5/22/2012 1 DR MD. JAHIDUL ISLAM e-mail: [email protected]Web: https://sites.google.com/a/mail.presidency.edu.bd/ce/faculty/jim Lecture # 03 Wind Load Calculation Wind Loads (W): are in the form of pressure or suction on the exterior surfaces of the building. They cause horizontal lateral loads (forces) on the structure. Wind loads also cause uplift of light roof systems. 2
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Wind Loads (W): are in the form of pressure or suction on the exterior surfaces of the building. They cause horizontal lateral loads (forces) on the structure. Wind loads also cause uplift of light roof systems.
The wind tunnel procedure consists of developing a small-scale model of the building and testing it in a wind tunnel to determine the expected wind pressure etc. It is expensive and may be utilized for difficult or special situations.
The analytical procedure is used in most design offices. It is fairly systematic but somewhat complicated to account for the various situations that can occur.
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According to BNBC (1993)
Basic wind speed: This is the maximum of wind speed (or velocity) to be used for specific location. It is based on recorded wind histories and adjusted for some statistical likely hood of occurrence.
Wind stagnation pressure / Sustained wind pressure (qz): This is the basic reference equivalent static pressure based on the critical local wind speed.
Design wind pressure (pz): This is the equivalent static pressure to be applied normal to the exterior surfaces of the building.
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Exposure: This refers to the conditions of the terrain
surrounding the building site. The terrain exposure in which a
building or structure is to be sited shall be assessed as being one
Figure 3: Wind direction, windward side & leeward side
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Windward side (may be thrust or suction)
Leeward side
(always suction)
Direction of wind
Direction of wind
Figure 4: Wind direction and pressure distribution on windward side & leeward side
Leeward side
(always suction)
Windward side (may be thrust or suction)
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Wind velocity will cause pressure on any surface in its path. The wind velocity and hence the velocity pressure depend on the height from the ground level. Following equation is recommended by BNBC for calculating the velocity pressure.
Where,
Vb = basic wind velocity in km/h
Cc = Velocity-to-pressure conversion co-efficient = 47.2x10-6.
CI = Structure importance factor (see table 3),
Cz = Combined height and exposure co-efficient (see table 4), varies with height z above the ground level.
For exposure category A, you can use the table or alternatively you
can use the following equation of combined height and exposure co-
efficient
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2b
Vz
CI
Cc
Cz
q
368.00.44350.1879(z)z
C
The velocity pressure qz is used to calculate the design pressure (pz) for the building structure as follows:
Where, CG = Gust response co-efficient (see table 5)
Cpe = External pressure co-efficient (see table 6)
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zq
peC
GC
zp
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11
L
B
Plan
Wind direction
θ
L
Wind directionz
h
Elevation
Figure 9: Rectangular building with sloped roof (plan, elevation & pressure distribution)
zq
peC
GC
zp
zq
peC
GC
zp
hq
peC
GC
zp
hq
peC
GC
zp
q = qz for the windward wall – varies with height z.
q = qh for the leeward wall.
qh is qz evaluated at z = h (mean height of building).
qh is constant.
qi = qh for windward, leeward, side walls and roofs.
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Table 2: Basic Wind Speeds (Vb) for selected locations in
Bangladesh
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LocationBasic Wind
Speed (Km/h)Location
Basic Wind
Speed (Km/h)
Angarpota 150 Lalmonirhat 204
Bagerhat 252 Madaripur 220
Bandarban 200 Magura 208
Barguna 260 Manikganj 185
Barisal 256 Meherpur 185
Bhola 225 Moheshkhali 260
Bogra 198 Moulvibazar 168
Brahmanbaria 180 Munshiganj 184
Chandpur 160 Mymensingh 217
Chapai
Nawabganj 130 Naogaon 175
Table 2: Basic Wind Speeds (Vb) for selected locations in
Bangladesh
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Dhaka 210 Nilphamari 140
Dinajpur 130 Noakhali 184
Faridpur 202 Pabna 202
Feni 205 Panchagar 130
Gaibandha 210 Patuakhali 260
Gazipur 215 Pirojpur 260
Gopalgonj 242 Rajbari 188
Habiganj 172 Rajshahi 155
Hatiya 260 Rangamati 180
Ishurdi 225 Rangpur 209
Joypurhat 180 Sakkhira 183
Jamalpur 180 Shariatpur 198
Jessore 205 Sherpur 200
Jhalakati 260 Sirajganj 160
Jhenaidah 208 Srimangal 160
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Table 3: Structure Importance Co-efficient (CI)
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Structure Importance CategoryStructure Importance
Coefficient (CI)
I. Essential facilities 1.25
II. Hazardous facilities 1.25
III. Special occupancy structures 1.00
IV. Standard occupancy structures 1.00
V. Low-risk structures 0.80
Table 4: Combined Height and Exposure Co-efficient (CZ)
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Height above
ground level, z
(meters)
Coefficient, CZ
Exposure A Exposure B Exposure C
0-4.5 0.368 0.801 1.196
6.0 0.415 0.866 1.2636
9.0 0.497 0.972 1.37
12.0 0.565 1.055 1.451
15.0 0.624 1.125 1.517
18.0 0.677 1.185 1.573
21.0 0.725 1.238 1.623
24.0 0.769 1.286 1.667
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Table 5: Gust Response Factor, Gh and Gz
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Height above
ground level,
(meters)
Gh and Gz
Exposure A Exposure B Exposure C
0-4.5 1.654 1.321 1.154
6.0 1.592 1.294 1.14
9.0 1.511 1.258 1.121
12.0 1.457 1.233 1.107
15.0 1.418 1.215 1.097
18.0 1.388 1.201 1.089
21.0 1.363 1.189 1.082
24.0 1.342 1.178 1.077
Table 6: External Pressure Coefficients, Cpe for Roof
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Win
d
Dir
ecti
on Windward Side
Leeward
Sideh/L
θ (degrees)
00 100-150 200 300 400 500 >600
No
rmal
to
rid
ge <0.3
-0.7 0.2 0.2 0.3 0.4 0.5 0.01×θ
-0.7 for
all values
of h/L
and θ
-0.9 0.01×θ
0.5 -0.7 -0.9 -0.75 -0.2 0.3 0.5 0.01×θ
1 -0.7 -0.9 -0.75 -0.2 0.3 0.5 0.01×θ
>1.5 -0.7 -0.9 -0.9 -0.9 -0.35 0.2 0.01×θ
Par
alle
l to
rid
ge
h/B or
h/L <2.5-0.7 -0.7 -0.7 -0.7 -0.7 -0.7 -0.7 -0.7
h/B or
h/L>2.5-0.8 -0.8 -0.8 -0.8 -0.8 -0.8 -0.8 -0.8
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12/Pratt truss (Roof)
6 @ 6.667/ = 40/
3 @ 3.333/ = 10/
GL
= 5.1829 m
Mean height of roof
Elevation
/172
/10/12h
Wind direction
L = 40 feet = 12.1951 m
= span of truss
B = 25 feet
= 7.6219 m
= bayB
rick
wal
l Brick w
all
Truss
Truss
Plan
Truss location: Dhaka
Vb= Basic wind speed in km/h = 210 km/h (see table 2)
B = Horizontal dimension of the building, in meters measured
normal to wind direction = bay distance (truss-to-truss spacing) =
25 feet = 7.6219 meter.
L = Horizontal dimension of the building, in meters measured
parallel to wind direction = span of truss = 40 feet = 12.1951
meter.
H = average/mean height of the roof in meters = 17 feet = 5.1829
meter.
z = Height above the ground in meters
θ = Angle of the plane of roof from horizontal in degrees