buildings, 4 Bailey, P.A. and Kwok, K.C.S. (985), Interference excitation of twin tall buildings, Journal of wind Engineering and Industrial Aerodynamics,, Cheng, C.M. and Lin, Y.C. (5), Interference effects on the design wind loads of tall buildings, proceedings of the sixth Asia-Pacific Conference on Wind Engineering, Seoul, Korea, English, E.C., (993), Shielding factors for paired rectangular prisms: an analysis of along-wind mean response data from several sources, Proceedings of the Seventh US National Conference on Wind Engineering, Los Angeles CA, 93- Gu, M., Xie, Z.N. and Huang, P. (5), Along-wind dynamic interference effects of tall buildings, Advance in Structural Engineering, 8(6), Khanduri, A.C., Stathopoulos T. and Bédard C. and (), Generalization of wind-induced interference effects for two buildings, Wind and Structures, 3(4), Korean Building Code Structural (KBC-S 5), Chapter 3-35: Wind Loads, Architectural Institute of Korea, 5 Minimum Design Loads for Buildings and Other Structures (ASCE7-5), Chapter 6: American Society of Civil Engineers, Reston, VA, USA, 5 Minimum Design Loads on Structures (SAA Loading Code), Part : Wind Loads, AS 7.. Standards Association of Australia, North Sydney, Australia, 989 Saunders, J.W. and Melbourne, W.H. (979), Buffeting effects of upstream buildings, Proceedings of the Fifth International Conference on Wind Engineering, Fort Collins, Colorado, Pergamon Press, Oxford, Xie, z.n. and Gu, M. (4), Mean interference effects among tall buildings, Engineering Structures, 6, Xie, Z.N and Gu, M. (7), Simplified formulas for evaluation of wind-induced interference effects among three tall building, Journal of wind Engineering and Industrial Aerodynamics 95, 3-5 Zhao, J.G. and Lam, K.M. (8), Interference effects in a group of tall buildings closely arranged in an L- or T-shaped pattern, Wind and Structures, (), -8