EFFECTS OF SIDEWALL OPENINGS ON THE WIND LOADS ON PIPE-FRAMED GREENHOUSES

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The Seventh Asia-Pacific Conference on Wind Engineering, November 8-12, 29, Taipei, Taiwan EFFECTS OF SIDEWALL OPENINGS ON THE WIND LOADS ON PIPE-FRAMED GREENHOUSES Yasushi Uematsu 1, Koichi Nakahara

1 The Seventh Asia-Pacific Conference on Wind Engineering, November 8-12, 29, Taipei, Taiwan EFFECTS OF SIDEWALL OPENINGS ON THE WIND LOADS ON PIPE-FRAMED GREENHOUSES Yasushi Uematsu 1, Koichi Nakahara 2, Satoshi Tanaka 3, Hideki Moriyama 4, Sadanori Sase 5 1 Professor, Department of Architecture and Building Science, Tohoku University, Sendai , Japan, yu@venus.str.archi.tohoku.ac.jp 2 Utsunomiya Branch, Daiwa House Industry Co. Ltd., Utsunomiya , Japan 3 Graduate Student, Department of Architecture and Building Science, Tohoku University, Sendai Japan, tanaka@venus.str.archi.tohoku.ac.jp 4 Senior Researcher, National Institute for Rural Engineering, Tsukuba , Japan 5 Team Head, National Institute for Rural Engineering, Tsukuba , Japan ABSTRACT Wind pressures acting on the external and internal surfaces of a 1:4 scale model of pipe-framed greenhouse with porous sidewalls are measured simultaneously at many points in a turbulent boundary layer. Wind speeds at several points outside and inside the greenhouse model are also measured. The porosity of the sidewalls is changed over a wide range. The equivalent static wind force coefficients generating the maximum peak response are evaluated by using the LRC (Load Response Correlation) method, in which the bending moment developed in the frame at the windward column base is regarded as the most important load effect. The effects of porosity on the wind-induced response of the frame as well as on the wind speeds inside the greenhouse are discussed. The results indicate that the porosity of approximately can reduce the maximum bending moment by approximately 2 percent without a significant increase in wind speed inside the greenhouse. KEYWORDS: PIPE-FRAMED GREENHOUSE, WIND LOAD, SIDEWALL OPENING, LOAD REDUCTION, DYNAMIC LOAD EFFECT, WIND-TUNNEL EXPERIMENT Introduction Pipe-framed greenhouses, as shown in Figure 1(a), are widely used in the agricultural and horticultural industries in Japan. Such greenhouses are designed to a lower level of structural safety than conventional structures, because of the need to minimize capital costs, the demand for a higher level of light transmission and so on. They are consequently very lightweight structures that are vulnerable to wind loading. In practice, they often experience damage during windstorms (Figure 1(b)). The wind resistance is one of the greatest concerns for structural engineers when designing these structures. For reducing the wind-induced damage, it may be common to strengthen the structures, e.g. by using members of larger size. However, this method causes an increase in construction cost. The alternative is to change the shape of cross section. Uematsu et al. (25), for example, proposed a new type of greenhouse with a cross section similar to a wing, which reduces the wind loads significantly. Making the sidewalls porous also provides a reduction in wind loads on commonly-used pipeframed greenhouses, even if the shape of cross section is not changed. The present paper investigates the effect of sidewall openings on the wind loads and the resultant responses of pipe-framed greenhouses. The bending moment developed in the frame at the windward column base is regarded as the most important load effect from the structural viewpoint. The effect on the wind speed inside the greenhouse is also discussed.

(a)typical shape (b)collapse by typhoon Figure 1: Pipe-framed greenhouse Experimental Arrangement and Procedure The experiments were carried out in the boundary layer wind tunnel with a working

The power spectrum of wind velocity fluctuations was consistent with the so-called Karman-type spectrum.

