Wind pressures in canopies with parapets

A. Poitevin1, B. Natalini

2, L. A. Godoy

3

1Research Assistant, Department of Civil Engineering and Surveying, University of Puerto Rico, poite@msn.com

2 Prof. of Mechanical Engineering, CONICET-UNNE, Resistencia, Argentina, bnatalini_2000@yahoo.com.ar 3 Professor, Department of Civil Engineering and Surveying, University of Puerto Rico, luis.godoy@upr.edu

ABSTRACT

A literature review shows that there is a lack of information on the wind load pressures

acting on canopy structures with a parapet on the roof perimeter. On the other hand, a large

number of those structures suffered catastrophic damages due to hurricanes, for example during

Hurricanes Katrina and Rita in 2005. The recent version of the predominant building code for

wind design in the eastern part of the US, the ASCE 7-05, recently introduced the issue of open

structures. However, the code recommendations do not address open structures with parapets. In

this research, the authors address the problem using two different (but complementary)

approaches: a numerical simulation using Computational Fluid Dynamics (CFD) and wind

tunnel testing. Values of pressure coefficients Cp were obtained by both methods, showing good

agreement between them. Further studies are in progress in order to gather data on Cp values

for canopies with parapets.

INTRODUCTION

Open canopies are frequently used in the construction of civil engineering facilities,

either as components of larger structures or as a self supported structures. An example of the

second type may be found in most gas stations throughout the US, in which the roof covers gas

pumps and vehicles. In small gas stations, canopies are supported by a single row of columns,

and do not include perimeter walls, whereas larger canopies are supported by columns aligned in

two rows.

Figure 1: Schematic view of a canopy used in gas stations.

The roof structure is formed by a system of beams in two directions to support the roof

panels. In some cases, the system is based on cantilever columns. In hurricane-prone areas, such

as the Caribbean islands and the coastal areas in the US, the most critical structural conditions

occur during high wind loads.

A large number of those structures suffered catastrophic damages due to hurricanes, for

example, during Hurricanes Katrina and Rita in 2005 [1]. Figure 2 illustrates the failure of one

canopy structure which had been recently constructed in Chalmette, close to the city of New

Orleans. The canopy rotated as a rigid body, with localized failure at the supports in the weak

direction of the columns.

Figure 2: Chalmette, New Orleans, Hurricane Katrina (photograph by L. A. Godoy).

The predominant specification for wind design in the eastern part of the US is ASCE 7.

The recent version of this code, the ASCE 7-05 [2] was the first one to address the issue of open

structures; however, the recommendations do not address the issue of open structures with

parapets. With such high wind velocities, it is surprising to find such lack of information on the

wind pressures acting on canopy structures with a parapet on the roof perimeter. Our conclusion

is that open canopy structures which include parapets have not been studied in detail in the

research literature or taken into account in the current design codes used in the US.

Canopy structures without parapets have been recently investigated in a small number of

previous investigations, leading to wind pressure distributions estimates and design coefficients

[3, 4, 5, 6, 7, 8], but, no testing has been reported regarding wind pressures in open canopies with

parapets. Because this may be a controversial topic (due to its engineering significance), it is

desirable to have some methodological redundancy to make sure that adequate pressure

coefficients are proposed. The situation calls for numerical and experimental investigations on

the same topic, in order to validate pressure coefficients using independent approaches. In this

research, the authors address the problem using two different (but complementary) approaches: a

numerical simulation using Computational Fluid Dynamics (CFD) and wind tunnel testing.

In this paper, mean pressure coefficients produced with a numerical model are presented

and compared with results of boundary-layer wind tunnel tests on a canopy roof with parapets,

being these values the first ones ever reported on this kind of structures.

EXPERIMENTAL ARRANGEMENT

MODELS

Experiments were conducted on one 1:50 scale model of a 7.5 m × 7.5 m square roof

with parapets of 1.2 m high, having an eave height of 3.6 m. First, experiments were carried out

with no parapets in order to compare with other wind tunnel results available in open literature.

Wind load coefficients were measured under wind blowing at angles of 0º and 30º relative to one

of the symmetry axis, since, as demonstrated by Gumley [3, 4, 7] among others, these directions

produce the most severe loads on planar canopy roofs with no parapets.

