Wind loads on low buildings with mono-sloped roofs

Journal of Wind Engineering and Industrial Aerodynamics, 23 (1986) 81--97 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

81

WIND LOADS ON LOW BUILDINGS WITH MONO~SLOPED ROOFS

T. STATHOPOULOS and A.R. MOHAMMADIAN

Centre for Building Studies, Concordia University, Montreal, Quebec H3G 1M8 (Canada)

Summary Wind pressure loads for mono-sloped roof buildings have been determined experimentally by testing a variety of models exposed to a simulated atmospheric boundary layer flow over an open country terrain. Geometrical parameters examined include the effect of height (3.6--12.2 m), width (12.2--24.4 m) and roof slope (1:12--4:12) on both local and area-averaged pressures measured for a variety of wind directions. The effect of scaling has also been examined by testing similar models of different size in the wind tunnel. Experimental data indicate that both mean and instantaneous peak wind pressures are higher than those found previously for buildings with gabled roofs. This is particularly true of roof corners and narrow buildings. Some trend with the height has also been found for corner points. This paper presents comparisons with previous studies and codes.

I. Introduction

D e s p i t e t h e significant p r o g r e s s m a d e in t h e research o f w i n d l o a d s o n l o w buildings d u r i n g t h e last t e n years, p e r t i n e n t q u e s t i o n s still r e m a i n . S o m e o f t h e s e q u e s t i o n s r e f e r t o t h e a p p r o p r i a t e testing a n d i n t e r p r e t a t i o n o f t h e m e a s u r e m e n t results (e.g. e f f e c t o f scaling, selection a n d d u r a t i o n o f p r e s s u r e p e a k s , etc.) w h e r e a s o t h e r s reflect t h e u n c e r t a i n t y involved w i t h t h e e x t e n t o f a p p l i c a t i o n o f d i f f e r e n t c o d e and s t a n d a r d s p e c i f i c a t i o n s t o various building f o r m s a n d g e o m e t r i e s . In p a r t i c u l a r , t h e w i n d l o a d s p e c i f i c a t i o n s f o r l o w buildings, i n c l u d e d in t h e 1 9 8 0 S u p p l e m e n t t o t h e N a t i o n a l Building C o d e o f C a n a d a [1] a f t e r a d e t a i l e d e x p e r i m e n t a l s t u d y carried o u t at t h e U n i v e r s i t y o f Western O n t a r i o , are a p p r o p r i a t e f o r flat o r gabled r o o f buildings w i t h w i d t h s g r e a t e r t h a n t w i c e t h e i r heights a n d f o r w h i c h t h e r e f e r e n c e h e i g h t (i.e. m e a n r o o f h e i g h t ) d o e s n o t e x c e e d 20 m. T h e S u p p l e m e n t also states t h a t t h e use o f t h e s p e c i f i c a t i o n s " m a y b e e x t e n d e d o u t s i d e t h e s e ranges w h e r e H / W is n o t g r e a t e r t h a n 2 " . N o guidelines are given f o r t h e validity o f t h e s p e c i f i c a t i o n s in case o f d i f f e r e n t building f o r m s . S o m e r e s e a r c h w o r k has b e e n f o c u s e d o n this p r o b l e m r e c e n t l y . H o l m e s , in Australia, has carried o u t a p r o j e c t o n w i n d l o a d i n g o f s a w - t o o t h r o o f buildings [ 2 ] . H e has f o u n d t h e r o o f s u c t i o n s t o b e d i f f e r e n t f r o m t h o s e

0167-6105/86/$03.50

© 1986 Elsevier Science Publishers B.V.

82 observed previously on gabled and flat roof buildings and generally higher than the worst values given in the current Australian Standard [3]. Rietdyk [4] and Burgers [5] have tested simple mono-sloped building geometries at the Boundary Layer Wind Tunnel of the University of Western Ontario. Their results indicate that the Canadian Code underestimates the loading on the ridge and its associated corner regions of the monosloped roof. A more extensive study on the wind loading of low buildings with monosloped roofs has been undertaken at Concordia University. The study is still underway and this paper refers only to the most significant experimental findings so far. In addition to the basic model geometries tested, the model used at the University of Western Ontario was kindly provided by Dave Surry for re-testing at Concordia. The latter exercise was extremely useful because it allowed not only to compare similar experimental data obtained from two different boundary layer wind tunnels but also to evaluate possible scaling effects and their influence to the results.