2 (a)typical shape (b)collapse by typhoon Figure 1: Pipe-framed greenhouse Experimental Arrangement and Procedure The experiments were carried out in the boundary layer wind tunnel with a working section 4 m wide, 3 m high and 2 m long at the National Institute for Rural Engineering. An open-country exposure was simulated in the wind tunnel. The power law exponent α of the mean velocity profile of the flow is 5, which corresponds to the Terrain Category II specified in the AIJ Recommendations for Loads on Buildings (24). The turbulence intensity I u and longitudinal scale L x of the flow at a height of z = 1 mm, nearly equal to the model height, are 9 and 3 m, respectively. The power spectrum of wind velocity fluctuations was consistent with the so-called Karman-type spectrum. The turbulence scale L x was obtained by fitting the curve of the spectrum to the experimental data. The wind tunnel model is shown in Figure 2. The cross section of the model is somewhat different from that of pipe-framed greenhouses commonly used in Japan. In the wind tunnel model, the sidewalls are vertical and the roof has no curvature. The geometric scale of the model is assumed 1:4. Therefore, the dimensions of the corresponding full-scale structure are span: S = 7.2 m, length: L = 21.6 m, and height: H = 3.9 m. The lower part of the sidewalls, up to a height of 22.5 mm (.9 m in full scale), is open. Fourteen and twelve pressure taps of mm I.D. are installed on the external and internal surfaces of the model, respectively, along each of the three lines (C, G and E); the taps are installed only on the external surface of the columns at the lowest level (z = 1 mm). Wind pressures at these taps are measured by a pressure scanning system (Kyowa Dengyo, F ) in parallel at a rate of 2 Hz on each channel for a period of 5 min. The tubing effects on the fluctuating pressures are numerically compensated by using the gain and phase-shift characteristics of the pressure measuring system. The wind pressure and force coefficients (a) Picture of a model θ = 9 O Line E 33.8 Line C 3 #4 6 #3 6 #2 3 3 #1 Line G Anemometer θ = O (b) Dimension of the wind tunnel model (unit: mm) Figure 2: Wind tunnel model

3 are defined in terms of the velocity pressure q H at the level of the roof top H. The wind speed U H at z = H is approximately 6.7 m/s. The corresponding Reynolds number Re, defined in terms of U H and H, is approximately The velocity scale of the wind tunnel flow is assumed 1:4.5 for typical strong wind events, which results in a time scale of 1:8.9. The porosity (φ W and φ L ) of the windward and leeward sidewalls is provided by attaching mm thick porous plates on the openings (see Figure 2(a)). The values of φ W and φ L tested are listed in Table 1. The wind direction θ is changed from o to 9 o at a step of 15 o for Lines E and C, and from o to 9 o for Line G. Regarding the results for Line G, the wind direction θ is replaced by θ, hereafter. The mean wind speeds at four locations (Points #1 #4) shown in Figure 2(b) are measured by a thermistor anemometer in order to investigate the effect of sidewall openings on the wind speeds outside and inside the greenhouse. The height of measurements is 12.5 mm ( m in full scale) above the wind tunnel floor. Table 1: Porosity of the windward and leeward sidewalls Porosity Case Windward φ W Leeward φ L Experimental Results and Discussion External and Internal Pressure Coefficients Figure 3 shows the distributions of the mean external pressure coefficients Cpe_mean along Lines C and G in oblique winds in the cases where φ W = φ L. In the figure, s represents the coordinate along the frame with the origin at the windward column base, and s max the total length of the frame. The wind pressure on the windward roof is minutely affected by the sidewall openings, while the effect on the leeward roof is significant. As the porosity increases, the values of C pe_mean on the leeward roof become less negative. Cpe_ mean (a)line C (θ = 3 o ) (b)line G (θ = 45 o ) Cpe_ mean Figure 3: Distributions of C pe_mean along Lines C and G

4 Cpe (rms).6.4 (eaves) (ridge) (eaves).6 Cpe (rms) Figure 4: Distributions of C pe_rms along Lines C and G (a)line C (θ = 3 o ) (b)line G (θ = 45 o ) Cpi_ mean - (eaves) (ridge) (eaves) Cpi_ mean (a)line C (θ = 3 o ) (b)line G (θ = 45 o ) Figure 5: Distributions of C pi_mean along Lines C and G Similar comparisons for the RMS external pressure coefficients C pe_rms are shown in Figure 4. It is found that the value of C pe_rms on the leeward roof decreases with an increase in porosity. These features on C pe_mean and C pe_rms may be related to the velocity of the separated flow at the ridge. That is, as the porosity increases, the velocity at the ridge decreases, which causes an increase (decrease in magnitude) in pressure. The distributions of the mean internal pressure coefficients C pi_mean along Lines C and G are shown in Figure 5. The value of C pi_mean changes significantly with φ W and φ L. However, the spatial variation of C pi_mean is relatively small, except the cases where the values of φ W and φ L are both large. When φ W = φ L (Cases 1, 3, 7 and 8), the value of C pi_mean generally decreases (increases in magnitude) with an increase in porosity. The value of C pi_rms is nearly constant over the whole area. Figure 6 shows the RMS internal pressure coefficients C pi_rms. As the porosity is increased, the values of C pi_rms increase. However, they are less sensitive to the porosity than the mean internal pressure coefficients C pi_mean. Equivalent Static Wind Force Coefficients Producing the Maximum Load Effect Uematsu et al. (28) investigated the load estimation for pipe-framed greenhouses. They suggested that the most important load effect from the structural viewpoint was the bending moment developed in the frame at the windward column base. Furthermore, it was found that the LRC (Load Resistance Correlation) method (Kasperski, 1992) provided reasonable load estimation for designing the structural frames. Therefore, this method is