The roof of the model and the parapets were made with a 2 mm thick aluminium plate,

and the columns with a 2.5 mm diameter steel rod. As the models have two axes of symmetry,

only a quarter of the models' roofs had pressure taps in place, thus reducing the number of tubes

needed. In addition, all the tubes were led towards the farthest corner, where they formed a

bundle that went into a horizontal pipe through which they went away from the model to finally

go under the floor. In this way, the scale distortion in both columns and roof thickness and the

possible interference of the tubes upon the measurements were minimized.

Sixteen pressure taps were spread on the roof and twenty on the parapets. Figure 3 shows

the position of the pressure taps on the model and Figure 4 shows the model placed in the

turntable.

z

x

y

z

Figure 3: Distribution of pressure taps on the model.

Figure 4: View the model with parapets on the turntable.

WIND SIMULATION

The wind tunnel testing was performed during 2008/2009 at Universidad Nacional del

Nordeste (UNNE) in Resistencia, Argentina. The facility is an open circuit tunnel with a length

of 22.8m (74.8ft), the testing chamber being a square section of 2.4m (7.87ft) width and 1.8m

(5.9ft) in height and uses a 2.25m (7.38ft) fan with a 92kw motor. The maximum wind velocity

is 25m/s (55.9 mph) when the testing section is empty. Further details regarding this wind

facility are given by Wittwer and Möller [9].

All the models were tested under a wind simulation corresponding to a suburban area.

The simulation hardware consisted of two modified Irwin’s spires [10] and 17.1 m of surface

roughness fetch downstream from the spires. In this way, a part-depth boundary layer simulation

of neutrally stable atmosphere was obtained. Mean velocities, when fitted to a potential law, give

an exponent of 0.24. The length scale factor determined according to Cook’s procedure [11] was

approximately 150. The local turbulence intensity at the level of the roof is 0.25. De Bortoli et al.

[12] provided further details of this turbulence simulation, including size, geometry, and

arrangement of the hardware and design criteria.

PRESSURE MEASUREMENT SYSTEM

Pressures were measured using a differential pressure electronic transducer Micro Switch

Honeywell 163 PC. A sequential switch Scanivale 48 D9-1/2, which was driven by means of a

CTLR2 / S2-S6 solenoid controller connected the pressure taps to the transducer through PVC

tubes of 1.5 mm in internal diameter and 650 mm in length. No resonance problems were

detected for tubes of that length (the gain factor being around one) therefore, restrictors of

section were not used for filtering. The DC transducer output was read with a Keithley 2000

digital multimeter. The integration time operation rate of the A/D converter was set to produce

mean values over 15 seconds of time integration.

Simultaneously to the pressure measurements being taken on the roof, the reference

dynamic pressure, qref, was measured at the eaves height with a Pitot-Prandtl tube connected to a

Van Essen 2500 Betz differential micromanometer of 1 Pa resolution. The probe stayed beside

the model at a distance of about 0.70 m to avoid mutual interference. The reference static

pressure was obtained from the static pressure tap of the same Pitot-Prandlt tube.

COMPUTIONAL FLUID DYNAMICS (CFD) SIMULATION

The wind flow on an open canopy structure with parapets can be represented and

visualized with the use of CFD. The specific CFD software package used in this research is

named EFD.Lab (Engineering Fluid Dynamics), developed by Flomerics Inc. [13]. EFD.Lab

solves the Navier-Stokes equations, which include the formulations of mass, momentum and

energy conservation laws for fluid flows. The equations are supplemented by fluid state

equations defining the nature of the fluid and by empirical dependencies of fluid density,

viscosity and thermal conductivity on temperature [13].

EFD.Lab solves the governing equations using the finite volume method (FVM) on a

prismatic rectangular computational mesh drawn in a 3D Cartesian coordinate system with the

planes orthogonal to its axes. To improve the results, the mesh is refined locally at the solid/fluid

interface. Turbulence is incorporated by means of turbulent intensity and turbulent length

parameters, they were obtained from the wind tunnel and used in the CFD computations order to

emulate the wind tunnel conditions.

The computational mesh used for the simulation is created in a prismatic domain, as

shown in Figure 5. The surface area on the left is defined as the inlet boundary, in which it is

possible to introduce the values for the wind profile. The specific profile adopted in the first

model investigated in this research is a power law (as defined by ASCE 7) for which an exponent

of 0.24 was used, as obtained from the wind tunnel tests. The transverse section of the flow in

Figure 5, showing different velocities on the left indicate that a wind profile was used as an

input. The canopy induces significant changes to the flow, and the program calculates pressures

and Cp values on its contours.

