2-1. Introduction to Wind Design

8/17/2019 2-1. Introduction to Wind Design

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INTRODUCTION TO WIND DESIGNDr. Henry LUK


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Wind Load on Structures

• Wind loads represent the most critical kinds of loads in the

design of a typical high-rise building in Hong Kong.

• The taller a building gets, the more the wind loads become the

controlling factor in the design.

• Wind pressure on a building surface depends many factors:

• E.g. velocity , the shape and surface texture of the building, the protection

from wind offered by surrounding natural terrain or man-made

structures and the density of air.

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Effects of Wind

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Bungale S.T. (2010). Reinforced Concrete Design of Tall Buildings. Taylor &Francis Group, LLC


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Wind-induced Motions

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• Wind-induced building motion can essentially be divided into

three types:

1. Along-wind motion

2. Across-wind motion

3. Torsional motion

• In tall buildings, the across-wind

and torsional motions are usually

more important.


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5

Translation

Translation + Twisting

Symmetric building Asymmetric building


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Nature of Wind

• Wind is the term used for air in motion and it is usually applied

to the natural horizontal motion of the atmosphere.

• Winds are produced by pressure differences in the atmosphere and

rotation of the earth.

• Air flowing over the earth’s surface is slowed down and madeturbulent by the roughness of the surface.

• Flow of wind unlike that of other fluids, is not steady and

fluctuates in a random fashion. The sudden variation in wind

speed, called gustiness or turbulence, is an important factor indetermining dynamic response of tall buildings.

• Because of this random nature, wind loads for building design are studied

statistically.

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Characteristics of Wind

• Wind flow is complex because numerous flow situations arise

from the interaction of wind with structures.

• In wind engineering, simplifications are made to arrive at the

design wind loads :

• Variation of wind velocity with height (velocity profile);

• Wind turbulence;

• Statistical probability;

• Vortex shedding;

• Dynamic nature of wind-structure interaction;

• Cladding pressure.

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Wind Velocity Profile

• The wind speed profile depends mainly on the degree of

surface roughness, caused by the overall drag effect of

buildings, trees, and other projections.

• The zone of wind turbulence due to surface roughness is often

refereed to as surface boundary layer .

• How wind effects are felt within this zone, where human construction

activity occurs, is of concern in building .

• The height at which the slowdown effect ceases to exist is

called gradient height , and the corresponding velocity, gradient velocity .

• At height of approximately 500 m above the ground, the wind

speed is virtually unaffected by surface friction.

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9

/1)/( g g z z z V V

where

V z is the mean wind speed at height z ;

V g is the gradient wind speed;

z is the height above-ground;

z g is the height of boundary layer;

α is the power law coefficient.

2

V p


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Wind Turbulence

• Air has a very low viscosity. Any movement of air at speeds

greater than 0.9 – 1.3 m/s is turbulent, causing particles of air

to move randomly in all directions (turbulent).

• The wind speeds can be decomposed into two components

• Quasi-steady mean wind speed that increase with height;

• Turbulent speed (Gust wind speed) remains the same over height.

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)(~)( t uut u

Wind speeds

)(~)( t p pt P t

Total pressure


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Probabilistic Approach

• The speed of wind is considered to be a function of the

duration of recurrence interval, i.e. return period .

• A 50 year return-period wind of 30 m/s means that on the

average, we will experience a wind faster than 30 m/s within a

period of 50 years.

• A return period of 50 years corresponds to a probability of

occurrence of 1/50 = 0.02 = 2% per year.

• Consider a building designed for a 50 year service life. The

probability of exceeding the design wind speed 30 m/s is

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%6464.036.01)02.01(1 50 P


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Vortex Shedding

• Along wind is used to refer to drag forces, while transverse

wind is the term used to describe cross-wind.

• In tall building design, the cross-wind motion is often more

critical than along-wind motion.

