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Journal of Wind Engineering

and Industrial Aerodynamics 93 (2005) 873888

Wind effects of parapets on low buildings: Part 4.

Mitigation of corner loads with alternative

geometries

Gregory A. Kopp, Christian Mans, David Surry

Alan G. Davenport Wind Engineering Group, Boundary Layer Wind Tunnel Laboratory,

University of Western Ontario, London, Ont., Canada N6A 5B9

Received 14 May 2004; received in revised form 23 August 2005; accepted 26 August 2005

Available online 10 October 2005

Abstract

This is the fourth paper in a series on the wind effects of parapets on low-rise buildings. This part

focuses on alternative parapet geometries which can mitigate local (component and cladding) loadingdue to the formation of corner vortices. It was found that spoilers and porous perimetric parapets

significantly reduce the loads for all areas in the corner, edge and interior zones considered. For solid

parapets, raising the corner, or putting a slot in the corner are also beneficial when compared to

uniform, continuous parapets. Removing the corner of a continuous parapet also lowers the loads as

compared to isolated (single) parapets which end at the side wall, indicating that shortened isolated

parapets could also be beneficial.

r 2005 Elsevier Ltd. All rights reserved.

Keywords: Wind loads; Low-rise buildings; Building codes; Parapets

1. Introduction

1.1. Background

Worst suction coefficients on the roofs of low buildings are known to occur in the

corner, caused by the development of conical vortices during cornering winds [1,2]. These

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Corresponding author. Tel.: +519 661 3338; fax: +519 661 3339.E-mail address: [email protected] (G.A. Kopp).
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vortices are triggered at the leading corner of the building, and have resulted in worst

suction coefficients of over 20 (referenced to the mean wind speed at roof height) being

measured [3]. Several researchers have examined different strategies for mitigating these

high suctions.

As described by Surry and Lin[4,5], there are several types of geometric configurationsthat can be used to alleviate the corner suction pressures, which they categorized as:

1. Displace the corner vortices using a solid parapet to raise the vortices away from the

roof surface.

2. Disrupt the formation of the vortices using a partial or porous parapet.

3. Disturb the vortices on the rooftop by placing a fence or object along the path of

formation.

4. Eliminate the straight sharp edges that create the separated flow, e.g. rounded edges.

Each of the above methods have been previously studied in wind tunnel experiments

[1,413], with a detailed listing of the configurations given in Table 1. These published

results are discussed in detail below.

1.2. Solid parapets

The solid parapet is the more popular of the configurations, primarily due to its large

architectural appeal. The solid parapet acts to raise corner vortices off the surface of the

roof so that the intense shear layers no longer interact with the roof surface. A detailed

study on the effect of a solid perimetric parapet was performed in Part 1 of the currentproject[13]. The investigation concluded that parapets with heights,h=H h40:23, mayreduce the corner and edge suction pressures by over 50%. However, it should also be

recalled that taller parapets also create substantial structural loading on the interior frames

and roof regions and severe positive (downward) loading upstream of the base of the

leeward parapet, as well as increased loading on the parapet itself.

1.3. Partial and porous parapets

A number of partial parapet configurations have been previously tested[5,7,8], differing

in geometry by either increasing or decreasing the parapet height in the corner region.These configurations included: (a) discontinued parapet at the corner[7,8], (b) increased

parapet height at the corner[7,8], (c) slots, allowing venting, placed in the lower half of the

parapet in the corner[7,8], (d) a partial parapet placed a small distance away from the roof

corner[4,5], and (e) a castellated parapet[6].

The results from Stathopoulos and Baskaran[7,8]suggest that, in most cases, the partial

parapets caused a reduction in loading compared to the solid parapet, but failed to reduce

the corner suction pressures compared to the no parapet case. Similar results were

observed in[6]for a castellated parapet, with the exception of 1.5 m (5 ft) high parapets,

which were found to reduce the suction coefficients in the corner region. However, for this

parapet height (1.5 m, with an eaves height of 96 m), the higher perimetric parapet may beequally effective. It should also be noted that the study by Stathopoulos and Baskaran[4,5]

was performed on a tall building H 96 m and as such the results may not be the same

as for low-rise buildings.

