Wind Loads on Low-Rise Building Models with Different Roof Configurations

Wind Loads on Low-Rise Building Models with Different Roof Configurations Deepak Prasad, Tuputa Uliate, and M. Rafiuddin Ahmed School of Engineering and Physics, Faculty of Science and Technology, The University of the South Pacific Suva, Fiji E-mail: ahmed r@usp.ac.fj Wind tunnel testing of low-rise building models with flat, gabled and hip roof configurations was carried out in a boundary layer wind tunnel. All the models had the same mean height. For the gabled and the hip roofs, the pitch angles investigated were 15, 20, 30 and 45. Pressure measurements were performed on all the walls and the roof of the building models facing a turbulent wind of 7 m/s and the values of pressure coefficient were calculated. It was found that the suction over the roof is significantly influenced by the roof configuration. The 45 gabled and hip building models performed the best under the same wind conditions. The peak suction over the roof reduces by 85 and 91%, respectively, compared to that over the flat roof. In addition to this, the hip roof models recorded less suction compared to their gabled counterparts. For the hip roof, the peak suction reduced by 42 % compared to the gabled roof. * * * Introduction Majority of the houses that are constructed all over the world are low-rise buildings [1]. According to American Society of Civil Engineers (Standard for Minimum Loads for Buildings and Other Structures, ASCE 7-02), a low rise building is defined as a structure with a mean roof height less than the least horizontal dimension and less than 18.3 m [2]. Most of the buildings constructed in the pacific region are low-rise buildings. These buildings are constructed in different types of terrain and topography with various planforms. The lateral strength of buildings in areas other than the high seismic zones is mainly governed by wind loads and this aspect is more evident in zones of severe winds such as coastal regions, open terrains and summit of hills. It is very important to study the wind loads acting on these buildings. Wind loads on structure are due to buffeting (fluctu- Received 09.08.2008 231 ISSN 1064-2277 c 2009 Begell House, Inc.

ating force produced by a fluctuating wind speed), turbulence generated between the structure and the incoming wind flow and also by strong winds. Measurements of the static pressure on low-rise building models in boundary layer wind tunnels provide vital information that can be used to design houses which are safer and more resistant to adverse weather conditions such as cyclones and hurricanes. 1. Background Wind tunnel testing of generic low-rise building models dates backs to the end of the nineteenth century. Jensen and Frank [3] in the 1960s established the boundary layer wind tunnel as a tool to conduct experiments that provided information on wind loads and helped to set up building codes. Following Jensen and Frank [3], a number of investigators such as Stathopoulos and Mohammadian [4], Krishna [5], Holmes [6] and Meecham et al. [7] studied the wind loads, provided results and presented reviews for low-rise buildings. Stathopoulos and Mohammadian [4] measured the local and area averaged pressures on buildings with mono-sloped roofs in a simulated atmospheric boundary layer and discussed the effects of the height, the width and the roof pitch on the magnitude of pressures. They found that both the mean and the peak pressures were higher than those for buildings with gabled roofs, in particular, at the roof corners. Krishna [5] in a review paper compared mean pressure coefficients on a gabled roof building with a roof pitch of 30. It was observed that the results differ from country to country. This variation is attributed to differences in the method of data acquisition, technological capabilities and the accuracy of the experiments. Holmes [6] conducted a detailed study of wind pressure on tropical houses in which he investigated the effects of the elevation of houses above the ground and the roof pitch. Results showed that for the same roof pitch, high set (elevated) houses performed worse than the low sets ones. On the windward side of the roof, 15 pitch recorded all negative pressure coefficients while 20 had near zero and 30 almost zero. In addition, the magnitude of the pressure coefficient on downwind half of the roof was more for 15 and almost the same for 20 and 30. The reason for this was that the flow did not reattach after the second separation at the ridge. The performances of gabled roof and hip roof building models were investigated by Meecham et al. [7]. They reported the dominance of hip roof over gabled roof. For hip roof, peak pressure was found to reduce by about 50 % compared to that of gabled roof. Holmes et al. [8] used statistical data on a 5 roof pitch low-rise industrial building to optimize the design. Blackmore [9] experimentally investigated the effects of chamfered roof edges on the pressures on flat roofs. A 30 chamfered edge reduced the area-averaged loads on the corner by 70 % and gave an overall load reduction of 30 %. The significant load reduction on the corner panel was due to narrower separation bubble and as a result a much narrower highly loaded edge region, resulting from the suppression of vortices generated at the windward corner. Hoxey [10] and Robertson [11] investigated the effect of curved eaves on wind pressure over the Silsoe structures building. There was a significant drop in the suction over the lower third of the windward side of the roof. The curved eaves prevented the flow from separating at the windward edge of the roof. However, the ridge region showed higher suction. Gerhardt et al. [12] studied the influence of relative building height (eave height/base width) on the pressure distribution over flat roof. It was observed that the roof pressure distribution is strongly affected by the relative building height for buildings with height to width ratio greater than or equal to 0.1. A high suction was recorded at the windward edge as well as windward corner, which increased with increasing relative building height. Leutheusser [13] investigated the effect of wall parapets on roof pressure coefficient. Kind [14] and Baskaran and Stathopoulos [15] also investigated the effects of parapet on roof pressures. 232