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• The originally parallel upwind streamlines are displaced on either side of

the building. This results in spiral vortices being shed periodically from the

sides into the downstream flow of wind, called the wake.

• When the vortices are shed, that is, break away from the surface of the

building, an impulse is applied in the transverse direction. At higher speeds,

vortices are shed alternately on both sides of building, causing vibration of

structures in the transverse direction.

• This phenomenon is called Vortex Shedding.

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• There is a simple formula to calculate the frequency of the transverse

pulsating forces caused by vortex shedding:

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• When the wind speed is such that the shedding frequency becomes

approximately the same as the natural frequency of the building, a

resonance condition is created.

• Further increases in wind speed by a few percent will not change the

shedding frequency, because the shedding is now controlled by the naturalfrequency of the structure.

• The vortex-shedding frequency has, so to speak, locked in with the natural

frequency.

D

S V f

where

f = frequency of vortex shedding (in Hz)

V = mean wind speed at the top of the building

S = a dimensionless parameter for the shape

D = diameter of the building


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Dynamic Nature of Wind

• Wind loads associated with gustiness or turbulence change

rapidly, creating effects much larger than if the same loads

were applied gradually.

• The action of a wind gust depends not only on how long it

takes the gust to reach its maximum intensity and decrease

again, but on the period of the building itself.

• If the wind gust reaches its maximum value and vanishes in a time much

shorter than the period of the building, its effects are dynamic .

•

On the other hand, the gusts can be considered as static loads if the windload increases and vanishes in a time much longer than the period for the

building.

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Cladding Pressures

• When air flows around a structure, the resulting pressures may

damage the local components such corner windows, eave and

ridge tiles, etc.

• The expense of replacement and hazards posed to pedestrians

is of major concern.

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1. Positive pressure zone on the

upstream face (Region 1)

2. Negative pressure zones at the

upstream corners (Region 2)3. Negative pressure zone on the

downstream face (Region 3).

The net load on cladding is the difference

between the external and internal pressures.


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CODE OF PRACTICE ON WIND

EFFECTS IN HONG KONG

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Code of Practice on Wind Effects

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Scope

• This code of practice gives general methods for calculating the

wind loads to be used in the structural design of buildings.

• The code does not apply to buildings of an unusual shape or

buildings situated at locations where complicated local

topography adversely affects the wind conditions.

• Experimental wind tunnel data may be used in place of the values given

in the Code.

• The design wind pressures have been determined from the

hourly mean wind velocities and peak gust wind velocitieshaving a return period of 50 years.

• Appendix B in wind code provides mortification factor for design wind

pressure with return period greater than 50 years.

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Calculation of Wind Loads

• Two conditions:

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• The building is considered to be one with significant resonant

dynamic response if it has either of the following properties:

1. The height exceeds five times the least horizontal dimension.2. The height of the building is greater than 100 m.

• Unless it could be justified that the fundamental natural

frequency of the building is greater than 1 Hz.

Without significant resonant dynamic response

With significant resonant dynamic response

Section 3

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• Breadth (b) means the horizontal dimension of the building normal to the direction of wind.

•Depth (d) means the horizontal dimension of the building parallel to the direction of thewind.

• Height (H) means the height of the building above the site-ground level in the immediate

vicinity of the building up to the top of the building. Masts and other appendages on top of

the building should not be include.

• Frontal projected area means the area of the shadow projection of the building on a plane

normal to the direction of the wind.

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Method 1: without significant resonant dynamic response

1. f n > 1 Hz; or2. H


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Design Wind Pressure

• For building without

significant resonant dynamic

response, the design wind

pressure qz at height z shall

be taken as the value given

in Table 1.

• Topography effect (see

Appendix C).

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Section 4

(open sea terrain)

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Forces on Buildings

• The total wind force F on a building shall be taken to be the

summation of the pressures acting on the effective projected

areas of the building

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z z f AqC F

where

C f is the force coefficient for the building (Appendix D);

qz is the design wind pressure at height z (Table 1);

Az is the effective projected area of that part of the building

corresponding to qz.

