Win Load Porous

*Corresponding author. Tel.: 61(07)3365-3511; fax: 61(07)3354-4599.E-mail address: [email protected] (C.W. Letchford)

Journal of Wind Engineeringand Industrial Aerodynamics 84 (2000) 197}213

Mean wind loads on porous canopy roofs

C.W. Letchford*, A. Row, A. Vitale, J. WolbersDepartment of Civil Engineering, The University of Queensland, Brisbane, Queensland 4072, Australia

Received 6 August 1997; accepted 7 June 1999

Abstract

Mean overall lift and drag forces on a range of canopy or open roof forms with varyingporosities are presented. In general, lift forces decrease while for low roof pitches (a(153) dragforces increase as porosity is increased in the range 0}23%. Resolution of these forces intoequivalent net roof pressures reveals that wind load may be transferred from the leeward to thewindward areas, leading to potential overloading of the supporting structure. Mean and#uctuating pressure measurements were undertaken to con"rm the inferred pressure distribu-tions on the roofs. ( 2000 Elsevier Science Ltd. All rights reserved.

Keywords: Wind loads; Porous canopy roofs

1. Introduction

Australia has the highest incidence of skin cancer in the world, with two out of threepeople developing skin cancers, many being life threatening [1]. The message in sunprotection programs to date has promoted personal protection, as evidenced by the`Slip, Slop, Slapa summer and `Slip, Slop, Slap has got Seriousa winter campaigns.However, making sun protection an integral part of community planning has beenacknowledged as just as important a preventative measure [1]. This involves theprovision of shade in public spaces where people gather, be it sportsgrounds, play-grounds, schoolgrounds or shopping areas. Motivation for the provision of shadestructures has also been aided by the litigation experiences of at least one localgovernment authority [1].

It is not only humans that su!er from lack of sun protection. The distress and deathof many beef cattle at the Whyalla feedlot, the largest in Queensland, in 1989,

0167-6105/00/$ - see front matter ( 2000 Elsevier Science Ltd. All rights reserved.PII: S 0 1 6 7 - 6 1 0 5 ( 9 9 ) 0 0 1 0 3 - 8

impacted both economically and publicity-wise on the beef cattle feedlot industry.The most recent research carried out at the Brigalow Research Station in CentralQueensland indicates that signi"cant improvement in liveweight gain and animalwelfare is achieved by the provision of shade [2]. Currently, animal welfare legislationis driving the need for greater provision of shade, but the economics of increasedproductivity are catching up.

Shade and weather (hail) protection have also become increasingly common in thefruit and vegetable growing industry where economic devastation can follow severecrop damage. This was the case for the plant nursery industry in North Queenslandfollowing Cyclone Winifred where sun damage to young plants was estimated to costup to one million dollars [3]. Similar weather protection is now being sought in otherareas, e.g. new car sales yards.

Whereas a large database of knowledge exists for solid suspended structures, e.g.roofs and bridges, there is very little information on the wind loading and structuralresponse of suspended porous shade structures. Donnan et al. [4] report wind tunneland structural analyses of a greenhouse structure constructed of porous shade clothsupported on cables. Their preliminary wind tunnel study indicated somewhat unex-pected results, viz. increasing drag force and decreasing lift force with increasingporosity. They went on to say that `If the results of this preliminary wind tunnelstudy are accurate, the implications for the design of such structures are extremelysigni"cant.a

The provision of sun protection has therefore become a signi"cant economic andhealth issue for humans, animals and plants. Typically of large span suspended porousroof form, these shade structures are wind sensitive and an ongoing research project atthe University of Queensland aims to develop a model of the response of this class ofstructure to #uctuating wind loads and implement this model as a rational designmethod. This design approach and newly obtained wind loading information willreplace the current largely ad hoc approach which has the possibility of allowingunsafe structures to be built.

This paper deals speci"cally with wind tunnel measurements on rigid models toinvestigate the e!ect of porosity and obtain loading coe$cients on porous canopy oropen roof forms. Future papers will examine other parameters in the response ofshade cloth structures under wind loading, including #exibility of fabrics and tension-ing system.