5 Cpi (rms) case Figure 6: Distributions of C pi_rms along Line G for θ = 45 o. employed in the present study, assuming that pipe frames are located along the lines of pressure measurements. For evaluating the equivalent static wind force coefficients C f_eq based on the LRC method, we need the influence coefficients that correlate the wind pressures at pressure taps with the bending moment developed in the frame at the windward column base. The influence coefficients are computed by using a finite element method (FEM). In the analysis, the outside diameter d and thickness t of the pipes and the spacing l of the frames are assumed d = 35 mm, t = 1.6 mm and l = 5 cm, respectively. These dimensions are typical for pipeframed greenhouses under consideration in the present study. Figure 7 shows the distributions of C f_eq along Lines C and G in the cases where φ W = φ L. Note that only the external pressures are considered in Case (enclosed structure). The distributions of C f_eq along the frame for various cases are similar in shape. As the porosity increases, however, the values of C f_eq generally increase. This feature is mainly due to the effect of the internal pressure on the C f_eq distribution. Cf _eq (a)line C (θ = 3 o ) (b)line G (θ = 45 o ) Figure 7: Distributions of C f_eq along Lines C and G Cf _eq Then, the distribution of the bending moment M developed in the frame is computed by using the C f_eq distribution. In general, the maximum value is observed at the windward column base of Frame G in an oblique wind. Figure 8 shows the maximum non-dimensional bending moment M * (= M/(q H lsh)) at the windward column base of Frame C, plotted as a function of wind direction θ. The results for Frame G are shown in Figure 9. The value of M * becomes the maximum when θ = 3 o 45 o, and the maximum value depends on both φ W and φ L.

6 The results in Figures 8 and 9 indicate that the effects of φ W and φ L on M * becomes the greatest when φ W = φ L. Under such a condition the maximum bending moment can be reduced by approximately 2 percent compared with that for the enclosed structures (Case ). case 2 case 3 (a) (b) Figure 8: Variation of M * with wind direction θ (Frame C) case 2 case 3 (a) (b) Figure 9: Variation of M * with wind direction θ (Frame G) Wind speed ratio R Wind direction Wind direction 15 Wind direction 3 Wind direction 45 Wind speed ratio R Wind direction Wind direction 15 Wind direction 3 Wind direction Point (a)case 1 (φ W = φ L = ) (b) Case 3 (φ W = φ L = ) Figure 1: Distributions of Mean Wind Speed Point Wind Speeds inside the Greenhouse Figure 1 shows the wind speed ratios R at Points #1 to #4 (see Figure 2(b)) for two cases where the sidewall opening significantly reduces the bending moment at the windward

7 column base; R is defined as the ratio of the mean wind speed at each measuring point to that of the approaching flow at the same height (z = 12.5 mm). At Point #1 outside the greenhouse, the value of R changes significantly with wind direction. On the other hand, at Points #2 to#4 inside the greenhouse, the value of R is less sensitive to the wind direction; the value decreases in the leeward direction. The value of R for oblique wind directions ranges from to.4 when φ W = φ L = and from to when φ W = φ L =. When the values of φ W and φ L are larger than.7, on the other hand, the wind speed ratio is higher than.8. Concluding Remarks The effects of sidewall openings on the wind-induced responses of pipe-framed greenhouses were investigated in a wind tunnel. When the porosity of the windward and leeward sidewalls is approximately, the maximum bending moment developed in the frame at the windward column base for oblique wind directions can be reduced by approximately 2 percent compared with that of the enclosed structures. Under such a condition, the wind speed inside the greenhouse ranges from 3 to 5 percent of that of the approaching flow at the same height. These results suggest that the load effect is decreased significantly by making the sidewalls porous. If this idea is applied successfully to practical pipe-framed greenhouses, the wind-induced damage may be reduced significantly without reinforcement of the frames. Acknowledgement Acknowledgement is due to the Japan Iron and Steel Federation and the National Institute for Rural Engineering for financial supports. References Architectural Institute of Japan (24), Recommendations for Loads on Buildings. Kasperski, M.(1992), Extreme wind load distributions for linear and non-linear design, Engineering Structures, 14(1), Uematsu, Y., Orimo, T., Watanabe, S., Kitamura, S. and Iwaya, M. (25), Wind loads on a steel greenhouse with a wing-like cross section, Proceedings of the Fourth European and African Conference on Wind Engineering, Prague, July, 25 (Paper 11). Uematsu, Y., Nakahara, K., Moriyama, H., Sase, S (28), Study of wind loads on pipe-framed greenhouse - External wind pressure coefficients on enclosed structure-, The Journal of the Society of Agricultural Structures, Japan, 39(2),

Pressure coefficient on flat roofs of rectangular buildings

Pressure coefficient on flat roofs of rectangular buildings T. Lipecki 1 1 Faculty of Civil Engineering and Architecture, Lublin University of Technology, Poland. t.lipecki@pollub.pl Abstract The paper