2. Experimental measurements All experiments were carried out at the boundary layer wind tunnel of the Centre for Building Studies (CBS) of Concordia University. The wind tunnel has a working section approximately 12.20 X 1.80 m wide, and has an adjustable roof height averaging roughly 1.60 m. A geometric scale of 1 : 400 has been suggested for the simulation of the most important variables of the atmospheric boundary layer under strong wind conditions. More details about this wind tunnel and its simulation characteristics are given elsewhere [6]. The present measurements were carried out in a simulated open country exposure since this is the case suggested by most codes and standards for the wind load specifications of low buildings. Figure 1 shows the velocity profile and the turbulence intensity characteristics measured at the location of the model (without the model in place), in the wind tunnel. The free stream or gradient velocity, Ug, used in the wind tunnel was approximately 13 m s -1. The basic model of the study is made of plexiglass. It has a roof angle of 4.8 ° and a constant length (L), whereas its width (W) and lower eave height (H) are variable. Typical dimensions of the model have been selected to be representative of real buildings at a relaxed geometric scale of 1:200. This slight relaxation o f scale is not expected to critically affect the experimental results as has been proposed by Lee [7] and also shown experimentally by Stathopoulos and Surry [8]. On the other hand larger models can be fabricated and instrumented more easily, pressure taps can be located closer to the edges and pressure measurements have better frequency response. Figure 2 shows an exploded plan view of the building model with the exact location of each of its 153 pressure taps. The model can be tested at

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f o u r different heights ranging bet w e e n 3.66 and 12.20 m and three different widths ranging be t w een 12.20 and 24.40 m. Pressure taps were drilled as close as 1.3 m m to t he r o o f edges corresponding t o a distance o f 0.26 m full scale. Measurements have been carried out for O, 30, 45, 60 and 90 ° azimuths for the wide (W = 24.40 m) building and for O, 30, 45, 60, 90, 120, 150 and 180 ° for t h e narrow (W = 12.20 m) building since azimuths greater th an 90 ° (wind towards t he high eave or ridge) have been f o u n d critical. Th e model provided by t h e University o f Western Ontario is a 1: 500 scaled building with L = 51 m, W = 20.5 m, H = 7.6 m, a variable r o o f slope f r o m 0 to 4:12 and a perimetrical eave of 1.2 m. This model has fewer pressure taps than t h e one o f t he present study and t h e closest distance between t h e m and the edge o f t he r o o f is equal t o 2.4 m m corresponding to a distance o f 1.20 m full scale. Pressure measurements were carried o u t by using t w o SETRA-237 pressure transducers set in a scanivalve. The tubing system connecting the taps to the scanivalve gives a f r e q u e n c y response virtually fiat t o a b o u t 100 Hz. Pneumatic averaging has been used for t he m e a s u r e m e n t o f area

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loads. Digitization of the pressure signals and analysis of the data using a MACSYM-2 data acquisition and control system on-line, gave the max, min, mean and RMS pressures over a 30-s period. The dynamic pressure o f the flow above the boundary layer was also measured and used to determine pressure coefficients referenced to gradient height. These were subsequently referenced to roof height, H, by using the velocity profile of Fig. 1. Further analysis of the data t o o k place using the Perkin-Elmer mini-computer of CBS.

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3. Local pressure loads Local pressure coefficients measured on the roof of the CBS model were compared with those found by Jensen and Franck [9] in their pioneering study on the wind loads of low buildings. Only mean values have been compared (dynamic pressures were not measured by Jensen and Franck) for three different wind directions. Results are shown in Fig. 3 for the

85 m os t similar model configuration available. N ot e t h a t t he model o f ref. 9 has s h o r t e r length, slightly higher r o o f slope and higher H/Zo value. Nevertheless, th e data of t he two studies agree reasonably well, particularly if o n e considers the different location o f t he pressure taps in each model. Higher values at r o o f edges f o u n d in t he present study can be partly attributed to this latter effect since pressure taps have been placed as close as possible to th e r o o f edges o f t he CBS model. Figure 4 shows c ont our s of the most critical values o f mean and instantaneous peak pressure coefficients f or a building 24.4 m wide exposed t o five wind directions between 0 and 90 °. Clearly, t here is suction everywhere on th e entire r o o f with t he highest values occurring near the windward