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Table1

Summaryof

previouslypublishedtestparameters

Author

Plandimensions

Eaves

height

Windangles

Terrain

Modelscale

Geometry

Banks[1]&W

u

[14]

12:2

9:1m;1:60

gableroofslope

4.0

m

501

open

1:25

Spoiler(

perimetric)

Porousparapets(perimetric)

6.0

"

9.0" 2.0

"overhang

10

(i)

wiremesh,50%sol

id

(ii)50%solidwithcircularholes

(iii)slattedfence

Stathopoulos

&

Baskaran[7,8

]

61:0

61:0m;flat

roof

4.6

m

0,30,45,60,

901

open&

urban

1:400

Partialparapets,

h=1.0m

solid

slotted

discontinuous

raised

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Table1(cont

inued)

Author

Plandimensions

Eaves

height

Windangles

Terrain

Modelscale

Geometry

Surry&Lin

[4,5]

12:2

9:1m;1:60

gableroofslope

4.0

m

0901(13

angles)

open

1:50

Sawtoothparapet

Cylindersonroof

Radialsplitters

Partialpa

rapet

50%s

olid(h=3.0m)

Curv

edeaves

Robertson[10]

24:1

12:8m;101

pitch,gableroof

5.3

m

N/A

open

Full-scale

R=2.1

"

Blackmore[9]

75:0

75:0m;flat

roof

25.0

m

0,15,30,45,

60,75,901

open

1:250

Slope=30

,45,60

w=20.5

',41.0

'

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Configuration (d), tested by Lin and Surry[4,5]with a building height of 4 m, was found

to reduce the peak and rms corner pressures by about 25%, but caused no significant

change in the mean corner pressures. The differing conclusions between the studies [46]

can probably be attributed to the differences in building height, in addition to variations in

the locations of the pressure taps. Baskaran and Stathopoulos [7,8]recorded a total of 87pressure taps over the entire roof surface, however, the majority of the presented results

were from a single corner tap. The study by Lin and Surry[4] recorded pressures from 30

taps at various locations around the building corner.

1.4. Fences and spoilers

Lin and Surry [4,5] examined three saw-tooth configurations (single, dual or triple

crowned saw-tooth parapets) positioned either at the leading corner or a short distance

away from the corner. The saw-tooth configuration reduced the peak corner pressures by

3040%, with the dual or triple saw-tooth parapets yielding no further reductions over

the single saw-tooth configuration. The largest reductions in loading occurred when the

parapet was positioned flush with the leading corner of the building.

The porous parapet has consistently been found to be the most effective parapet

configuration, reducing the corner pressures by up to 70%[5]. As described by Surry and

Lin, the effectiveness of the porous parapet is due to three factors: (a) the porous screen

disrupts the vortex formation along the roof edge, (b) the screen absorbs some of the

energy of the flow over the roof edge, and (c) the weakened vortices that do form, do so

over the top edge of the parapet and are displaced away from the roof-top. Recent work by

Banks [1], who examined a number of porous parapet geometries, found similar results.Banks studied three variations: a mesh of50% porosity, a circular hole porous parapet

with 50% porosity and a slatted fence. The effectiveness of the three parapet

configurations were similar (around 50% reduction in peak pressures), with the circular

holed parapet providing marginally better results.

Banks [1] and Wu [14], in a collaborative project between Colorado State University

(CSU) and Texas Tech University (TTU), also created a novel device, a spoiler, specifically

designed to be easily added to existing structures. (See Table 1 for details of its

appearance.) This spoiler is relatively small in full-scale, 10 cm by 2 cm, but was still found

to reduce the peak coefficients by about 50% in wind tunnel testing at CSU and full-scale

testing at TTU. It does this by allowing air to pass beneath the device, disrupting theformation of vortices at the sharp corners and edges. Melbourne [15] used similar

principles to alleviate the high suctions on the leading edges of cantilever stadium roofs.

