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Wind Tunnel Experiments on a Patio Cover Attached to a Low-Rise Building

Ioannis Zisis1, Ted Stathopoulos2

1Research Assistant, Centre for Building Studies, Concordia University, Montreal, QC, Canada,i_zisis@live.concordia.ca

2Professor, Centre for Building Studies, Concordia University, Montreal, QC, Canada,statho@bcee.concordia.ca

ABSTRACT

The paper presents the methodology and findings of a set of wind tunnel experiments onbuilding models with an attached patio cover. Both testing procedure and data analysis arediscussed and detailed results are presented. Three different configurations have beenconsidered in order to assess the effect of building and patio cover height on the design windpressures for patio covers. The wind tunnel experiments were conducted on a 1:100 geometricscale model tested in the Building Aerodynamics Laboratory of Concordia University. The windpressure distribution is presented in the form of pressure coefficients for both the top and thebottom surface of the patio. In addition, critical net wind pressures on the patio cover and thecorrelation of pressures for the two patio cover surfaces are evaluated. The experimentalfindings are also compared to those from previous studies (roof-eaves) and to the eave /overhang provisions suggested by the current codes of practice. The paper identifies thedifferences and shows that building codes and standards may lead to over-conservative design.

INTRODUCTION

A widely used element of residential low-rise buildings is the patio cover, which in most cases isattached to houses and provides shelter to residents for outdoor activities. Patio covers lightweight makes them vulnerable to wind-induced loads requiring proper design and specialattention during construction for both safety and economical reasons.

The available wind provisions include insufficient information regarding design loads forsuch structural components. In some cases improper interpretation of current building codesmislead engineers to treat patio covers as a subsection of canopies or eaves, i.e. roof extensions.Applied design pressures may thus be over conservative or simply inadequate. In reality, patiocovers are a completely independent group of structures which cannot fall under any of theabove categories for the purpose of wind load evaluation.

Currently, codes that clearly refer to patio covers are the International Building Code(IBC 2000) and the International Residential Code (IRC 2000) accompanied by AcceptanceCriteria AC340 (ICC-ES). Despite the fact that these standards/criteria use the exact terminologyfor patio covers, they all refer to ASCE 7-05 (ASCE/SEI) for specific wind load requirements.The ASCE 7-05 standard nevertheless, does not have any direct reference on patio coversbesides sections 6.5.12 and 6.5.13 that present wind load details for enclosed, partially enclosedand open buildings. Suggested values for net pressure coefficients for a number of different roofconfigurations are also shown in figures 6-18 A-D (main wind force resisting system) and 6-19A-C (component and cladding) of ASCE 7-05. These net pressure coefficients includecontributions from both roof surfaces. Discussion made in chapter C6 of ASCE 7-05 indicatesthat Figures 6-18 and 6-19 are mainly based on the Australian code provisions.

The Australian Standard (AS/NZS 1170.2:2002) includes a section for attached canopies,awnings and carports in which net force coefficients are suggested. It appears that an attached

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canopy should have similar wind load design requirements to an attached patio cover. The designcases covered on this standard include both the uplift and down-lift wind action scenarios for twobasic directions, parallel and perpendicular to the building’s orientation. Moreover, the ratio ofthe patio to the roof height is taken into consideration along with the size (width) of the patiocover.

Other codes (e.g. National Building Code of Canada 2005, Eurocode 1 / prEN 1991-1-4,British Standards / BS 6399-2 1997) do not include any section referring directly to wind loadrequirements for patio covers attached on buildings. Instead they present in detail designrequirements for roof canopies.

WIND TUNNEL EXPERIMENTS

The case of a low-rise building with an attached patio cover was examined through detailed windtunnel tests. The experiments conducted in the Building Aerodynamics Laboratory located in theEngineering Complex at Concordia University. The 1:100 geometric scale building model usedfor the tests has external dimensions of 15 cm by 10 cm by 9 cm (length – width – ridge height)and roof slope of 4:12. The model is equipped with 65 roof pressure taps located both on the topand the bottom of the roof eave. For the current study only specific pressure taps wereconsidered. In addition a metallic patio cover model of the same geometric scale (1:100) wasconstructed and mounted on the existing building model creating the final test model. The patiocover model is 15 cm by 3.65 cm and 0.2 cm thick. In order to consider different buildingconfigurations, the patio cover could be attached to two different building heights. Threeconfigurations were examined; two-storey house with the patio cover at the first floor level, two-storey house with the patio cover at the roof height and single-storey house with the patio coverat the roof height (see Figure 1). The patio model was equipped with 30 pressure taps, 15 on thetop surface and another 15 on the bottom surface (Figure 2). It should be noted that odd numbers(1.3.5 etc.) are used for the top surface pressure taps and even numbers (2.4.6 etc.) for those atthe bottom surface.

Figure 1: Isometric, front and side views of the building models - Configurations I, II and III.

I

II

III

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The tests were conducted using open terrain simulation characteristics. The value of thepower law exponent alpha (α) was 0.14 and the corresponding turbulence intensity at the roofheight was approximately 17.5% for the one storey model and 14.0% for the two storey model.The later satisfies previously addressed importance of proper simulation of turbulence intensityduring wind tunnel testing of low-rise buildings (Tieleman 1998).