• Every building shall be designed for the effects of wind

pressures acting along each of the critical directions.

Section 5

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Appendix D

• The force coefficient C f for an enclosed building is given as

• where the height aspect factor C h and the shape factor C s given in Table

D1 and Table D2 respectively; or

• International Codes acceptable to the Building Authority may be used.

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sh f C C C

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• If the frontal projected area is greater than 500 m2, the force

coefficient may be multiplied by a reduction factor RA given in

Table D3 .

• This is applicable for structures without significant resonant

dynamic response.

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• The force coefficient C f for an open framework building shall

be the value given in Table D4; or the appropriate value

specified in other International Codes acceptable to the

Building Authority.

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Example 1

Determine the design wind pressure distribution along the

height of building. Compute the base shear and overturning

moment. Assume that the building is sitting on a smooth surface

(open sea terrain).

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10

20

40

Along wind direction

Unit: m

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Solution

Checking for resonant effect

30

1020

40

Along wind

direction

5410/40/

m100m40

d H

H

Without significant resonant

dynamic response (Clause 3.3)

Topography effect

Insignificant

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Compute design wind pressure

qz = 1.82 kPa

qz = 2.01 kPa

qz = 2.23 kPa

qz = 2.37 kPa

qz = 2.57 kPa

5m

10m

20m

30m

40m

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Height/Breadth aspect factor

220/40/ b H

0.1h

C

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Shape factor

210/20/ d b

1.1 s

C

Force coefficient factor

1.1

1.10.1

sh f C C C

Frontal projected area = 10 x 40 = 400 m2 < 500 m2

Hence, reduction factor RA = 1.0

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Determine base shear and moment

Fz1 = 182 kN

Fz2 = 201 kN

Fz3 = 446 kN

Fz4 = 474 kN

Fz5 = 514 kN

5m

10m

20m

30m

40mBase shear force

Overturning moment

kN1999

)182201446474514(1.1

z z f AqC F

kNm42342

)5.21825.7201154462547435514(1.1

z z z f z AqC M

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Dynamic Effects

• For building with significant resonant dynamic response, the

total along-wind force F on an enclosed building with

significant resonant dynamic response shall be determined by

z z f AqGC F

where

G is the dynamic magnification factor, or gust factor (Appendix F)

C f is the force coefficient for the structure (Appendix D);

is the design hourly mean wind pressure at height

z (Table 2);

Az is the effective projective area.

Section 7

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Appendix F

• The dynamic magnification factor G may be taken as the values

from Table F1 or Table F2, or by

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• Alternatively, the dynamic magnification factor G may be taken

as follows

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• Special cases

• an open framed building with significant resonant dynamic response; or

• a building for which the fundamental natural frequency is less than 0.2

Hz, or

• the cross wind resonant response / torsional resonant response may be

significant.

• The dynamic effects should be investigated in accordance withrecommendations given in published literature and/or through

dynamic wind tunnel model studies.

• The combination total response of such a building would

usually be calculated from the of the response in the threefundamental modes of vibration.

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Wind Tunnels

• Wind tunnels are used to provide accurate distributions of

wind pressure on buildings as well as investigate aero-elastic

behaviour of slender and light weight structures.

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• Services provided by a wind tunnel consultant typically offer the

following benefits:

• Provides an accurate distribution of wind loads, especially for structures in a

built-up environment by determining directly the impact of surrounding

structures.

• Provides predictions of wind-induced building motions (accelerations and

torsional velocities) likely to be experienced by occupants of the top floors,

and compares the test results to available serviceability criteria.

• Estimates cladding pressures and overall loads which can help the engineer,

the architect, and the facade engineer to develop a preliminary foundation

design and initial cost estimate for the curtain wall.