2. Experimental procedure

Porosity was deemed the dominant dimensionless parameter for the "rst stage ofthis project. The porosity (p) or solidity (d) of the materials studied was calculatedfrom

p"1!d" open}areatotal}enclosed}area

. (1)

198 C.W. Letchford et al. / J. Wind Eng. Ind. Aerodyn. 84 (2000) 197}213

Total enclosed area refers to the overall roof surface in this context. For shade clothfabrics however, porosity is di$cult to de"ne and indeed these fabrics are classi"ed ina number of ways; by weight, weaving type, or most commonly by cover factor. Thecover factor is equivalent to solidity and is estimated by measuring the amount of350 nm wavelength solar radiation (i.e. middle of the ultra-violet region) transmittedthrough the shade cloth and does depend on the colour of the shade cloth, due todi!erent degrees of opaqueness of the "bres. Indeed, wind forces acting on a porousstructure will depend not only on the porosity but also on the shape of the &pores orholes' making up the porous surface, for example, sharp-edged holes will havedi!erent characteristics to woven "bres. An alternative to porosity is the pressure losscoe$cient, which is de"ned as

K"P6!P$o;2/2

(2)

where P6

and P$

are the upstream and downstream static pressures on either side ofthe mesh and ;M is the average approach velocity. The pressure loss coe$cient isa measure of the resistance to #ow through a porous surface and includes the e!ects ofporosity as well as shape of `holesa. Thus similarity of wind loading will be bestachieved by equality of pressure loss characteristic (K). Here, the pressure losscharacteristics of a range of shade cloth fabrics will be compared with those measuredfor various perforated metal plates of known porosity to select a suitable rigidmaterial for the wind tunnel tests.

The pressure loss measurements were performed in a small wind tunnel, approxim-ately 300 mm square, in which the entire cross-section was covered by the variousmaterials being tested. Fig. 1 shows the experimental results plotted as K vs. Re. TheReynolds number (Re) was de"ned in terms of ; and dominant "bre diameter forfabrics and hole size for porous metal plates. The perforated metal plates of porosity11% and 23% bracketted the commonly used high UV reduction (solidity) shadecloths and were selected for the wind tunnel study. The 11% porous plate had 2.4 mmdiameter holes at 6.4 mm spacing while the 23% porous plate had 0.8 mm diameterholes at 1.5 mm spacing.

Generic canopy roof forms of hip, gable and monoslope were chosen for the studyand two are sketched in Fig. 2. Three roof pitch angles (a) were selected for study: 73,153 and 273. The models were constructed from thin (1 mm for solid and 0.5 mm forporous) metal sheets 300 mm square and thus di!erent roof pitches had di!erentprojected plan areas. This arrangement is identical to earlier pressure measurementstudies of canopy roofs [5,6]. All were mounted at a lower eaves height (h) of 100 mmon four 6 mm-diameter legs. The nominal model scale was 1 : 50.

A simple, one component force balance was constructed [7] to measure the verysmall loads. This force balance could be mounted in several ways to obtain separately,measures of the overall drag and lift forces on the models for various angles of attack.A paddle in a container of a viscous #uid was used to dampen the #uctuating loads.Only mean values of force are presented here which represent the average of betweenthree and "ve runs of 30 s duration at a sampling frequency of 100 Hz. The drag forces

C.W. Letchford et al. / J. Wind Eng. Ind. Aerodyn. 84 (2000) 197}213 199

Fig. 1. Pressure loss coe$cient K as a function of Reynolds number for various porous materials.Perforated metal plates speci"ed by porosity, p (%), and shade cloth fabrics speci"ed by cover factor whichis approximately the solidity d (%).