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Fig. 5. Most critical mean and instantaneous peak pressure coefficients for the narrow building (W = 12.2 m). are equal or generally lower f or the narrow building as compared t o those f o u n d for the wide building. Previous studies o f wind loads on flat and gabled r o o f buildings (.e.g. ref. 10) have shown t h a t t he effect o f building height on pressure coefficients may be virtually eliminated if these are referenced t o the dynamic velocity pressure measured at the building height. Based on this finding, t h e pressure coefficients of t he National Building Code of Canada [1] are specified regardless of t he height of the low building. This trend of height independence was f ound less p r o n o u n c e d in t he case o f buildings with mono~sloped roofs, particularly for r o o f corner points. Figure 6 shows mean and peak pressure coefficients measured for the ridge c o m e r tap of buildings o f f our heights f or a n u m b e r o f wind directions. Although this t r en d o f height independence is generally true for t he non-critical azimuths, it becomes clearly invalid for the critical azimuth of 150 °, for which b o t h mean and peak pressure coefficients do increase with height. Th e effect o f height on b o t h mean and peak pressure coefficients in a m or e global sense is presented in Fig. 7 for buildings of two different widths. Results obtained from the lowest (H1 = 3.66 m) and the highest (H4 = 12.20 m) building are compared. Clearly the pressure coefficients are higher f o r th e higher building and the difference is m ore distinct for t he wider building. By applying linear regression it has been f o u n d that, on average,

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the mean pressure coefficients for the high building are higher by approximately 0.12 (0.20) for the narrow (wide) building whereas peak values are more scattered. Note that the three points standing out in all graphs of Fig. 7, as having much higher values for H4 than for H1, correspond to the r o o f corner taps. The graphs include pressure data for the most critical wind direction for the entire building envelope (roof and walls) but further analyses carried out separately for roofs and walls have indicated that r o o f pressures are mostly affected by height whereas wall pressures are rather insensitive to the building height. Figure 8 shows the effect of building width on r o o f pressure coefficients.

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Only corresponding pressure taps have been considered for the narrowest (W1 --- 12.20 m) and the widest (W3 = 24.40 m) building roof and the most critical values recorded for any wind direction have been plotted. By using linear regression it has been found again that, on average, both mean and peak pressure coefficients are approximately 20% higher for the narrow as compared with the wide building. As previously mentioned, the 1:500 scaled model of the UWO was provided to be tested at CBS. Data were obtained and compared for the 7.62 m high building at corresponding pressure taps. Results for the most critical mean and peak pressure coefficients are shown in Fig. 9, which clearly indicates that pressure coefficients measured with the smaller size building are lower than those obtained from the larger model. These data on scaling effects, however, should be considered only tentative for two reasons: first, there are differences in the correspondence of pressure taps between the two models; and second, the data for the UWO model are incomplete ,

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91 because of a computer malfunction due to which the 30 ° azimuth results were lost. The effect of r o o f slope on pressure coefficients was examined by testing the UWO model at three different roof slopes, namely 1:12, 2:12 and 4:12. Contours of mean and peak instantaneous pressure coefficients for the most critical wind direction are presented in Fig. 10. Mean values appear to decrease at the lower eave and increase at the ridge with increasing r o o f slope whereas peak values are unaffected at the lower eave but t h e y also increase at the ridge with increasing roof slope. Wall suctions appear rather unaffected by roof slope. Since only most critical (worst) values of pressure coefficients are presented here, it should be mentioned that detailed comparisons between the data obtained from different oblique wind directions have indicated the azimuth of 30 or 150 ° as giving, by and large, the most critical suctions for the building geometries examined in this study. Finally, Fig. 11 shows some preliminary comparisons between the gust pressure coefficients specified in the National Building Code of Canada [1] for flat or gabled r o o f buildings with roof angles smaller than 10 ° and the present experimental data for buildings with mono-sloped roofs. The Code seems to predict rather adequately the wind loads on a wide (W = 24.40 m) building but it clearly underestimates the loads affecting a narrow (W = 12.20 m) building. Some similar remarks have also been made in refs. 4 and 5.