Tamura et al. [16] investigated the wind pressure distribution causing maximum quasi-static wind loads at the base of low-rise building models with a square and a rectangular plan. Ahmad and Kumar [17] tested the Texas Tech University (TTU) building at a geometric scale of 1 : 50. Their results were generally in agreement with the prototype. In addition to this the authors carried out a detailed study on hip roof building models with roof pitch of 30 and different overhang ratios. The effect of height aspect ratio on wind pressure distribution was similar to the findings of Gerhardt et al. [12] and Holmes [6]. The worse loaded regions were the windward edges, corners and the hip ridge near this corner. Endo et al. [18] also tested the TTU building at a geometric scale of 1 : 50. They paid attention to the external point pressures at the mid-plane and roof corner pressures for a wider range of the wind azimuth than previously reported. For the mid-plane locations, a good agreement between the model and full-scale pressures was observed and was attributed to a close matching of the laboratory and field flows. Ho et al. [19] reported that the steeper roof slope leads to a significant drop in suction. Their results indicated similar aerodynamic behaviour for roof slopes less than 10 but significant changes were recorded for roof slopes between 10 and 20. Kasperski [20] provided information on design wind loads on a portal frame for low-rise buildings taking into account the directional effects. He provided information on mean pressure coefficient for a point on the edge on the center bay and reported strong dependence of it on wind azimuth angle. Cope et al. [21] explored the effects of the spatial and probabilistic characteristics of pressure fields on the net uplift acting on the roof panels of low-rise gabled roof building. The authors provided information on correlation coefficient, the average magnitude of the loads and mean pressure coefficients. The mean pressure coefficient on the windward roof portion was higher for lower pitched roof models. Ginger and Letchford [22] conducted experiments on low-rise gabled roof buildings and made similar observations. Wagaman et al. [23], Gao and Chow [24], and Richards and Hoxey [25] provided information on the flow over cubes. In these papers, they investigated flow separation and the formation of separation bubbles. Thus, it can be seen that a lot of work has been done on the aerodynamics of low-rise buildings. However, there is a wide scatter in the results and there are many questions remaining unanswered. Recently, significant improvements in experimental techniques have been made. With the help of better instrumentation, accurate measurements can be performed that can certainly enhance our understanding of the flow structure and help us design buildings with better configurations that can withstand strong winds. The present paper specifically looks at houses of common geometry that are built in the Pacific and provides aerodynamic information. 2. Experimental Setup and Procedure 2.1. Wind tunnel. The building models were tested in a boundary layer wind tunnel in the Thermo-fluids laboratory of the University of the South Pacific. The test section has a length of 0.5 m, a width of 0.42 m and a height of 4 m and is located 2 m downstream of a centrifugal blower which generates the flow. All the experiments were performed at a flow velocity of U = 7 m/s. For testing of buildings, the correct simulation of the atmospheric boundary layer is required. The wind characteristics are generally expressed in the form of suitable mean velocity and turbulence intensity profiles. Together, these represent the correct variation of wind speed with height, of the gustiness of wind with height and the size of wind gusts in relation to the building or the building model. 233