Fig. 2. Sketch of model roof details.

on the four supporting legs were measured separately and were subtracted from theoverall loads to produce loads on the roof alone. The forces were reduced tocoe$cient form by dividing by the mean dynamic pressure at eaves height (the upperheight for the monoslope roof ) and the projected plan area A

p(" roof area]cos(a)):

CF" F

1/2o;M 2A1

. (3)

F is the force, lift or drag, with lift de"ned as positive downwards to be consistent withAS1170.2 [8]. The 03 wind direction was de"ned as normal to the ridge line or roofedge.


Fig. 3. Model and pressure tapping details.

The pressure tapped models had a single row of 10 taps spaced evenly along theroof centreline. The pressure tap arrangements are illustrated in Fig. 3. The uppersurface tappings were all type A, while for the underside measurements severaldi!erent tapping variants, types A reversed, B and C, were employed in an e!ort toobtain a true wake pressure without interference. Each tapping has a 1.0 mm internaldiameter tube with a base mounting approximately 4 mm in diameter. Type A wasmounted on the underside for upper surface measurements and generally producedthe most consistent results when reversed, ie., placed on the upper surface, formeasurement of underside pressures. Type B su!ered from azimuth e!ects, picking upsome stagnation pressure when turned into the wind on the gable roof models, whiletype C su!ered larger wake interference e!ects compared with type A. Net pressureswere measured with type A taps on the upper surface and type B taps on the lowersurface but displaced laterally by 10 mm.

Point and area-averaged net and separate top and bottom surface pressuremeasurements were obtained using Scanivalves and Honeywell pressure transducersand a 1.5 mm tubing system with a near linear frequency response to 150 Hz.Pressures were sampled at 400 Hz for 15 s and repeated 10 times. A Fisher}Tippetttype-1 extreme value distribution was "tted to these data and mean extremes (maximaand minima) estimated. Pressure coe$cients were obtained by dividing by the meandynamic pressure at eaves height.

The tests were conducted in the Department of Civil Engineering's Boundary LayerWind Tunnel which is 3 m wide ] 2 m high and has some 12 m of upstream fetch forboundary layer simulation. A 300 mm fence and uniform carpet roughness wereemployed in the smoother simulation, while a grid of 100 mm beams at 300 mmcentres was added immediately upstream of the fence for the rougher simulation.Except where stated only results from the smoother simulation, where the turbulenceintensity at eaves height was approximately 15%, are presented here. The meanvelocity and turbulence intensity pro"les are compared with AS1170.2 [8] values inFig. 4(a) and (b) at 1 : 50 scale. The mean dynamic pressure was measured bya pitot-static tube mounted at eaves height away from the in#uence of the model. It isexpected that this will lead to approximately 4% overestimate of the true dynamicpressure [9] for the turbulence intensities in this study.


Fig. 4. (a) Mean velocity pro"les for the two simulations compared with AS1170.2 values. (b) Turbulenceintensity pro"les for the two simulations compared with AS1170.2 values.

3. Results and discussion

3.1. Force measurements

The drag and lift coe$cients for the gable roof at an azimuth of 03 for three roofpitches are plotted against porosity in Fig. 5. The drag coe$cient increases slightlywith increasing porosity for the shallower pitches but reduces for the 273 pitch roof.


Fig. 5. Drag and lift coe$cients for three pitches of a gable roof at 03 azimuth.

Fig. 6. Drag and lift coe$cients for three pitches of a hip roof (pyramid) at 03 azimuth.

The uplift (negative Cl) reduces as porosity increases and for the 273 pitch roof is no

longer an uplift but a downward load which increases with porosity.The drag and lift coe$cients for hip roofs, in e!ect pyramids for the two shallower

pitches, for an azimuth of 03 for three roof pitches are presented in Fig. 6. Again withincreasing porosity, drag increases for the shallower pitches but decreases for thesteepest pitch while uplift changes to a downward load.

The drag and lift coe$cients for monoslope roofs of three pitches at azimuths of 03and 1803 are presented in Figs. 7 and 8. Like the gable roof, for increasing porosity,


Fig. 7. Drag and lift coe$cients for three pitches of a monoslope roof at 03 azimuth.

