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WIND LOADS ON GABLE TYPE CANOPY ROOF
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“Recent Developments in Structural Engineering (RDSE-2007)”, Department of Civil Engineering, Manipal Institute of Technology, Manipal, India,
30, 31st August and 1st September 2007.
WIND LOADS ON GABLE TYPE CANOPY ROOF
A. K. Roy1, A. K. Ahuja2 and V. K. Gupta3
1Research Scholar, Deptt. of Civil Engineering, I. I. T. R., Roorkee, India, [email protected] 2Associate Professor, Deptt. of Civil Engineering, I. I. T. R., Roorkee, India, [email protected]
3Professor, Deptt. of Civil Engineering, I. I. T. R., Roorkee, India, [email protected]
Abstract
Low-rise buildings are designed against wind using pressure coefficients on wall and roof surfaces as available in relevant code of practices. In case of canopy roofs very limited information is available in code of practices. The present paper describes the efforts taken to collect more information in this area. Wind pressure distribution on 150 mm x 300 mm gable type canopy roof was determined in the wind tunnel, in a simulated, terrain category 2 (as per IS:875-1987 [3]) boundary layer flow at a length scale of 1/40. Gable type canopy roof models made of perspex sheets tested in the closed-circuit wind tunnel having cross section 1.3 m (width) x 0.85 m (height). Mean, R.M.S, Maximum and Minimum pressures has been measured on the surfaces of the model for 7 wind directions namely 00, 150, 300, 450, 600, 750
and 900 in order to study the influence of wind incidence angle. Contour plots of local point pressures indicate that the largest pressures are experienced close to the leading edges and the ridge line of both the upper and lower surface of the roof at a wind orientation of 30°, 60° and 75°. The largest area averaged pressure coefficients are obtained on the leading sections of both the upper and lower surface of the roof at a wind orientation of between 30° and 75°. Area averaged pressures on rectangular strips in regions close to the leading edges and ridge line experience pressures substantially larger than those on rest of the roof.
Keywords : Canopies, Pitch (inclination), Wind tunnel tests, Different wind incidence angle, Pressure distribution, Contours.
INTRODUCTION
Calculation of occasional loads like wind loads on structures require information on design wind speed and pressure or force coefficients, which can be obtained from the relevant codes of practice or literature to the extent it is available [AS/NZS: 1170.2 (2002) [1], ASCE:7-02 (2002) [2], IS:875 (part-3)-(1987) [3], NBC (Part-4) (1995) [4]].
According to Indian code of practices for design wind loads for building and structures IS: 875 (Part-3)-(1987) [3], the pressure coefficients (Cp) for gable type canopy roof are available only for few parameters. For different wind incidence angle the pressure coefficients are not available. Further, available information does not include wind pressure coefficients (Cp) on corners where high pressure or suction is expected especially in case of skew angles. Gumley (1984) [5] carried out a comprehensive study of wind pressures on various types of canopy-roofed structures. However, available information is yet not sufficient for the designer
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“Recent Developments in Structural Engineering (RDSE-2007)”, Department of Civil Engineering, Manipal Institute of Technology, Manipal, India,
30, 31st August and 1st September 2007.
to design a canopy roof for wind loads. Therefore, an effort has been made in the present study to investigate experimentally the distribution of wind loads acting on canopy roof for different wind incidence angle.
EXPERIMENTAL PROGRAM
The experiment was conducted on the gable type canopy roofed building model in the closed-circuit wind tunnel at IIT Roorkee. For the benefit of the readers, a brief introduction of the experimental setup is given next.
Details of Test Models
In the present study full-scale building with gable type canopy roof is taken as length=12m, width=6m and height=3m. The experimental study is conducted by making 1:40 scale models of canopy-roof building using perspex sheets with pressure points on upper as well as lower surfaces. Dimensions of the model are 300 mm (length) × 150 mm (width) × 75 mm (height) [Fig. 1(a)].
The corner pressure taps were densely arranged close to the short roof edge as it is shown in Fig. 1(b).
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5
5
P1 P2 P3
P4 P5 P6
“Recent Developments in Structural Engineering (RDSE-2007)”, Department of Civil Engineering, Manipal Institute of Technology, Manipal, India,
30, 31st August and 1st September 2007.
Fig. 1. Schematic Diagram of (a) Typical Experimental Model and (b) Pressure Tap Layout.Flow Characteristics
The model is tested in boundary layer flow corresponding to terrain category 2 as per IS: 875 (part-3)-(1987) [3] in the closed-circuit wind tunnel having cross section 1.3 m (width) x 0.85 m (height) [Fig. 2].
(a) (b)
Fig. 2. Wind Tunnel Model of Gable Roof. (a) Schematic Diagram Showing Model Placed at Test Section in Wind Tunnel and (b) Setup in Wind Tunnel.