Fig. 1. Normalized mean velocity in the test section. The velocity profile in the wind tunnel at the middle of the test section is shown in Fig. 1. Because of the turbulent nature of the boundary layer, the pressure readings were fluctuating, which resulted in a little scatter in the velocity distribution in the boundary layer. The correlation between the axial component of velocity u and the height y at this location is found to be: u/u = 1.53y 1/7. (1) 2.2. House models. The building models were fabricated with 4 mm thick Perspex. In all, nine models were constructed: a flat roof model, gabled and hip roof models with roof pitch angles of 15, 20, 30, and 45. Perspex pieces were cut and then milled to ensure smooth edges and accurate dimensions. The length, width and mean height, H, of all the models are 75, 50 and 45 mm respectively. Fig. 2a shows the gabled roof model configuration, Fig. 2b stands for hip roof, and Fig. 2c presents the flat roof building model. The locations of the pressure taps are shown on the walls and roofs of the models. Due to the small size of the models, pressure taps could not be provided right at the ridge and at the corners. The size was kept small to ensure that the entire model was fully submerged in the boundary layer and to keep the blockage ratio small. The blockage ratio for the models in the wind tunnel ranged from 2.57 to 3.68 %. Metal sleeves of 10 mm length, 1 mm internal diameter and 1.5 mm external diameter were inserted into the holes drilled in the Perspex. Vinyl tubes each having an inner diameter of 1.5 mm and a length of 1 m were fixed on these sleeves. 2.3. Experimental procedure. The building model was placed at the middle of the test section with its base flush with the lower wall of the test section and the ridge of the model normal to the flow direction. Each pressure tap on the building model was connected to a Furness Controls channel box, model FCS421, with the help of a 1 m long vinyl tube. The channel box allowed simultaneous monitoring of 20 pressure tap readings. The channel box was in turn connected to a Furness Controls digital micromanometer, model FCO510 having a range of ±200 mm of water. The measured values of static pressure were converted to non-dimensional pressure coefficient, C p, which is defined as C p = P P 0.5ρU 2, (2) where P is the pressure at a given location, P is the free-stream static pressure, U the free-stream 234

Fig. 2. Building models and the locations of pressure taps. F ß C E D WIND F J E WIND A G B (a) (b) Fig. 3. Orientation of building model in the test section (a) gabled (b) hip. velocity and ρ the air density. The point pressure measurements were processed with the help of a contour plotting software to obtain surface contour plots on an entire surface of the building. Fig. 3 shows the orientation of the gabled and hip roof building models with respect to the direction of wind. In Fig. 3a, A is the windward wall, B and D are the side walls, C the leeward wall, E is the windward half of the roof and F the leeward half, β is the roof pitch angle. Fig. 3b shows the additional roof faces G and J for the hip roof model; all other sides are the same as for the gabled roof building model. 235