Fig. 8. Drag and lift coe$cients for three pitches of a monoslope roof at 1803 azimuth.

drag slightly increases for the shallower pitches but reduces slightly for the 273 pitchroof, while lift reduces in magnitude with porosity.

A regression of the mean drag coe$cients for the various roof con"gurations,pitches, porosities and azimuths for the smoother TC1 (15%) and rougher TC2 (20%)


Fig. 9. Regression of mean drag coe$cients for the two di!erent simulations.

Fig. 10. Regression of mean lift coe$cients for the two di!erent simulations.

simulations is presented in Fig. 9 and for lift coe$cients in Fig. 10. The increasedturbulence leads to an increase in drag and lift coe$cient of approximately 9%. Thisresult somewhat contradicts the general observation that increased turbulence leadsto earlier reattachment (for elongated bodies) and hence reduced wake pressures and


Table 1Comparison between the present study, earlier pressure studies and AS1170.2 [8] for the meanwindward and leeward net pressure coe$cients on solid gable roofs

Roof type and pitch Source Azimuth C18

C1-

Gable 73 AS1170.2 03 !0.48 to 0.32 !0.56Gumley [5] 03 !0.08 !0.52Letchford et al. [6] 03 0.07 !0.31Present study 03 0.14 !0.30

Gable 153 AS1170.2 03 !0.32 to 0.48 !0.80Gumley [5] 03 0.034 !0.80Letchford et al. [6] 03 0.12 !0.41Present study 03 0.19 !0.65

Gable 273 AS1170.2 03 !0.32 to 0.68 !0.92Gumley [5] 03 0.59 !0.59Letchford et al. [6] 03 0.60 !0.60Present study 03 0.69 !0.57

consequently lower overall drag. Clearly the #ow mechanisms here are more complex,with net (upper!lower pressures) and windward/leeward interactions making extra-polation of simple arguments unacceptable.

Table 1 presents for solid gable roofs the mean lift and drag force measurementsresolved into windward and leeward net pressure coe$cients. Positive values ofC

1are de"ned as downward for both windward and leeward faces. The present results

are compared with those from the Australian wind load code AS1170.2 [8] and earliermean pressure measurement studies of gable canopy roofs by Gumley [5] andLetchford and Ginger [6]. The 273 roof pitch results have been interpolated from22.53 and 303 results for both pressure measurement studies. The code values repres-ent envelope results and were largely derived from the yuctuating pressure measure-ments of Gumley as indicated by the range of pressure coe$cients on the windwardroof and large suctions on the leeward roof. Here the code values have been multipliedby an area reduction factor K

A"0.8. In addition, the force measurements include

both normal and tangential stresses while the code and pressure studies cover onlynormal stresses or pressures.

The agreement between the di!erent studies is encouraging given the di!erenttechniques used. The steeper pitch has the best agreement while there is rather a lot ofscatter for both windward and leeward coe$cients for the 153 pitch roof. Discrepan-cies between the pressure studies [5,6] have been attributed by Letchford et al. [6] tointerference from the overly large supports on the Gumley model. This is not the casefor the force measurements where the legs were in correct scale. Discrepancies betweenforce and pressure studies can also arise from the distorted roof thickness (&8 mm)required to conceal tubing for both pressure measurement models whereas the forcebalance models were more realistically scaled being only 1 mm thick. Additionally,


Table 2Comparison between the present study, Gumley [5] and AS1170.2 [8] for the mean lift and drag on solidmonoslope roofs. Bold indicates in excess of AS1170.2 values

Roof type and pitch Source Azimuth C$

C-

Monoslope 73 AS1170.2 03 0 to 0.06 !0.52 to 0.16Gumley [5] 03 0.06 !0.48Present study 03 0.04 !0.17AS1170.2 1803 0 to 0.05 !0.20 to 0.40Gumley [5] 1803 0.03 0.26Present study 1803 0.05 0.32