Roof-top splitters, either solid or porous, have been positioned along the approximate

lines of the corner vortices in a direct effort to interfere with the formation and

development of the vortex. Lin and Surry [4,5] found that this is an effective method of

eliminating the formation of the vortices, reducing the corner pressures by 60%.

1.5. Curved and chamfered eaves edges

The effect of rounding the eaves of industrial buildings is known to significantly reducethe worst suction pressures at the leading edge. Full-scale studies by Robertson [10]

recorded a reduction of about 40% in the mean pressure coefficients along a line of taps at

the mid-span (i.e., L/2) of a low-rise building with a curved edge at the eaves. However,

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wind loads behind the ridge of the building are increased by about 30%, possibly because

there is less energy lost at the building edge resulting in higher velocities at the ridge.

Unfortunately, no results are presented for winds approaching the leading corner of the

building where the worst suction pressures are expected to occur. However, Lin and Surry

[5] recorded the effect of rounded edge profiles on the corner pressures, reporting areduction of about 60% in the peak and mean pressure coefficients. Work by Cooper[16]

on rounding the leading edges of truck trailers provides a useful guide to minimum radius

requirements, although this should probably be repeated for low buildings in thick

turbulent boundary layers.

Similar to the rounded edge profile, Blackmore[9]examined the effect of chamfering the

edge profile of a building as a means of preventing the formation of the corner vortices. Six

alternative configurations were examined: three chamfer angles, 30o, 45o and 60o, with two

chamfer widths. Results suggest the largest load reductions of 70% were observed for the

30o chamfer, which was the least steep of the three tested.

1.6. Objectives of the current study

The above considerations suggest that the porous parapet may be the most effective

parapet geometry for mitigating corner suction pressures. Independent experiments by Lin

and Surry [5] and Banks [1] concluded that porous parapets, with about 50% porosity,

reduced the worst loading by around 5070% in the corner region. Saw-tooth parapets

were less effective, reducing the corner pressures by about 3040%. Solid and partial

parapets were the least useful, reducing the loading only if the height of the parapet was

greater than 1.5 m (5 ft) for the building heights they used. When reviewing the literaturewe noted that many of the previous experiments were performed with a limited number of

pressure taps and that little area-averaged information, the type of loading information

designers require, was available. The high-resolution module that we used in Part 1 [13]

allows detailed area-averaged loads to be obtained without the inherent uncertainty of

relying on single point pressures or distributions of point pressures. Thus, the objective of

the current work is to determine the area-averaged (component and cladding) loads in a

corner panel using alternative parapet geometries which have been shown to work, but

which would be possible and practical for industry to implement.

2. Experimental set-up

Testing was conducted in the Boundary Layer Wind Tunnel II at the University of

Western Ontario. The 1:50 scale model used in Parts 1 [13] and 3 [17]of this study, was

used for the current experiments. The building model has equivalent full-scale plan

dimensions of 31.1 m (102 ft) by 46.3 m (152 ft) with a 14

on 12 gable roof slope. Testing was

repeated for the eight configurations listed inTable 2. These were chosen in consultation

with Dr. Lee Shoemaker of the Metal Building Manufacturers Association as to which

types may possibly be used in industry. Note also that the isolated parapet (i.e., a parapet

along a single wall) data, reported in Part 1 [13], are also used herein. About 700 pressure

taps were used on the high resolution module, shown in Fig. 1(a). The high-resolutionmodule was placed in the corner (see Fig. 1b) with its edge the thickness of the parapet

(0.3 m) from the outside of the wall, as in Part 1 [13]. Photographs of the building model

with the slotted parapet and the spoiler are given in Fig. 2.