A total number of 28 wind attack angles were examined - see Figure 3. The scanningfrequency was 250 Hz and the duration of the record was 36 seconds.

Figure 2: Pressure tap location and notationon the roof and the patio cover model.

Figure 3: Wind directions tested in the wind tunnel.

All acquired data were analyzed and transformed into dimensionless pressure coefficientsbased on the following equation:

mean(peak)ap,mean(peak)

2h

p pc 1 V

2

(1)

where: : the air density (kg/m3)

hV : the mean wind speed at the mid-roof height (m/s)ap : the ambient atmospheric pressure (kPa)mean(peak)p : the actual surface pressure (kPa)

Correlation coefficients were evaluated by the following equation:

i iX,Y

x y

x x (y y)r

(n 1)s s

(2)

where:i ix , y : sample random variables

x, y : sample means of random variables

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x ys ,s : sample standard deviations of random variablesn : total number of samplesNote that positive correlation indicates that in absolute terms, large values of “x” variable

tend to be associated with large values of “y” variable, negative correlation indicates that smallvalues of “x” variable tend to be associated with large values of “y” and zero correlationindicates that the two variables tend to be unrelated.

EXPERIMENTAL RESULTS AND DISCUSSION

NET PRESSURE COEFFICIENTS

All three building model configurations were tested in the wind tunnel for 28 wind angles ofattack (see Figure 3). An integration of maximum mean, minimum mean, maximum andminimum net pressure coefficients for all tested directions was performed and the most critical(worst) net pressure coefficients are presented in Table 1. The pressure contour patterns showmore similarities for Configurations II and III for which the patio is closer to the roof eave.Configuration I experiences the highest positive peak pressure coefficients (+1.40) whereas meanand peak suctions for Configuration II are significantly higher (minimum net pressure coefficientof -2.80) compare to the other two examined cases.

Table 1: Most critical mean and peak net pressure coefficients.

Cp,mean (max) Cp,mean (min) Cp,max Cp,min

Configuration I

Configuration II

Configuration III

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PRESSURE CORRELATIONS BETWEEN UPPER AND LOWER SURFACES

Patio covers experience wind-induced pressures on both upper and lower surfaces. This type ofwind-structure interaction is a highly sensitive phenomenon closely related to the correlation ofthe upper and lower pressure traces. The combined effect on both surfaces is of great interest andin most cases the guide for the design of such structures. Peak values are not likely to occursimultaneously for both surfaces therefore the applied net pressure is in some cases significantlylower compared to the algebraic sum of individually observed most critical top and bottompressures.

Two representative azimuths were selected and correlation coefficients calculated foreach pair of top and bottom pressure taps. The results are presented as contour plots for eachconfiguration, for both 30 and 135 degrees wind direction (Figure 4). In agreement to theprevious results, Configurations II and III show a similar pattern which is more pronounced forthe case of 135 degrees wind direction. As expected, the two traces are better correlated for thecase of 30 degrees rather than the 135 degrees wind direction. Another interesting finding is thatwhen the patio is located on the windward side (30 degrees) higher correlations occur for regionscloser to the wall the patio is attached to, oppose to the case of 135 degrees wind direction thatresults into higher correlations for the front edge region of the patio.

In addition to the results for these two wind directions, the variation of the correlationcoefficient versus the wind azimuth for three representative sets of top and bottom pressure tapsis presented in Figure 5. The location of these pressure tap pairs is on the corner (pressure taps 1-2), the front edge (pressure taps 13-14) and the patio-to-wall edge (pressure taps 17-18) regions.The results indicate that, in general, Configuration I shows higher correlation values compared tothe other two configurations. Moreover, the presence of the patio cover upstream of the actualbuilding results in lower correlation values, especially for the front edge pressure tap sets (1-2and 13-14). The set of pressure taps located on the patio-to-wall region (17-18) shows the mostuniform behaviour and least dependent on wind direction.

Figure 4: Correlation coefficient contour plots for top and bottom pressure signals (30 and 135 degrees winddirection).

30o–

Con

f. I

30o–

Con

f. II

30o–

Con

f. II

I

135o–

Con

f. I

135o–

Con

f. II

135o–

Con

f. II

I

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Pressure Taps 1-2

-1-0.75-0.5

-0.250

0.250.5

0.751

0 45 90 135 180 225 270 315 360

Conf. I Conf. II Conf. III

Pressure Taps 13-14

-1-0.75-0.5

-0.250

0.250.5

0.751

0 45 90 135 180 225 270 315 360

Conf. I Conf. II Conf. III

Pressure Taps 17-18

-1-0.75-0.5

-0.250

0.250.5

0.751

0 45 90 135 180 225 270 315 360

Conf. I Conf. II Conf. III

Figure 5: Correlation coefficient variation with the wind direction (pressure taps 1-2, 13-14 and 17-18).