• Provides an assessment of expected pedestrian wind comfort along with any

conceptual recommendations for improvement to key pedestrian areas.• The overall design wind loads are generally (not always) lower than code

wind loads resulting in lower cost.

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• In determining the effects of wind for a particular building,

there are two main components to consider.

• The first comprises the aerodynamic characteristics of the building.

These are simply the effects of the wind when it blows from various

directions.

• This climatological information, in the form of a probability distribution

of wind speed and direction, is the second main ingredient needed fordetermining wind effects for a particular development.

• Appendix A in wind code states the necessary provisions for

wind tunnel testing.

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• Types of wind-tunnel test

• Rigid pressure model (PM)

• Obtains cladding design pressures, storey shear forces and base shear and

overturning moment.

• Rigid high-frequency base balance model (HFBB/HFFB)

• Determine the effects of wind load on a flexible building with the

consideration of mean wind, fluctuating wind and inertia effect.

• Aero-elastic model (AM)

• Investigate the instabilities of structure or capture the resonant behaviour of

building.

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Rigid pressure model

Rigid high-frequency base balance model

Aero-elastic model

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Appendix B

• The design wind pressures on buildings where the period of

exposure to wind is longer than 50 years shall be multiplied by

the following factor:

where R is the period of exposure to wind in years.

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Appendix C

• Wind code states that local

topography is considered

significant for a site located

within the topography

significant zone as defined

in Figure C1.

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• The relative dimensions of

the topography are defined

in Figure C2.

• For shallow upwind slopes

0.05 < αu < 0.3

• For steep upwind slopes αu

> 0.3

ue

ue L L

3.0e

3.0/ H Le

whereαe = effective slope

Le = effective slope length

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• The design wind pressure at height z shall be multiplied by a

topography factor S a

at that height.

• The topography factor S a at height z above site ground level

shall be determined by

2)2.11( sS ea

where

αe = effective slope

s = a topography location factor (Figure C3 for hills and ridges, Figure C4 for

cliffs and escarpments)

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Forces on Elements

• The total wind force F p acting in a direction normal to the

individual elements such as walls, roofs, cladding panels or

members of open framework structures shall be determined by

m z p p

AqC F

where

C p is the total pressure coefficient for individual elements (Appendix E);

qz is the design wind pressure at height z ;

Am is the surface area of the element.

• Except for members of open framework structures, the design

wind pressure shall be a constant value over the lower part of

the building.

Section 6

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Appendix E

• The total pressure coefficient C p for individual elements in a

particular area of an enclosed building:

• where there is only a negligible probability of a dominant opening

occurring during a severe storm, the value given in Table E1; and

•

where a dominant opening is likely to occur during a severe storm, thevalue determined with the aid of other published materials acceptable to

the Building Authority or through the use of wind tunnel model studies.

• The total pressure coefficient C p for individual elements of an

open framework building shall be

• 2.0; or

• appropriate value specified in other International Codes acceptable to

Building Authority.

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References

• Bungale S. Taranath (2004). Wind and Earthquake Resistant Buildings:

Structural Analysis and Design. CRC Press, Taylor & Francis Group.

• Bungale S. Taranath (2010). Reinforced Concrete Design of Tall Buildings.

CRC Press, Taylor & Francis Group.

• Hong Kong Buildings Department. (2004). Code of Practice on Wind Effects

in Hong Kong 2004.

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Example 2

Determine the base shear and overturning moment.