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Measurements were obtained in a single terrain condition, an open country terrain

zo 0:03 m for a single wind angle (3251), consistent with the worst wind direction forcorner vortices for buildings with solid perimetric parapets of height h=H ho0:17[13].The open country terrain was identical to that in [13,17](although the current data were

obtained 3 years later than the earliest data reported in[13]).

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Table 2

Configurations in the present experiments

Plan dimensions 31:1 46:3 m; 1:48 gable roof slopeEaves height 4.6 m

Model scale 1:50Terrain Open zo 0:03 mWind angles 3251

Geometry of parapets 1. no parapet

2. solid,h 0:9 m, 0.30 m thick, perimetric parapet3. 50% solid screen,h 0:9 m, perimetric parapet4. slotted parapet,h 0:9 m

5. solid parapet ending h from side wall (no corner)

6. as in 5, with 50% solid screen corner (porous corner)

7. solid parapet with raised corner

8. perimetric spoiler


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Pressure measurements were made at a wind tunnel reference speed of14 m/s. Thisleads to an eaves height speed of 8.6 m/s in the open country terrain. The reference location

was at the mid-height of the wind tunnel, just upstream of the model. Each pressure tap

was sampled essentially simultaneously for 120 s at a rate of 400 samples per second and

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0.076 m0.38 m

7.6 m

3.7m

0.0

76m

21@

0.1

52m

Y

X

CORNER

EDGE

EDGE

INTERIOR

0.3

8m

47 @ 0.152 m

Y

X

3.7m

7.6m

31.1m

46.3 m

90

180

270

0

L

W

Corner Edge

13.4m

(b)

(a)

Fig. 1. (a) pressure tap layout for the high-resolution module, and (b) over model dimensions and definition of the

wind angle. Note that only the corner location of the high-resolution model is tested in this part.


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digitally low-pass filtered at 200 Hz. The measurements recorded within the sampling cycle

have a maximum lag time of about 15/16 of the sampling rate, which is15=16 0:002 1:875 ms. This time lag is corrected by linear interpolation of the datawithin the same sample cycle. The resulting pressure time series were then referenced to the

mean dynamic velocity pressure at eaves height, H, using the previously measured velocity

profile, as in[13].

The peak pressure and area-averaged load coefficients presented in the study

are not the absolute worst coefficients recorded within the sample time, but are

Lieblein-fitted statistical peaks. This involves dividing the recorded time series into ten

equal segments and performing the Lieblein BLUE formulation [18] with the

peak values taken from each of 10 segments. The resulting mode and dispersion

of the Type I extreme value distribution were used to determine the mean 12 hour(full scale) peak value for each pressure and load coefficient reported herein.

These are believed to be more statistically stable quantities, than the actual recorded

peaks.

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Fig. 2. (a) Close-up photographs of the high-resolution module with the raised parapet in the corner. Machine

nuts on the floor of the wind tunnel provide roughness right up to the edge of the model. Tubing can be seen

through the clear acrylic roof below the high-resolution module. (b) Photograph of the spoiler at one corner of the

model.


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3. Results and discussion

3.1. Distribution of point pressures

Fig. 3depicts the distribution of the mean,Cp, and root-mean-square1 (rms),Cp0, point

pressure coefficients along the line of taps nearest the boundary between the ASCE 7-02

[19]defined corner, edge and interior zones (see Fig. 1a), i.e.,y=H 0:42, for the eight testconfigurations. Perhaps the most striking observation that can be made is that the different

parapet geometries lead to mean Cp distributions which can be categorized into two

groups. One group has mean distributions which cluster around the data from the solid,

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-2.0

-1.6

-1.2

-0.8

-0.4

0.0

0.0 0.3 0.9 1.2 1.5

x/H

Cp

no parapet

0.9m parapet (perimetric)

spoiler

raised corner

slotted corner

no corner

porous corner

porous perimeter

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.0 0.3 0.6 0.9 1.2 1.5

x/H

Cp'

-6.0

-5.0

-4.0

-3.0

-2.0

-1.0

0.0

0.0 0.3 0.6 0.9 1.2 1.5

x/H

Cp

min

0.0

4.0

8.0

12.0

16.0

20.0

0.0 0.3 0.6 0.9 1.2 1.5

x/H

g

0.6(a)

(c)

(b)

(d)

Fig. 3. (a) mean, (b) rms, (c) minimum pressure coefficients, and (d) peak factor distributions along the line

y=H 0:42 for a wind angle of 3251 for the various parapet geometries.