The accuracy of estimation of C p was estimated by calculating dc p from the expression for C p, Eq. (2). Thus, the values of C p in the present studies were estimated with an accuracy of 1.72 %. The repeatability of pressure measurements was within ±1.9 % for a given model as well as for different models. 3. Results and Discussion 3.1. Pressure distributions. As described earlier, the pressure readings were normalized to obtain C p values. Iso-pressure contours on the walls and the roof were plotted with these values of C p. The coordinates x and y here are the horizontal and vertical distances from the lower left edge while looking at that particular surface from its front (all the walls) and from the top (roof). Both x and y are normalized by the mean eave height of the building, H. The pressure pattern observed on the front wall for nearly all the models was similar; the iso-pressure contours for the flat roof model are shown is Fig. 4. It can be seen that the pressure is very high on the front wall and the C p values are above 0.9 at nearly all the points. Fig. 5 shows the iso-pressure contours on the side wall of the 30 gabled roof building model. It was observed that the pressures on the side walls were insensitive to the roof pitch. The pressure distributions on the side walls for all the models were similar and the C p values varied only a little. Significant differences were observed in the suction on the back wall for different models. It was found that the suction over the back wall eases with increasing roof pitch. The strongest suction was recorded (not shown) for the flat roof building model with a mean C p of 0.94. Fig. 6 shows the iso-pressure contours on the back wall of the 30 gabled roof building model. The value of mean C p reduced to 0.51 for this case. There was some asymmetry observed in the pressure distributions on the back walls, due to the interaction of the separated flows from the side walls and the roof, vortex shedding and intense three-dimensional mixing in the near-wake region. Iso-pressure contours over the roof of the flat roof and the 30 gabled roof building models for the windward side and the leeward side are shown in Figs. 7 to 9 respectively. The suction over the flat roof was found to be very high due to the flow separation taking place at the corner. A significant reduction in roof suction on the 30 gabled roof building model compared to the flat roof model can be seen in Figs. 8 and 9. The region and the extent of flow separation are expected to reduce for this case resulting in a reduced suction. Higher C p values are recorded near the leading edge as well as near the windward corners. Gerhardt and Kramer [12] also made similar observations. In Figs. 8 and 10 the contours are shown on the windward roof side starting from the leading edge to the ridge and in Fig. 9, it is shown from the ridge to the rear end. It is observed that for 30 model, the suction decreases from the leading edge to the ridge on the windward side as the separated flow tends to reattach, but on the other side of the ridge it continues to increase along the entire leeward roof side. The increase in suction behind the ridge is due to the second flow separation at the ridge. Fig. 10 shows the iso-pressure contours on the windward side of the roof for the 45 gabled roof building model. It is interesting to see that the suction has disappeared for this case and there is a positive pressure over the entire windward side of the roof. The pressure is observed to decrease towards the ridge due to the convergence of the streamlines as a result of a reduction in the flow area. 3.2. Mean pressure coefficient for the gabled roof models. The average values of C p were estimated on all the faces of the gabled roof building models. These values are listed in Table 1. For the 30 gabled roof building model, comparison is made with the average values available in the literature [5]. The values with an asterisk ( ) correspond to the codes practiced in New Zealand 236

0.93 0.94 0.8 0.92 0.7 0.95 0.91 0.6 0.5 0.4 0.92 0.93 0.9 0.91 0.95 0.94 0.96 0.97 0.98 0.94 0.96 0.97 0.99 1 0.98 0.98 0.99 0.95 0.92 0.96 0.97 0.91 0.93 0.2 0.4 0.6 0.8 1 1.2 1.4 Fig. 4. Iso-pressure contours on the front wall of flat roof building model. 1-0.875-0.9-0.775-0.675-0.7-0.725-0.75-0.825-0.7-0.85-0.775-0.9 0.8-0.85-0.65-0.8-0.65-0.8 0.6-0.625-0.6-0.725-0.625-0.6 0.4 0.2-0.725-0.575-0.825-0.925-0.625-0.75-0.675-0.85-0.95-0.875-0.925-0.95-0.8-0.875-0.7-0.65-0.9-0.775-0.925-0.75-0.675-0.825 0.2 0.4 0.5 0.6 0.7 0.8 Fig. 5. Iso-pressure contours on the side wall of the 30 gabled roof building model. 237

-1.4 0.8-0.625-0.6-0.55-0.525-0.5-0.575-0.625 0.7-0.65-0.525 0.6 0.5 0.4-0.475-0.5-0.55-0.45-0.425-0.575-0.6-0.625-0.4-75 - -0.425-0.45-25 -0.475-0.5-5 -0.25-0.4-0.275-75 -0.475 - -0.4-0.525-0.55-0.45-0.425-0.575-0.6 0.2 0.4 0.6 0.8 1 1.2 1.4 Fig. 6. Iso-pressure contours on the back wall of the 30 gabled roof building model. 0.8-1.45-1.425-1.3-1.375-1.35-1.275-1.3-1.275-1.325-1.375-1.35 0.7-1.35-1.325-1.275-1.325-1.3 0.6 0.5 0.4 0.2-1.6-1.425-1.45-1.5-1.625-1.475-1.525-1.575-1.5-1.475-1.525-1.55-1.4-1.575-1.375-1.5-1.6-1.425-1.45-1.525-1.475-1.4-1.625-1.65 0.2 0.4 0.6 0.8 1 1.2 1.4 Wind Fig. 7. Iso-pressure contours on the roof of the flat roof building model. 238