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4. Area pressure loads Spatially averaged pressure coefficients were measured by using the pneumatic averaging technique. Areas considered covered a wide spectrum of sizes and locations. A typical configuration consists of 20 panels of approximately equal size for the 61 × 24.40 m roof and 10 panels for the 61 X 12.20 m. Sixteen panels were considered for the UWO model. Some

93

typical results for the area averaging effect appear in Fig. 12, in which the arithmetic average (AA) of the most critical pressure coefficients measured over all r o o f panels of a building 24.40 m wide and 7.62 m high is compared with the pneumatically averaged (PA) values. Data indicate close similarity between mean values and the area averaging alleviation effect for suction peaks. Note that this alleviation effect becomes more distinct for the highest suction values, as has also been found in previous studies. -3=0

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Table 1 shows peak (and mean in parentheses) values of spatially averaged pressure coefficients measured for roof panels of buildings of various heights and widths. As in the case of local pressure coefficients, only worst values from all different wind directions tested were presented. For all different configurations the ridge corner panel appears more severely loaded. The effect of building width is much less pronounced here with the wider building showing somewhat higher loads. The corner panel suctions do increase marginally with height whereas other panel suctions show little dependence on height. This is further shown in Fig. 13 which indicates the azimuthal variation of both peak and mean corner panel pressure coefficients for two building heights and widths. Once again, the height effect appears negligible for most of the non-critical azimuths. The effect of r o o f slope on the most critically affected r o o f c o m e r panels is presented in Fig. 14 for the UWO model tested at CBS. Data indicate that spatially averaged coefficients measured at the ridge corner area tend to increase with increasing r o o f slope whereas those measured in the lower eave c o m e r area of the r o o f decrease with increasing r o o f slope. It should also be mentioned that the suctions obtained from the UWO model are lower

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than those found by testing the CBS model (see Table 1). This may possibly indicate some scaling effect but more detailed analysis is required o n this issue. Finally, Fig. 15 shows the effect o f azimuth and building height and width o n area corner loads. The r o o f corner considered is the lower eave one and area loads are determined by pneumatically averaging the o u t p u t of the

96

f o u r corner taps indicated in Fig. 2. This corner area has the same size fo r the wide and narrow buildings but it corresponds to the NBCC r o o f corner size [1] for the wide building. Results, however, indicate t hat the most critical suctions are almost independent of the building width and, once more, t h e y tend to increase with height, particularly for the critical wind directions. The azimuth of 0 or 90 ° is generally critical for the suction -3.0

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on the corner area whereas, in contrast with the local loads, azimuths between 0 and 90 ° generally provide lower suctions.

5. Concluding remarks Despite the fact that the project on wind loads on mono-sloped r o o f buildings is still underway, some tentative conclusions may be drawn from the overall findings so far. R o o f loads appear somewhat higher than those found previously for buildings with fiat or gabled roofs. The effect of width is clearly pronounced indicating higher suctions for narrower buildings. Experimental data show some dependence on height particularly for corner points, whose suctions increase significantly with increasing building height. A number of wind directions tested indicated that wind towards the ridge of the r o o f gives generally more critical loads. Scaling effects have been detected particularly for area loads. This, however, requires further investigation.

Acknowledgements Financial support for this project has been provided by the Natural Sciences and Engineering Research Council of Canada under grant No. A1213. The writers are also grateful to Dr. Dave Surry of the University o f Western Ontario for providing the UWO model for testing and copies of refs. 4 and 5.

References 1 Associate Committee on the National Building Code, The supplement to the National Building Code of Canada 1980, National Research Council, Ottawa, NRCC 17724. 2 J.D. Holmes, private communication, 1984. 3 Standards Association of Australia, S.A.A. loading code part 2, wind forces, AS1170 Part 2, 1983. 4 P. Rietdyk, Wind loads for low-rise buildings with mono-sloped roofs, Report submitted in partial fulfillment of the requirements for the degree of Bachelor of Engineering Science, The University of Western Ontario, 1982. 5 N.B. Burgers, Wind loads o n low-rise buildings having mono-sloped roofs, The Boundary Layer Wind Tunnel Laboratory, The University of Western Ontario, 1982. 6 T. Stathopoulos, Design and fabrication of wind tunnel for building aerodynamics, J. Wind Eng. Ind. Aerodyn., 16 (1984) 361--376. 7 B.E. Lee, Force and pressure measurements in wind tunnels: a prepared critique, Proc. Int. Workshop on Wind Tunnel Modelling for Civil Engineering Applications, U.S. National Bureau of Standards, Washington, DC, 1982. 8 T. Stathopoulos and D. Surry, Scale effects in wind tunnel testing of low buildings, J. Wind Eng. Ind. Aerodyn., 13 (1983) 313--326. 9 M. Jensen and N. Franck, Model Scale Tests in Turbulent Wind, Part II, The Danish Technical Press, Copenhagen, 1965. 10 D. Surry, T. Stathopoulos and A.G. Davenport, The wind loading of low-rise buildings, Proc. Canadian Structural Engineering Conf., Canadian Steel Industries Construction Council, 1978.