COMPARISON WITH WIND CODES AND STANDARDS

The experimental findings were compared with the provisions available in some wind standardsand building codes of practice. Following the common practice to consider attached patio coversat the eave level as roof extensions and design them accordingly, Figure 6-11C of ASCE 7-05overhang provisions were compared to local pressure coefficients obtained from the currentstudy. Using single or multiple sets of pressure taps the variations of the maximum and minimumpeak net pressure coefficient were derived with respect to the corresponding effective surfacearea. The resulting polylines are superimposed as shown in Figure 6. It should be noted that theASCE 7-05 proposed values refer to gust pressure coefficients based on the 3-sec gust windvelocity pressure, therefore appropriate transformation was made to the wind tunnel pressurecoefficient (see Equation 1) results (i.e. divided by 1.532 – Figure C6-4, ASCE 7-05). Thecomparison clearly shows that code values overestimate net wind suctions (uplift behavior) andconform to the net wind pressures. For the negative pressure coefficients all configurations testedare significantly lower than the code values.

The Australian Standard is the only source that refers directly to attached patio covers. Itsprovisions are based on wind tunnel studies (Jancauskas and Holmes 1985) and refer to netpressure coefficients integrated over the total area of the patio. The comparison of wind tunnelvalues with the Australian Standards (net pressure coefficients) is presented in Tables 2 (0degrees wind direction – see Figure 3) and 3 (90/270 degrees wind direction – see Figure 3).Similarly to the ASCE standard, the AS/NZS proposed values are referenced to the 3-sec gustreference pressure, therefore appropriate transformation was also made to the wind tunnelresults. Furthermore, the wind tunnel measured net pressure coefficients were integrated over theentire area of patio cover in order to determine the total force coefficient for each wind direction.

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The agreement in most cases is satisfactory, with some discrepancies observed mainly forConfiguration III and wind direction parallel to the attached wall.

Figure 6: Variation of the peak net pressure coefficient with the area – Configurations I, II and III.

Table 2: Comparison of the AS/NZS suggested force coefficient values with the wind tunnel results, for 0degrees wind direction.

Wind Direction 0

Configuration I II IIIAS/NZS 1170.2:2002 0.46 -0.20 0.35 -1.50 0.43 -1.12

Wind Tunnel 0.45 -0.19 0.29 -1.03 0.25 -0.59

Table 3: Comparison of the AS/NZS suggested force coefficient values with the wind tunnel results, for 90/270degrees wind direction.

Wind Direction 90 (A-B-C) / 270 (C-B-A)

Configuration IRegion A B C

AS/NZS 1170.2:2002 0.40 -0.30 0.00 -0.40 0.20 -0.20

Wind Tunnel 0.52 -0.66 0.40 -0.42 0.32 -0.19

Configuration IIRegion A B

AS/NZS 1170.2:2002 0.40 -0.30 - - 0.00 -0.40

Wind Tunnel 0.47 -0.80 - - 0.32 -0.11

Configuration IIIRegion A B C

AS/NZS 1170.2:2002 0.40 -0.30 0.00 -0.40 0.00 -0.20

Wind Tunnel 0.20 -0.59 0.33 -0.18 0.33 -0.14

OOvveerrhhaanngg

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CONCLUSIONS

A set of wind tunnel tests were conducted to evaluate wind-induced pressures on patio coversattached to low-rise buildings. Mean and peak local net wind loads were presented for three patioconfigurations and correlation between the top and bottom surface was examined. Moreoverexperimental findings were compared to current wind provisions, indicating a generallyconservative design approach followed by these limited code references. In particular, thesignificant differences found from the overhang design provision of ASCE 7-05 should beaddressed. The results of the study can be codified and proposed for adoption to several windcode and standard committees.

REFERENCES

AC340, 2007. Acceptance Criteria for Patio Covers, ICC Evaluation Service, INC, Whittier, CA.

ASCE/SEI 7-05, 2005. Minimum Design Loads for Building and Other Structures, American Society ofCivil Engineers, New York, NY.

AS/NZS 1170.2, 2002. Australian/New Zealand Standards, Structural design actions – Part 2, WindActions, Standards Australia International Ltd (Sydney) – Standards New Zealand (Wellington).

BS 6399-2, 1997. Loading for Buildings – Part 2: Code of Practice for Wind Loads, British Standard.

International Building Code (IBC) 2000. International Code Council, Fall Church, VA.

International Residential Code (IRC) 2000. International Code Council, Fall Church, VA.

Jancauskas, L., Holmes, J., 1985. Wind Loads on Attached Canopies, Proc. Of the National Conferenceon Wind Engineering, Texas Engineering University, Lubbock TX.

National Building Code of Canada (NBCC), 2005. National Research Council of Canada (NRCC),Ottawa, Canada.

prEN 1991-1-4, 2004. Eurocode 1: Actions on Structures – General Actions – Part 1.4: Wind Actions,European Standard.

Tieleman, H. W., Hajj, M.R,. Reinhold, T.A., 1998. Wind tunnel simulation requirements to asses windloads on low-rise buildings, Journal of Wind Engineering and Industrial Aerodynamics, 74-76, pp.675-685.