Mean, R.M.S, Maximum and Minimum pressures has been measured on both the upper and lower surfaces of the models for 7 wind directions namely 00, 150, 300, 450, 600, 750 and 900
in order to study the influence of wind incidence angle.
The pressure taping installation of the experimental gable type roof model is shown in Fig. 3. In this figure P1, P2 and P3 are the pressure tapping point on the upper surface of the roof model and P4, P5 and P6 are the pressure taping points on the lower surface of the model.
Fig. 3. Pressure Tubes Arrangements in Roof Section.
RESULTS AND DISCUSSION
Systematic measurements were conducted at the experimental model. The collected data then used to calculate Mean, R.M.S, Maximum and Minimum pressures.
The mean pressure contours superimposed on the typical structural framing patterns on both the upper and lower surface of the roof for different wind directions are shown in Fig. 4 to 7. It is observed that largest pressures are experienced close to the leading edges and the ridge line of both the upper and lower surface of the roof at a wind orientation of 30°, 60° and 75°. This is also noticeable in that figures of contour plots of local point pressures that the ‘largest area averaged’ pressure coefficients are obtained on the leading sections of both the upper and lower surface of the roof at a wind orientation of between 30° and 75°. Rectangular strips in
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Model
Ref. Pitot TubeFlow
Grid
3.5 m 2.6 m500
6.1 m
Flow
“Recent Developments in Structural Engineering (RDSE-2007)”, Department of Civil Engineering, Manipal Institute of Technology, Manipal, India,
30, 31st August and 1st September 2007.
regions close to the leading edges and ridge line experience pressures substantially larger than those on rest of the roof.
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80 100 120 140
F a ce B
10 30 50 70
W id th (m m )
10
30
50
70
90
110
130
150
170
190
210
230
250
270
290
Len
gth
(mm
)
F a ce A
Wind 0° to Ridge
(a)
1 0 3 0 5 0 7 0
W id th (m m )
1 0
3 0
5 0
7 0
9 0
11 0
1 3 0
1 5 0
1 7 0
1 9 0
2 1 0
2 3 0
2 5 0
2 7 0
2 9 0
Len
gth
(m
m)
F a ce A
8 0 1 0 0 1 2 0 1 4 0
F a ce B
Wind 0° to Ridge
(b)
80 100 120 140
F a ce B
1 0 3 0 5 0 7 0
W id th (m m )
10
30
50
70
90
110
130
150
170
190
210
230
250
270
290
Len
gth
(m
m)
F a ce A
Wind Direction
(a)
8 0 1 0 0 1 2 0 1 4 010 30 50 70
W id th (m m )
1 0
3 0
5 0
7 0
9 0
11 0
1 3 0
1 5 0
1 7 0
1 9 0
2 1 0
2 3 0
2 5 0
2 7 0
2 9 0
Len
gth
(m
m)
F a ce A
Wind Direction
(b)
“Recent Developments in Structural Engineering (RDSE-2007)”, Department of Civil Engineering, Manipal Institute of Technology, Manipal, India,
30, 31st August and 1st September 2007.
Fig. 4. Mean Pressure Coefficient Contours of the Gable Roof on the (a) Upper and (b) Lower Surface for Wind Flow along 0° to Ridge and on the (c) Upper and (d) Lower Surface for Wind Flow along 15° to Ridge.
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1 0 3 0 5 0 7 0
W id th (m m )
1 0
3 0
5 0
7 0
9 0
11 0
1 3 0
1 5 0
1 7 0
1 9 0
2 1 0
2 3 0
2 5 0
2 7 0
2 9 0
Len
gth
(m
m)
F a ce A
80 100 120 140
F a ce B
Wind direction
(a)
8 0 1 0 0 1 2 0 1 4 0
F a ce B
1 0 3 0 5 0 7 0
Width (mm)
1 0
3 0
5 0
7 0
9 0
11 0
1 3 0
1 5 0
1 7 0
1 9 0
2 1 0
2 3 0
2 5 0
2 7 0
2 9 0
Len
gth
(m
m)
F a ce A
Wind Direction
(b)
10 30 50 70
W id th (m m )
1 0
3 0
5 0
7 0
9 0
11 0
1 3 0
1 5 0
1 7 0
1 9 0
2 1 0
2 3 0
2 5 0
2 7 0
2 9 0
Len
gth
(m
m)
F a ce A
8 0 1 0 0 1 2 0 1 4 0
F a ce B
Wind Direction
(a)
8 0 1 0 0 1 2 0 1 4 0
F a ce B
1 0 3 0 5 0 7 0
W id th (m m )
1 0
3 0
5 0
7 0
9 0
11 0
1 3 0
1 5 0
1 7 0
1 9 0
2 1 0
2 3 0
2 5 0
2 7 0
2 9 0
Len
tgth
(m
m) F a ce A
Wind Direction
(b)
“Recent Developments in Structural Engineering (RDSE-2007)”, Department of Civil Engineering, Manipal Institute of Technology, Manipal, India,
30, 31st August and 1st September 2007.