5 25-15 -3-0.255-0.24-45 -0.21-0.27-0.195-0.165-0.24-0.18-0.195-0.255-0.27-0.285-0.21 - -0.255-0.24-0.27-15 0.275 0.25 - -3 0.225-45 0.2 0.175 - -0.285-45 -3-15 -0.285 0.4 0.6 0.8 1 1.2 1.4 Wind Fig. 8. Iso-pressure contours on the windward portion of the roof for the 30 gabled roof building model. 75-0.56 5 25-0.56-0.55-0.56 0.275 0.25 0.225-0.55-0.54-0.53-0.54-0.51-0.52-0.53-0.55-0.52-0.54 0.2-0.53-0.52-0.5-0.49-0.48-0.5 0.4 0.6 0.8 1 1.2 1.4 Wind Fig. 9. Iso-pressure contours on the leeward portion of the roof for the 30 gabled roof building model. 239

0.5 0.135 0.15 0.165 0.12 0.135 0.105 0.09 0.12 0.105 0.09 0.06 0.075 0.45 0.4 5 0.18 0.21 0.24 0.225 0.255 0.27 0.285 0.195 0.15 0.225 0.165 0.24 0.255 0.27 0.18 0.195 0.21 0.27 0.12 0.135 0.165 0.105 0.18 0.225 0.24 0.15 0.195 0.21 0.4 0.6 0.8 1 1.2 Wind 0.255 Fig. 10. Iso-pressure contours over the windward roof portion of the 45 gabled roof model. and the values with two asterisks ( ) correspond to the codes practices in UK [5]. The suction over the roof was found to be high for the flat roof building model, but decreased with increasing roof pitch, as discussed earlier. Similar observations were made by Uematsu and Isyumov [1] and Holmes [6] who reported reducing suction on the roof with increasing pitch. The wall facing the flow, A, recorded high positive pressures and the magnitude of C p increased slightly with roof pitch. The side walls B and D generally recorded very little changes in the pressure coefficient. The windward edges of the side wall recorded slightly higher negative pressure coefficients due to the flow separation at the edge. The back wall, C, for all the models recorded less suction than the side walls and the magnitude of pressure coefficient decreased significantly with increasing roof pitch. However, pressure recorded over the back wall was fairly constant in each case due to the fact Table 1. Mean pressure coefficients for the flat and gabled roof building models Model Mean pressure coefficient (C p ) A B C D E F Flat roof 0.93 1.10 0.94 1.10 1.40 15 gabled 0.92 1.04 0.78 1.04 1.01 1.10 0.93 0.80 0.51 0.94 3 0.55 30 gabled 0.90 0.70 0.50 0.70 0.50 0.70 0.70 0.60 0.25 0.60 0.20 0.40 45 gabled 0.93 0.90 0.50 0.90 0.24 0.51 240