Monoslope 153 AS1170.2 03 !0.11 to 0.17 !0.4 to !0.64Gumley [5] 03 0.21 !0.79Present study 03 0.16 !0.50AS1170.2 1803 0.13 0.48Gumley [5] 1803 1.12 0.46Present study 1803 0.19 0.60

Monoslope 273 AS1170.2 03 !0.43 to 0.60 !0.85 to !1.18Gumley [5] 03 0.79 !0.46Present study 03 0.44 !0.87AS1170.2 1803 0.44 0.85Gumley [5] 1803 0.49 1.05Present study 1803 0.53 1.01

#ow visualization indicated that reattachment was possible on the underside of thewindward half of the shallower canopy roofs and di!erences in #ow simulationbetween the three studies, particularly eaves height turbulence intensities &22% inRef. [6], &20% in Ref. [5] and &15% here, could explain the observed di!erences.

As the mean lift and drag forces cannot be resolved into windward and leeward netpressure coe$cients for a monoslope roof, Table 2 presents comparisons of lift anddrag coe$cients with the earlier pressure measurement results of Gumley [5] and thecode [8] resolved into these force coe$cients. The 73 results for Gumley have beeninterpolated from 03 and 153. Again the code values have had a K

Afactor of 0.80

applied. For this roof con"guration the mean force coe$cients from the present studylie within the bounds of the code values for all pitches studied for the 03 azimuth (highside windward). However, for the 1803 azimuth (high side leeward) the present resultsare nearly 50% greater for the two steeper roof pitches. A satisfactory explanation forthis large di!erence has yet to be advanced. Surface oil #ow visualization wasundertaken on 1 mm thick monoslope roof models. Along the roof centreline, theseparated #ow region extends further, by approximately 12%, on the underside for the1803 azimuth monoslope roof (&0.45 of roof length) than on the topside for the 03azimuth case (&0.33 of roof length). These surface #ow patterns are consistent witha greater resultant force (hence lift and drag) for the 1803 azimuth. Typically theGumley results di!er from the code values by a constant 0.8 factor, probably K

A,

except for the 153 pitch roof at 1803, where they are almost equal.


Table 3Net pressure coe$cient representation for porous gable roofsat 03

Roof type & pitch Porosity C18

C1-

Gable 73 Solid 0.14 !0.3011% 0.35 !0.3823% 0.71 !0.64

Gable 153 Solid 0.19 !0.6511% 0.45 !0.4923% 0.58 !0.48

Gable 273 Solid 0.69 !0.5711% 0.66 !0.4823% 0.67 !0.39

Table 3 presents the pressure coe$cient representation of the lift and drag on gableroofs as a function of porosity. It shows that, with the exception of C

1-for the 73 roof,

there is e!ectively a transfer of wind load from the leeward to the windward face ofporous roofs which is most evident at moderate pitches with the e!ect increasing withporosity. Identical trends were evident in the rougher (TC2) simulation results. Thistransfer of load is of great signi"cance because although the overall drag changes arerelatively small for increasing porosity, the doubling of load on the windward face formoderate pitch roofs will have signi"cant consequences for the roof substructuredesign. The results for the 73 pitch are somewhat inconsistent and it must be notedthat the conversion of force coe$cients to pressure coe$cients is sensitive to pitchangle and relative magnitude of lift and drag forces.

3.2. Pressure measurements

Pressure measurements were undertaken in order to estimate the wind load distri-bution. Table 4 compares the results of the centreline mean net area-averaged pressuredistribution for a 153 pitch gable roof at an azimuth of 03 for the three di!erentunderside tapping arrangements shown in Fig. 2. An average of the three results is alsopresented.