H = 60 m

h = 3 m

40 m

30 m

Wind direction

LU = 500 m LD = 1000 m

x = 50 m

HU = 100 m

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Solution

Checking for resonant effect

5230/60/

m100m60

d H

H Without significant resonant

dynamic response (Clause 3.3)

Topography effect

Significant

upwind slop =100

500= 0.2

significant zone Case (a) hill & ridge

downwind slop =100

1000= 0.1

0.5 × slope length = 0.5 × 500 = 250 m

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Topography factor (Figure C2)

For case Hill & ridge and 0.05 < = 0.2 < 0.3

Effective slope = = 0.2

Effective slope length = = 500 m

Topography factor

= (1 + 1.2)

A function of z/Le and x /LD

obtained in Figure C4

Modified Design wind pressure

′ = ×

=

50

1000= 0.05

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Compute design wind pressure

Height z

(m)

Pressure

(kPa)

Moment

arm

(m)

z/Le s Sa Modified

pressure

(kPa)

5 1.82 2.5 0.01 1.0 1.54 2.8010 2.01 7.5 0.02 1.0 1.54 3.09

20 2.23 15 0.03 1.0 1.54 3.43

30 2.37 25 0.05 0.9 1.48 3.50

50 2.57 40 0.08 0.85 1.45 3.73

60 2.73 55 0.11 0.8 1.42 3.88

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Determine force coefficient factor

5.140/60/ b H

0.1h

C

1. Height/breadth aspect factor

2. Shape factor

33.130/40/ d b

1.1 s

C

3. Force coefficient factor

1.1 sh f C C C

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Force distribution

Height

(m)

Modified

pressure

(kPa)

Force

(kN)

Moment

(kNm)

5 2.80 616 1539

10 3.09 680 509920 3.43 1509 22630

30 3.50 1542 38549

50 3.73 3278 131138

60 3.88 1707 93871

= ( × × )

Breadth = 40 m

Projected area = 40 x 60 = 2400 m2

Reduction factor = 0.94

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Determine base shear and moment

Base shear force

Overturning moment

kN9331

z a z f AS qC F

kNm292826

z z a z f z AS qC M

kN8772

z a z f A AS qC R F

kNm275257

z z a z f A z AS qC R M

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Further Question

If the building height is now modified to 100 m with damping

ratio of 2%, dynamic resonant effect should be considered.

• Dynamic magnification factor (Table F1 or F2)

For RC building, H = 100 m, b = 40 m 808.1G

• Force coefficient factor

5.240/100/ b H 03.1 hC

1. Height/breadth aspect factor

2. Shape factor

33.130/40/ d b 1.1 sC

13.1 sh f C C C

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Design hourly mean wind pressure (Table 2)

Height

(m)

Pressure

(kPa)

Sa Modified

pressure

(kPa)

Force

(kN)

Moment

(kNm)

5 0.77 1.54 1.19 483 1207

10 0.90 1.54 1.39 594 423120 1.05 1.54 1.62 1316 19747

30 1.15 1.48 1.70 1387 34664

50 1.28 1.45 1.86 3026 121040

75 1.40 1.42 1.99 4055 253440

100 1.49 1.36 2.03 4144 362573

′ = ×

Base shear force

Overturning moment

kN14975 z a z f AS qGC F

kNm796902

z z a z f

z AS qGC M

Cannot apply

reduction factor !!

′ = × × ′ ×

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Summary

• (Method 1) Without significantresonant dynamic response

1. Calculate design wind

pressure (Table 1)

2. Determine topography factor

Sa (Appendix C)

3. Calculate force coefficients Cf

(Appendix D)

4. Calculate total wind force

• (Method 2) With significantresonant dynamic response

1. Calculate design hourly mean

wind pressure (Table 2)

2. Compute gust response factor

G (Appendix F)

3. Determine topography factor

Sa (Appendix C)

4. Calculate force coefficients Cf

(Appendix D)

5. Calculate total wind force z z f AqC F

z z f AqGC F

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Remark 1

Centre of the frontal

projected area

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Remark 2 Storey height = 3 m

2.80 kPa

3.09 kPa

kN370

)40)(3(8.21.11

F

kN402

)40)](5.2(09.3)5.0(8.2[1.12

F

kN4.415

)40)](5.0(43.3)5.2(09.3[1.13

F

3.43 kPa Storey force

Documents

2-1. Introduction to Wind Design