Pressure coefficient, -Cp 2 1.75 1.5 1.25 1 0.75 0.5 0.25 0 Gable (Windward) Gable (Leeward) Hipped (Windward) Hipped (Leeward) Flat Roof -0.25-0.5 0 5 10 15 20 25 30 35 40 45 Roof pitch, Fig. 11. Variation of the minimum pressure coefficient over the roof. that the flow is fully separated. The back wall is in the near wake, which is known to be a region of low velocity and high turbulence. If the roof is steep enough to protrude into the flow boundary formed by the windward wall, the streamlined flow is pushed up even further, resulting in positive pressures over the windward roof portion. Mean pressure coefficient on the roof is profoundly influenced by the roof pitch and this is evident from columns E and F in Table 1. It was observed that suction over the leeward roof side was more than that over the windward roof for all the gabled roof building models. Holmes [6] and Ho et al. [19] made similar observations of a significant increase in the suction behind the ridge. However, Ho et al. [19] recorded higher suction near the ridge compared to the rear end. The windward roof portion recorded suction for flat roof, 15, 20 and 30 models and the magnitude of suction reduced significantly with increasing roof pitch. On the other hand, 45 models recorded positive pressure over the windward roof portion. Suction was recorded on the leeward roof potion for all the gabled roof building models. Higher values of negative pressure coefficient were recorded in the vicinity of the leading edge of the flat roof. This is due to flow separation at the leading edge which causes high suction in this region. Gerhardt and Kramer [12] also recorded high suction near the corners and the leading edge. Numerical simulation by Gao et al. [24] showed flow separation at the leading edge of the cube which in the context of the present work can be treated as a flat roof. Ho et al. [19] observed similar trend on gabled roof. They found flow separation at the leading edge and a second separation for some models at the ridge. Flow was found to separate at the ridge for roof pitch angles greater than 10. 3.3. Minimum pressure coefficient over the roof. The variation of the minimum pressure coefficient over the roof of different building models with the roof pitch angle is shown in Fig. 11. It is clear that the flat roof recorded the worst suction with a minimum C p value of 1.72. 241

Looking at the windward and the leeward portions of the roof, the peak suction over the leeward side is higher compared to that over the windward side with the difference between the two increasing with an increase in the roof pitch. It is clear from this figure that the worst suction reduces continuously when the roof pitch is increased. Similar trends are observed on the windward side for both the gabled and hip roof building models. Interesting observations are made for the 45 pitch building models. The minimum C p value is positive for both the gabled and the hip roof models. It is also interesting to note that there is a slight increase in the suction on the leeward side for the gabled roof compared to the 30 case. Moreover, it is observed that the suction on the leeward portion of the roof is lower for hip roof models than their gabled counterparts. In general, the hip roof models performed better than their gabled counterparts under the same wind conditions. There is a 42 % reduction in the peak suction over the hip roof models compared to their gabled counterparts. Meecham et al. [7] also reported the dominance of hip roof over gabled roof. For the 45 gabled and hip models, the peak suction over the roof reduced by 85 and 91 % respectively compared to that over the flat roof. Conclusions Wind load information for nine different models is presented in this paper. Non-dimensional pressure coefficient, C p, was experimentally obtained and used to identify the best roof configuration amongst the nine models. The suction was found to be worst for the flat roof. The 45 gabled and hip roof building models performed the best under the same wind conditions, with the peak suction over the roof reducing by 85 and 91 % respectively compared to that over the flat roof. Furthermore, the hip roof models performed better than the gabled models and a 42 % reduction in the peak suction over the hip roof models was recorded compared to their gabled counterparts. Acknowledgements The authors wish to thank Mr. Maurice Nonipitu, Mr. Sanjay Singh and Mr. Shiu Dayal for their contribution during the fabrication work of building models used in the experiments. REFERENCES 1. Uematsu, Y. and Isyumov, N., Wind Pressures Acting on Low-Rise Buildings A Review, J. Wind Eng. Industr. Aerodyn., 1999, 82, No. 1-3, pp. 1 25. 2. American Society of Civil Engineers (ASCE) Standard, Minimum Design Loads for Buildings and Other Structures, ASCE 7-02, New York, USA, 2002. 3. Jensen, M. and Frank, N., Model Scale Tests in Turbulent Wind, Danish Technical Press, Copenhagen, 1965. 4. Stathopoulos, T. and Mohammadian, A. R., Wind Loads on Low-Rise Buildings with Mono- Sloped Roofs, J. Wind Eng. Industr. Aerodyn., 1986, 23, No. 1, pp. 81 97. 5. Krishna, P., Wind Loads on Low-Rise Buildings A Review, J. Wind Eng. Industr. Aerodyn., 1995, 54-55, No. 1, pp. 383 396. 6. Holmes, J. D., Wind Pressures on Tropical Building Low-Rise Building, J. Wind Eng. Industr. Aerodyn., 1986, 53, No. 1-2, pp. 105 123. 242

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