1Root-mean-square (rms) is used as a synonym for standard deviation in the current work, i.e., it is always

calculated with the mean component removed.


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perimetric parapet with parapet height, h 0:9 m. This group consists of parapets whichcan be viewed as perturbations to the solid, perimetric parapet case, namely, the parapets

with slotted corners, porous corners, raised corners and no corner. It is clear that this

group would not be expected to significantly alter the large-scale wind loading, although

particular features of the local loading near the corner are clearly affected. This will beexamined in detail in the following section. The second group is defined by configurations

having common mean pressure distributions which are clustered away from those in the

first group. The group contains the no parapet, the spoiler and the porous, perimetric

parapet cases. This second group clearly has much lower mean loads associated with it, at

least for this particular line of taps. (It should be noted that no attempt, either here or in

Banks study, has been made to measure the loads on the spoiler itself, but it is likely to be

attracting high lift loads as it re-directs the corner flow, so that its attachment to the roof

needs to be equally considered.)

The categories of behaviour defined by the mean pressure distributions in Fig. 3(a)also

appear to hold for the rms pressure coefficients, Cp0, distributions presented inFig. 3(b).

However, within these two groups, differences in the resulting corner flow become

apparent. The group of five parapets that are perturbations of the solid parapet, the first

group described above, exhibits interesting behaviour. Clearly, the worst configuration is

the solid, perimetric parapet which has elevated rms levels due to a strong and stable

vortex core [13]and its expanded aerial extent on the roof surface. The three geometries

with openings in the corner of the parapet (the slotted corner, porous corner and no

corner) all exhibit similar rms distributions which are consistent with the presence of

corner vortices. The resulting surface pressures are smaller in magnitude than for the solid

parapet, but still larger than the no parapet case within 1Hof the edge. The raised corneralso gives indications that the corner vortex remains, but that it is raised higher from the

surface, much like a taller solid parapet (e.g., see Fig. 6(b) in Part 1). One should note

though that rms levels are significantly higher for the raised parapet, so that one would

expect higher area-averaged loads for this configuration, all else being equal. All of these

geometries then lead to worse peak pressures than the non-parapet case, but they are better

than the solid perimetric parapet case, at least for this line of taps. Thus, it appears that

corner treatments to these parapet configurations will ease the local loading if a parapet is

required, but that for h 0:9 m on a building with H 4:6 m (h=H h 0:17), this isworse than having no parapet. However, analysis of area-averaged loads will be

needed to confirm this. Finally, it is interesting to note that beyond x=H1, Cp andCp0asymptote to the same values for all five configurations in the group, as doesCp0 forh 0.

Thus, the effects of the perturbations to the parapet geometry are truly local, as one

would expect.

For the second group of parapets defined above (i.e., the spoiler, porous perimetric

parapet and no parapet), the classification based on the mean distribution still holds. Fig. 3

clearly indicates that the corner vortices are significantly weakened so that overall loads are

much lower than even the no parapet case, although the typical footprints of the corner

vortices are still observed.

