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DR MD. JAHIDUL ISLAMe-mail: jahiduli@mail.presidency.edu.bd

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.

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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

of the following categories:

Exposure A: Urban and sub-urban areas, industrial area, wooded areas, hilly

or other terrain covering at least 20% of the area with obstructions of 6

meters or more in height and extending from the site at least 500 meters or

10 times the height of the structure, whichever is greater.

Exposure B: Open terrain with scattered obstructions having heights

generally less than 10 meter extending 800 meter or more from the site in

any full quadrant. This category includes air fields, open park lands, sparsely

built-up outskirts of towns, flat open country and grasslands.

Exposure C: Flat and un-obstructed open terrain, coastal areas and riversides

facing large bodies of water, over 1.5 km or more in width. Exposure C

extends inland from the shoreline 400 meter or 10 times the height of the

structure, whichever is greater.

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Figure 1: Effect of

physical location on

wind exposure

Figure 2: Influence of

size of the structure

on design wind speed.

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Windward side: Direction of wind is called windward side. May

be thrust (compression) or suction / uplift pressure (tension).

Leeward side: Opposite to the direction of wind is called

leeward side. Always suction / uplift pressure (tension).

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Windward side Leeward side

Direction of wind

Leeward side Windward side

Direction of wind

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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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

=tan-1 (10/20) = 26.56510 (degree)

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Cc= Velocity-to-pressure conversion co-efficient = 47.2x10-6.

CI= Structure importance co-efficient (a factor that accounts for

the degree of hazard to hazard to human life and damage to

property) = 1.00 for standard occupancy structures (see table 3).

Cz= Combined height and exposure co-efficient = 0.3897 for

exposure A (see table 4).

*For exposure category A, you can use the table 4 or alternatively

you can use the following equation of combined height and

exposure co-efficient

z = height above ground in meters

Here, z = h = 5.1829 meter (17 feet)

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368.00.44350.1879(z)z

C

0.368)( 0.38970.4435829)0.1879(5.1z

C

CG = Gust response co-efficient = 1.6257 exposure A (see table 5)

Cpe = External pressure co-efficient = -0.3889 for windward side,

wind direction normal to ridge (see table 6)

Cpe = External pressure co-efficient = -0.70 for leeward side,

wind direction normal to ridge (see table 6)

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qz = Sustained wind pressure in kN/m2,

qz = (4.72x10-6) × (1) × (0.3897) × (210)2

qz = 0.81137 kN/m2

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2b

Vz

CI

Cc

Cz

q

pz = Design wind pressure in kN/m2,

Design wind pressure for windward side:

pz = (1.6257) × (-0.3889) × (0.81137) = -0.513 kN/m2 = -10.71 psf

(suction)

Design wind pressure for leeward side:

pz = (1.6257) × (-0.70) × (0.81137) = 0.923330946 kN/m2 = -

19.26419306 psf (suction)

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zq

peC

GC

zp

1 ksf = 47.89 kN/m2

1 kN/m2 = 20.88 psf

1 MPa = 1 MN/m2 = 1 N/mm2

= 145 psi

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Leeward side

(-19.2642 psf)

Direction of wind

(left to right)

Figure: Wind direction and pressure distribution on windward side & leeward side

Leeward side

(-19.2642 psf)

Windward side

(-10.71 psf)

Windward side

(-10.71 psf)

210 km/h

210 km/h

Direction of wind

(right to left)