A second computational model was also built, in which laminar flow was assumed with a

constant velocity profile in height. Although the inlet conditions between a power law and a

uniform velocity profile are very different, in the present case in which the canopy is a thin body

at a given distance from the terrain, the results in terms of pressures acting on the canopy are

remarkably similar. The reasons for this similarity are that the canopy is located away from the

turbulence generated at the ground level, and the pressures are dominated by the pressures and

velocities that occur at the canopy height. Such similarity is only specific of this case and would

not occur in enclosed structures.

The results from the initial computational domain were compared to a smaller domain

without a boundary condition. The number of cells for the initial domain was 303,123. The

smaller domain used was 54,954 cells. The results from the comparison produced similar results

between both simulations, whereas the computational time was reduced dramatically. For this

reason, further analysis was performed with the smaller domain using a uniform velocity profile

at the inlet surface as shown in Figure 6.

The CFD software yields computations of wind pressures on the external (top and

bottom) surfaces of the structure, with a wind velocity similar to those specified by design codes.

CFD simulations are extremely useful to identify and define the geometric characteristics of

canopies that will be investigated prior to the construction of physical models to be tested in a

wind tunnel facility. Preliminary investigations allow determining the high pressure areas for

determination of the wind pressure taps to be used on the scale model.

Figure 5: Computational domain with boundary element and wind profile. The inlet condition is defined in

the plane on the left.

The initial model geometry analyzed consists of a 7.6 m (25 ft) x 7.6m (25ft) with a 1.2m

(4ft) parapet. The roof height is located at 3.6m (12ft). The mesh cells sides are orthogonal to the

specified axes on the coordinate system. The mesh can be automatically created using a mesh

generator, but the mesh in this simulation was specified to be refined on the exposed surfaces.

Further sub-meshing was assigned for refinement on all exposed surfaces to the turbulent flow.

Figure 6: Computational domain and meshing used in the present CFD simulations.

Wind pressures were obtained from the CFD simulations. To better represent the results

obtained from the wind tunnel, data were obtained at the same places where the pressure taps

were located in the physical model. Specifically, pressure taps were located on the top surface,

bottom surface and on all parapets, inside and outside surfaces. Results of the Cp (pressure

coefficient factor) were obtained from the simulation, and have been represented in Figure 7.

Figure 7: Location of pressure taps and measured wind pressures on the top surface. Wind direction acting

from the right (0 degrees).

Two wind directions were used for the computational model, which were the same wind

directions used in the wind tunnel [3,4,7]: One direction at 0 degrees and the second one at 30

degrees of the long axis of the canopy (see Figure 8).

Figure 8: Wind direction in CFD model at 0 degrees and 30 degrees.

RESULTS

Data from the wind tunnel and computational models were obtained on the top and

bottom surfaces at 64 pressure points leading to Cp values for each surface. Two wind conditions

were considered: one with incident wind oriented in the long axis direction (0°), whereas a

second case was considered by rotating the wind to 30° with respect to the structural axis.

Figure 9: Cp results from top surface (a) wind tunnel, and (b) CFD model for incident wind at 0 degrees from

the structural axis.

Figure 9 shows the Cp values for the top surface of the canopy. Figure 9a shows the wind

tunnel Cp results and Figure 9b shows the CFD Cp results for the wind at 0°.On both, the wind

tunnel and the CFD simulation the top surface shows an uplift almost until the end of the surface

where a downward pressure is clearly shown on both figures.

Figure 10: Cp results from bottom surface (a) wind tunnel, and (b) CFD model for incident wind at 0 degrees

from the structural axis.

Figure 10 shows the Cp values for the bottom surface of the canopy. Figure 10a shows

the wind tunnel Cp results and Figure 10b shows the CFD Cp results for the wind at 0°. The

bottom surface shows on both, the wind tunnel and the CFD simulation an uniform downward

pressure through all the bottom surface.

Figure 11: Cp results from top surface (a) wind tunnel, and (b) CFD model for incident wind at 30 degrees

from the structural axis.