Some di$culty was experienced in obtaining undisturbed pressures, particularly onthe underneath roof surface and this led to the trial of three pressure tappingarrangements as discussed in Section 2. Although there is some scatter in the data it isevident that the trends in the force measurements are reproduced. Clearly there is aninitial increase in windward net pressure coe$cient with porosity while there isa signi"cant decrease in the leeward net pressure with increasing porosity. Directcomparison with force measurements is not really possible as only centreline pressureswere measured and there would be signi"cant three-dimensional e!ects over suchshort breadth roofs. However the trends in Table 3, C

18and C

1-from the force


Table 4Mean windward and leeward area-averaged net roof pressures on a 153 pitch gable roof at an azimuth of 03

Tapping type A B C Average

Porosity C18

C1-

C18

C1-

C18

C1-

C18

C1-

solid 0.20 !0.93 0.30 !0.82 0.29 !0.80 0.26 !0.8511% 0.48 !0.47 0.48 !0.34 0.42 !0.44 0.46 !0.4223% 0.37 !0.28 0.43 !0.14 0.37 !0.23 0.39 !0.22

Fig. 11. Centreline mean net area-averaged pressure coe$cient for one half of the solid and 23% porous,273 pitch, hip roof as a function of wind direction.

measurements, are reproduced in Table 4 apart from the decrease in windwardcoe$cient for the 23% porosity roof.

Fig. 11 shows the centreline mean net area-averaged pressure distribution acrossone half of the roof for the solid and 23% porous, 273 pitch, hip roof as a function ofwind direction. It is evident that the porous roof experiences larger net positivepressures (downwards) than the equivalent solid roof and this phenomenon is reversedfor suctions (upward loads). The largest downward load for each porosity occurs forwinds normal to the ridge line (azimuth"03), while the largest uplifts occur for eachporosity for a wind direction of about 1503, i.e., on the leeward roof half. Meanmaximum and minimum net area-averaged pressures showed similar trends. The lackof symmetry about 1803 indicates the level of interference caused by the tappingarrangement } here type B for underneath pressures.


Fig. 12. Centreline mean net pressure coe$cient for solid and 23% porous, 273 pitch, hip roof.

Fig. 12 shows the mean net point and area-averaged pressure distribution acrossthe centreline of the solid and 23% porous 273 pitch hip roof for an azimuth of 03. yis the distance from the leading edge and = is the in-wind roof length as detailedin Fig. 3. The area-averaged pressures over each roof half are signi"ed by p.a. It isevident in both the point and area-averaged measurements that there is an increase inwindward mean net pressure and a decrease in leeward mean net pressure along thecentreline.

Fig. 13 shows the RMS net point and area-averaged pressure distribution across thecentreline of the solid and 23% porous 273 pitch hip roof for an azimuth of 03. Againthe area-averaged pressures over each roof half are signi"ed by p.a. Here the #uctuat-ing area-averaged windward pressures are only slightly less for the porous roofwhereas there is a signi"cant reduction in leeward #uctuating pressures for theporous roof.

Fig. 14 shows the maximum net point and area-averaged pressure distributionacross the centreline of the solid and 23% porous 273 pitch hip roof for an azimuth of03. Surprisingly the area-averaged windward pressure maxima are slightly larger forthe porous roof. Leeward pressure maxima are of little interest in design.

Fig. 15 shows the minimum net point and area-averaged pressure distributionacross the centreline of the solid and 23% porous 273 pitch hip roof for an azimuth of03. Here there is a signi"cant reduction in area-averaged minima on the leewardporous roof when compared with the solid roof. As might be expected the average ofthe point pressure minima and maxima are greater in magnitude than the corre-sponding area-averaged pressures due to reduced correlation of the #uctuatingpressures.


Fig. 13. Centreline RMS net pressure coe$cient for solid and 23% porous, 273 pitch, hip roof.

Fig. 14. Centreline maximum net pressure coe$cient for solid and 23% porous, 273 pitch, hip roof.


Fig. 15. Centreline minimum net pressure coe$cient for solid and 23% porous, 273 pitch, hip roof.

These pressure results, although for only one roof type and pitch and subject tosome criticism in terms of tapping wake interference, clearly illustrate that thewindward roof region experiences increased mean and peak maxima loading while theleeward region experiences reduced mean and peak minima loading. These resultssupport the inferred load transfer to windward regions made from the overall forcemeasurements.