Fig. 5. Mean Pressure Coefficient Contours of the Gable Roof on the (a) Upper and (b) Lower Surface for Wind Flow along 30° to Ridge and on the (c) Upper and (d) Lower Surface for Wind Flow along 45° to Ridge.
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(a)
10 30 50 70
W id th (m m )
1 0
3 0
5 0
7 0
9 0
11 0
1 3 0
1 5 0
1 7 0
1 9 0
2 1 0
2 3 0
2 5 0
2 7 0
2 9 0
Len
gth
(m
m)
F a ce A
8 0 1 0 0 1 2 0 1 4 0
F a ce B
Wind Direction
8 0 1 0 0 1 2 0 1 4 0
F a ce B
1 0 3 0 5 0 7 0
W id th (m m )
1 0
3 0
5 0
7 0
9 0
11 0
1 3 0
1 5 0
1 7 0
1 9 0
2 1 0
2 3 0
2 5 0
2 7 0
2 9 0
Len
gth
(m
m)
F a ce A
Wind Direction
(b)
10 30 50 70
W id th (m m )
10
30
50
70
90
110
130
150
170
190
210
230
250
270
290
Len
gth
(m
m)
F a ce A
80 100 120 140
F a ce B
Wind Direction
(a)
8 0 1 0 0 1 2 0 1 4 0
F a ce B
1 0 3 0 5 0 7 0
W id th (m m )
1 0
3 0
5 0
7 0
9 0
11 0
1 3 0
1 5 0
1 7 0
1 9 0
2 1 0
2 3 0
2 5 0
2 7 0
2 9 0
Len
gth
(m
m)
F a ce A
Wind Direction
(b)
“Recent Developments in Structural Engineering (RDSE-2007)”, Department of Civil Engineering, Manipal Institute of Technology, Manipal, India,
30, 31st August and 1st September 2007.
Fig. 6. Mean Pressure Coefficient Contours of the Gable Roof on the (a) Upper and (b) Lower Surface for Wind Flow along 60° to Ridge and on the (c) Upper and (d) Lower Surface for Wind Flow along 75° to Ridge.
Fig. 7. Mean Pressure Coefficient Contours on the (a) Upper and (b) Lower Surface of the Gable Roof for Wind Flow along 90° to Ridge.
CONCLUSIONS
The effect of gable type canopy-roof construction in extreme wind environments is significant in mitigating wind damage.
The following conclusions are drawn from the study reported in this paper.
(i) Rectangular strips in regions close to the leading edges and ridge line experience pressures substantially larger than those on rest of the roof.
(ii) Largest pressures are experienced close to the leading edges and the ridge line of both the upper and lower surface of the roof at a wind orientation between 30° and 75°.
(iii) The largest ‘area averaged pressure coefficients’ are obtained on the leading sections of both the upper and lower surface of the roof at a wind orientation of between 30° and 75°.
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80 100 120 140
F ace B
1 0 3 0 5 0 7 0
W id th (m m )
1 0
3 0
5 0
7 0
9 0
11 0
1 3 0
1 5 0
1 7 0
1 9 0
2 1 0
2 3 0
2 5 0
2 7 0
2 9 0
Len
gth
(m
m)
F a ce A
Wind 90°to Ridge
(a) (b)
8 0 1 0 0 1 2 0 1 4 0
F a ce B
1 0 3 0 5 0 7 0
W id th (m m )
1 0
3 0
5 0
7 0
9 0
11 0
1 3 0
1 5 0
1 7 0
1 9 0
2 1 0
2 3 0
2 5 0
2 7 0
2 9 0
Len
gth
(m
m)
F a ce A
Wind 90°to Ridge
“Recent Developments in Structural Engineering (RDSE-2007)”, Department of Civil Engineering, Manipal Institute of Technology, Manipal, India,
30, 31st August and 1st September 2007.
(iv) Pressure coefficients for gable type canopy-roof should be incorporated in future revisions of wind loading codes so that designers may directly utilize the amplitude reductions and spatial distributions that improve the survival of gable type canopy-roofed structures.
REFERENCES
[1] AS/NZS: 1170.2, Structure Design Actions, Part-2: Wind Actions, Standard Australia/Standards New Zealand, Sydney, Wellington, 2002.
[2] ASCE: 7-02, Minimum Design Loads for Buildings and other Structure, Str. Engg. Institute of the American Society of Civil Engineers, Reston, 2002.
[3] IS: 875 (part-3), Code of practice for the design loads (other than Earthquake) for buildings and structures, B. I. S. New Delhi, India, 1987.
[4] NBC (Part-4), Structural Commentaries, National Research Council of Canada, Canada, 1995.
[5] S. J. Gumley, “A parametric study of extreme pressure for the static design of canopy structures”, J. of Wind Engg. and Ind. Aerodyn., 1984, pp. 43-56.
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