Fig. 3(c), which depicts the minimum pressure coefficients, shows the same

trends. Also, the minima are roughly bounded by the no parapet and solid parapetdata (except for the spoiler and the porous parapet), with a few exceptions. Not

surprisingly, the boundary between the two groups is still clear, although it would

appear that the no parapet case fits much more with the first group. What all of this implies


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is that the peak factor

g Cpmin Cp

Cp0 (1)

is strongly affected by the parapet geometry. This is shown in Fig. 3(d). Corner vortices(for h 0) have a large quiescent zone between them, as can be seen by the low mean

suction for 0:2ox=Ho0:6 inFig. 3(a), with the smallest rms value at x=H 0:3. Thepressure gradients due to the corner vortices are clearly large. Since the vortex cores will

move randomly about a well-defined mean location [1], large suctions will occur in the

quiescent region from time to time due to a vortex core moving slightly or being

strengthened from a large gust. This behaviour, with different flow phenomena passing a

particular point, results in large peak factors (up to 18!) in Fig. 3(d). Solid perimetric

parapets stabilize this phenomenon, most likely because, as the core is further from the

roof surface, the pressure gradient is reduced and a slight movement of the core has

relatively less effect. In the range 0:2ox=Ho0:6, the peak factor is in the range of 4 to 5forh 0:9 m. These values, together with the raised corner, are the lowest observed alongthis line.

3.2. Local, area-averaged loads

Within each zone, a series of area-averaged loading coefficients were developed by

simultaneously combining pressures from a number of surrounding taps. The Lieblein

method, discussed above, was then applied to obtain statistically reliable peak values from

the time series of the area-averaged coefficients. For simplification, a square-shapedgeometry was consistently used in the analysis, ranging in full-scale area from about 0.1 to

5 m2, as listed in Table 3 of [13]. A few rectangular areas were considered for the larger

areas as well. The roof surface was separated into three regions (corner, edge and interior

zones), as defined by the current ASCE 7-02 standard. For the tested building geometry,

this corresponds to an edge zone width of 1.8 m (6 ft). Dimensional areas will be discussed,

since this is the format currently applied in building codes such as the ASCE 7-02, even

though normalizing areas byH2 has been demonstrated to collapse such data, as discussed

in Part 1. The coefficients obtained from this method have been converted to effective GCp

values for direct comparison with the coefficients used in the ASCE 7-02. St. Pierre et al.

[20] have described in detail the procedure we are using for comparing wind tunnel andcode loads. In particular, referring the wind tunnel coefficients to the ASCE coefficients

requires

GCpeq qH

^Cp

q10 m;3 sKztKhKdI FWT ^Cp, (2)

where (GCp)eqis the equivalent wind tunnel pressure coefficient, ^Cp is the peak coefficient

based on the mean (e.g., hourly) wind pressure at the eaves height,qH,q10m,3sis the basic

wind pressure in the ASCE 7-02, Kzt is the topographic factor, Kh converts the pressure

coefficient to roof height (terrain factor), Kd is the directionality factor and I is the

importance factor. All factors were set to the same values as used Part 1 [13].Figs. 46depict the worst area averages obtained in the ASCE 7-02 defined corner, edge

and interior zones, respectively. The coefficients,GCp, in the figures are in a form suitable

for direct comparison with the coefficients in ASCE 7. The conversion of the experimental


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coefficients was done in precisely the same manner as by St. Pierre et al. [20], and in Part 1

[13]; the reader is referred there for details. One major difference between Part 1 and the

present work is that herein, only one wind angle is used, whereas in Part 1, the worst values

ARTICLE IN PRESS

-4

-3.2

-2.4

-1.6

-0.8

0

0.01 0.1 1 10

Area (m2)

GCp

no parapet 0.9m parapet (perimetric)raised parapet no cornerslotted corner porous cornerporous perimeter spoiler0.9 m parapet (isolated) ASCE 7-02 corner

Fig. 4. Variation of worst pressure coefficients with loading area in the corner region of the high-resolution

module.

-3.2

-2.4

-1.6

-0.8

0

0.01 0.1 1 10

Area (m2)

GCp

no parapet 0.9m parapet (perimetric)raised parapet no cornerslotted corner porous cornerporous perimeter spoiler0.9 m parapet (isolated) ASCE 7-02 edge

Fig. 5. Variation of worst pressure coefficients with loading area in the edge region of the high-resolution module.