Figure 11 shows the Cp values for the top surface of the canopy. Figure 11a shows the

wind tunnel Cp results and Figure 11b shows the CFD Cp results for the wind at 30°.On both,

the wind tunnel and the CFD simulation the top surface shows an uplift for the first half of the

top surface and a downward pressure on the other half. This effect is clearly shown on both

figures.

Figure 12 shows the Cp values for the bottom surface of the canopy. Figure 12a shows

the wind tunnel Cp results and Figure 12b shows the CFD Cp results for the wind at 30°. The

bottom surface shows on both, the wind tunnel and the CFD simulation a uniform downward

pressure through all the bottom surface. The Cp values on both figures are very similar.

Net values, denoted by Cn, were calculated as the difference between the Cp values

obtained at the same location and on top and bottom surfaces. Figure 13a and Figure 13b are the

Cn results for the wind at 0° for the wind tunnel and the CFD simulation. Both figures showed an

initial downward pressure through the first 1/3 of the length, and uplift for the next 1/3 of the

length and a downward pressure for the last 1/3.

Cn results for the wind at 30° are shown on Figure 14. Figure 14a and Figure 14b shows

as before the wind tunnel test and the CFD simulation. Both figures showed an uplift area

located in the center of the surface, with a downward pressure around that center area.

Figure 12: Cp results from bottom surface (a) wind tunnel, and (b) CFD model for incident wind at 30

degrees from the structural axis.

Figure 13: Cn results from (a) wind tunnel, and (b) CFD model for incident wind at 0 degrees from the

structural axis.

Figure 14: Cn results from (a) wind tunnel, and (b) CFD model for 30 degrees from the structural axis.

The extreme values for Cp and Cn along the wind axis are shown on Figure 15. Values of

Cp TP (Cp at the top surface), Cp BP (Cp at the bottom surface), and Cn for the canopy with

wind at 0° are plotted. The Cn values show a Cp of +1.3 maximum behind the parapet. The Cn

values changes along the center of the roof to a Cn of -0.5, through the end of the surface it

changes to a Cn of +0.5. The Cn values changes from + (positive) to – (negative) and to +

(positive) Cn values through the roof geometry.

Figure 15: Cn results from different parapets heights on CFD simulation.

CONCLUSIONS

There is conclusive evidence that a large number of open canopy structures suffered

catastrophic damages due to hurricanes in the US during the last decade. The most recent version

of the ASCE [2] includes considerations for open structures; however, the recommendations do

not account for cases of open structures with parapets. For the specific geometry used in this

investigation, the calculated Cn values for an open structure without parapets using ASCE 7 are

negative on the complete surface and range from -0.8 to -0.3.

But, based on the evidence presented in this paper, those values cannot be trusted to

perform the design or verification of actual canopy structures used in gas stations. Our Cn results

from physical and computational simulations show that there is a strong influence of the parapets

on the pressure coefficients of a canopy structure; this influence was observed in both, wind

tunnel testing and CFD simulations, and good agreement has been obtained between the two

independent methodologies.

For the case studied here, the Cn values obtained in this investigation vary from +1.3, -

0.5 and +0.5. The mean roof pressures vary from pressure downward to upward pressure

depending on the specific locations considered in the roof along the long axis. This significantly

differs from the recommended code values, which predict that only suction, will occur in the

roof.

We conclude that ASCE recommendations for this type of structures are not sufficient to carry

out safe designs of canopy structures with parapets. It is suggested that current code

recommendations should be modified for open structures and should include new coefficients for

open structures with parapets, such as those presented in this paper. Further investigation [14] is

in progress to include the influence of different parapet heights and different roof geometries on

Cp values.

ACKNOWLEDGEMENTS

The authors wish to acknowledge the help of José A. Iturri in making the models and

preparing the experimental setup. Research in Argentina is part of an area supported by the

Facultad de Ingeniería of the Universidad Nacional del Nordeste . Research in Puerto Rico was

supported in part by NSF-CCLI grant DUE-0736828: “A computer-based simulated environment

to learn on structural failures in engineering” (Program Director: Sheryl Sorby).

REFERENCES

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[2] ASCE 7 Standard, 2005, Minimum Design Loads for Buildings and Other Structures, American

Society of Civil Engineers, Reston, VA.

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[14] A. Poitevin, 2009, Analysis of canopy structures with parapets under wind, Ph. D. Thesis, University

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