4. Conclusions

Mean wind loading coe$cients, both lift and drag, have been determined fora range of rigid porous canopy roof forms. Similarity of the pressure loss characteristicwas used to match the range of typical shade cloth materials used in the constructionof these structures to a range of perforated metal plates used to construct the windtunnel models. Hip, gable and monoslope roof forms were studied for three pitchangles, 73, 153 and 273, for porosity's ranging from 0% (solid) to 23%. The results areapplicable to shade cloths with cover factors (UV reduction rating) ranging from 80%to 100%. In using this data for the design of other porous roof materials the pressureloss characteristic K should be measured "rst to determine the applicability of theseresults.

In general windward loads increase and leeward loads reduce with increasingporosity. Flow visualization on a 153 pitch gable roof model revealed that porosityinduces #ow through the windward roof preventing reattachment beneath this sectionof the roof and thereby increasing both the upper surface load through increased


stagnation area and lower surface load through prevention of pressure recovery afterreattachment. This regime occurs on shallower pitch roofs. For steeper pitch roofsreattachment does not occur and the possibility of increased loading on the windwardroof is signi"cantly reduced. The leeward roof experiences reduced loading becausethe separation bubble formed on the upper surface at the ridge line is ventedsomewhat while the lower surface experiences a much more signi"cant wake e!ectfrom the #ow through the windward roof section. Pressure measurements, althoughdi$cult in porous materials, con"rmed the trend for an increase in windward roofload with increasing porosity and although mean and #uctuating suctions weresigni"cantly reduced with the introduction of porosity, mean and #uctuating pres-sures were actually maintained or increased with the addition of porosity.

For solid monoslope roofs, the force measurements indicated greater lift and dragfor the 1803 azimuth, high end leeward, than for the 03 azimuth. Surface oil #owvisualization con"rmed that the separation was larger on the underneath side for the1803 azimuth than on the top side of the 03 azimuth which would support theobserved force measurements. However, this "nding is opposite to earlier pressuremeasurement studies and this discrepancy remains to be clari"ed.

Signi"cant work remains to be undertaken to examine #uctuating loads and inparticular the e!ect of #exibility of porous roofs on the structural response. This is thesubject of an ongoing research program at the University of Queensland.

Acknowledgements

The authors wish to acknowledge support for this study from the Department ofCivil Engineering and an Australian Research Council small grant in 1996 and 1997.

References

[1] Civil Engineers Australia, Skin cancer is an engineering problem, Vol. 38, 19 February 1993.[2] R. Clarke, A. Lloyd, A. Whyte, Feedlot design as it a!ects the welfare and productivity of feedlot

cattle } e!ect of shade on British breed cattle, Annual Report, Brigalow Research Station,QDPI 1994.

[3] G.F. Reardon, G.R. Walker, E.D. Jancauskas, E!ects of Cyclone Winifred on buildings, Tech. Rep. 7,James Cook Cyclone Testing Station, Townsville, 1986.

[4] R.L. Donnan, G.R. Walker, E.D. Jancauskas, Design of a low cost wind resistant shade cloth structure,First National Structural Engineering Conference, Melbourne, Australia, 26}28 August 1987,pp. 384}387.

[5] S.J. Gumley, Panel loading mean pressure study for canopy roofs, OUEL Report 1380/81, Universityof Oxford, 1981.

[6] C.W. Letchford, J.D. Ginger, Wind loads on planar canopy roofs, Part 1: mean loads, J. Wind. Eng.Ind. Aerodyn. 45 (1992) 25}45.

[7] A. Row, J. Wolbers, Wind loads on shade cloth structures, BE Thesis, Department of Civil Engineering,The University of Queensland, 1996.

[8] Standards Australia, AS-1170.2, SAA loading code Part 2: Wind Loads, 1989.[9] C.J. Wood, On the use of static tubes in architectural aerodynamics, J. Wind. Eng. Ind. Aerodyn.

3 (1978) 374}378.


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