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from all wind angles considered are presented. Since the actual worst values may come

from other wind angles, the present figures are primarily useful for comparison purposes

between the different parapet configurations. In addition, data from the isolated parapet

configuration, as reported in Part 1, has been included to aid the discussion.

In the corner region (Fig. 4), it is clear that both the spoiler and porous parapet

significantly reduce the local area-averaged loads compared to the no-parapet configura-

tion, as expected from the point pressures presented in Fig. 3. Thus, these devices are useful

in reducing the loads on buildings which would otherwise not have parapets. (The reader is

also reminded, from the results in Part 1, that high, solid parapets with h=H hX0:23,

also reduce the loads.) For the other group of parapets, the corner perturbations to thesolid, perimetric parapet, the results are interesting. In the corner zone, the raised corner is

an effective method for reducing the loads, as compared to the solid (uniform) parapet,

and, for areas less than 1 m2, perhaps even for buildings without parapets (which would

then be similar to a saw-tooth parapet[4,5]). The worst cases are the porous corner and the

no corner configurations which are also higher than the solid parapet. This appears to

occur because the flow behaves somewhat like it would for the case of an isolated parapet,

although the isolated parapet data from [13] indicate that the latter configuration is the

worst case. Thus, one could conclude that ending a parapet before the corner reduces the

corner zone loads, as compared to continuing it all the way to the corner.

Fig. 5shows the results for the edge zone, as defined by the ASCE 7-02. Once again, thespoiler and porous parapet are seen to yield local loads considerably below all other

configurations, including the no-parapet case. For all other configurations, there appears

to be real benefit to modifying the parapet corner geometry, as all data are well below the

ARTICLE IN PRESS

-2

-1.5

-1

-0.5

00.01 0.1 1 10

Area (m2)

GCp

no parapet 0.9 m parapet (perimetric)

raised parapet no corner

slotted corner porous corner

porous perimeter spoiler

0.9 m parapet (isolated) ASCE 7-02 interior

Fig. 6. Variation of worst pressure coefficients with loading area in the interior region of the high-resolution

module.


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solid parapet case. For areas less than 0.5 m2, these configurations are even better than

the no parapet case, somewhat surprisingly. Once again, the isolated parapet is seen to be

the worse case, although for larger areas, the data overlap with the solid parapet data as

corner effects are reduced.

In the interior zone (Fig. 6), the spoiler has the lowest loads, followed by the porousparapet. The no-parapet case is next, but drops more rapidly with area so that for areas of

2 m2, the same values as the porous parapet are obtained. The group of parapets which

have corner geometry modifications, all behave similarly in the interior zone. And, once

again, the isolated parapet configuration is the worst, but with loads approaching the solid

perimetric parapet case for areas 2 m2.

4. Conclusions

In the current study, alternative parapet geometries were studied in order to findeffective means for reducing the significant area-averaged loads associated with corner

vortices. The major conclusions resulting from this work are:

1. Spoilers and porous perimetric parapets are effective means of mitigating roof

suctions in the corner, edge and interior zones as defined by ASCE 7-02. These result in

lower area-averaged component and cladding loads, than not having a parapet at all.

2. Given a choice, raising a solid parapet at the corner will reduce loads in all zones

compared to a solid continuous parapet. A parapet with a slotted corner would also be

beneficial, but less so.

3. Single, isolated parapets (i.e., parapets spanning one complete wall of the building)

are the worst configuration.

4. Leaving the corner open is worse than a continuous solid parapet for small areas.

However, if an isolated parapet is used, ending it before the side wall will reduce loads

compared to an isolated parapet brought all the way to the corner.

Acknowledgements

This project was initiated by the Metal Building Manufacturers Association and the

American Iron and Steel Institute. Their financial support and continued interest is greatly

appreciated. The on-going interest of Dr. Lee Shoemaker is greatly appreciated. One of the

authors (G.A. Kopp) gratefully acknowledges the support of the Canada Research Chairs

Program. The authors also wish to acknowledge the contribution of Ms. L.M. St. Pierre

who performed some of the data analysis.

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