Wind Load

Introduction •Wind is moving air. The air has a particular mass and moves in a particular direction at a particular direction at a particular velocity. It thus has kinetic energy of the from expressed as, E=1/2mv2 •The radiation effects are mainly responsible for Convection current either upwards or downwards. •It is generally blows horizontal to the ground at high speeds. 1

CHARACTERISTIC OF WIND • • • • •

Variation of wind velocity with height Turbulent nature of wind Probabilistic approach Vortex shedding Phenomenon Dynamic nature of wind structure interaction

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Classification wind as per speed

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Wind classification as per saffir-Simpson scale Category

Velocity 1 minute

Pressure

Damages

(mb)

(km/hr)

1

120 - 150

> 980

Minimum

2

150 – 175

965 – 980

Moderate

3

175 – 210

945 - 965

Extensive

4

210 – 250

920 - 945

Extreme

5

> 250

< 920

Catastrophic 4

Relation of wind velocity to pressure on a stationary object.

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How wind force governing for tall structure? • For tall structure more than 15 stories ,with innovations in architectural treatment, increases in strength of materials and advance in method of analysis, tall building structures have become more efficient and lighter and consequently more prone to deflect and even to sway under wind loading. 6

• If decrease cross sec. property of structural element with increase height of building • Construction cost per unit area decrease • Increasing lightness in weight per unit area • More danger against high velocity of wind force at high level

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Typical flow of wind for tall structure-Turbulent flow of wind on longitudinal and transverse sides

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Turbulent flow on tall structure due to upwind obstructions

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Wind velocity increase due to large openings at lower floors

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Different types of forces acting on structural element

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Pressure coefficient on tall structure - 0.6

- 0.5

- 0.6 - 0.6

- 0.6

Pressure keeps constant with height (Leeward)

- 0.6

ROOF

0.9 0.8 - 0.5

WIND

0.7 - 0.6 - 0.6

0.6 - 0.5 - 0.5

0.5 - 0.6 - 0.7

Pressure varies with height (Wind ward)

0.4

0.3

0.4 SIDE

0.3 0.3

FRONT

0.4 BACK

D IN W

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Effect of wind load on tall structure • Overturning due to wind overturning moment because of larger drift.

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Example • Building toppled other side due to more overturning moment.

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When there is a lack of symmetry among resisting elements, wind will be induce more torsional effect

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Example of failure of structure due to torsional effect

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Analyses of wind force • Wind Tunnel Experimental Method for static and dynamic effect • Detailed Analytical Method as per code IS:875-Part-3,Draft IS:875-Part-3. for static and dynamic effect.

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OBJECTIVE OF WIND TUNNEL TEST • Dynamic Response • Drag, Vortex shedding and wind separation from building surface. • To decide building configurations (shape)

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Wind Tunnel test • World Trade Center – New York 1973-2001

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•Empire State Building - full-scale and wind-tunnel studies in 1930’s N-S

1.0

∆

Uh

3

x 2 1 0 Y (N-S)

0.5

wind E-W

α

x x

x

X (E-W) 0

10x

20

30

40

50

60

70

80

90

Angle of attack - degrees

∆ - Mean deflection (inches) Uh - Mean wind speed at 381m in MPH (uncorrected) 20

Wind Analyses by code IS:875 (Part-3) •Static Analyses by regular &draft code •Dynamic Analyses (Gust factor method) by draft code

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23

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Static Analyses of wind force • General steps • Design wind speed at height z (Vz) 2 • Wind pressure at height (pz)=0.6 Vz

• Design wind pressure (pd) • Design wind force (F)

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Comparisons

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27

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Selection of k2 Factor • As per regular code • Class-A,B and C • Terrain category

• As per draft code • Terrain category • Terrain roughness and height factor • Fetch and developed Height

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Sketch showing effects of topography on wind velocity on a hilly island Vg

100

Speed up Vs Vg

10 m

Vs

100

Vg

80

Open sea

120

100

Vg

60

Wind ward Coast

100

Vs 40

Speed up over hill crest

Sheltered leeward coast

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Terrain category-1

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Terrain category-2

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Terrain category-3

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Terrain category-4

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Boundary Layer Profile for Diff. Approach Terrain

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36

37

38

39

40

41

42

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Dynamic Analyses by Gust Factor method

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Codel criteria

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Fundamental Time pereod

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Design Pressure for Dynamic Analyses

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48

49

50

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Excel worksheets • Static Analyses by Regular code • Static and dynamic Analyses by draft code

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Different International standards Standard

Identification

ISO

International Standard Organization

CUBiC

Caribbean Uniform Building Code

ENV

Eurocode

DRBC

Dominican Republic Building Code

AIJ

Japan Standard

AS

Australian Standard

BNSCP

Barbados Standard 53

Different calculations for design wind speeds and dynamic pressures Standard

Speed

Pressure

Building Pressure/Force

ISO 4354

V

q ref =12 ρV2

W = ( qref ) ( Cexp )( C fig )( C dyn )

CUBiC

V

q ref =12 ρV2

W = q ref Cexp Cfig C dyn

ENV 1991-2-4

Vref = CdirC temCaltC ref ,0

q ref =12 ρ( Vref ) 2

We = q ref Cexp ( Ze ) C pe

DRBC-03

V( 3s − gust)

q z = 12 ρ K z K zt K d IV 2

p = q z GCp − q h GCpi

AIJ

UH = UgEf EgR

q h = 12 ρU H2

Wf = q h Cf G f A

AS1170.2-89

Vz = Vz,cat M s M t M i q h =1 ρVz2 2

BNSCP28

V

(

(

)( )

)

(

Pe = C p,e K a K l K pq z

q =12 ρ( VS 1S2S3 ) 2 P = qCpe 54

)

Building types in seven international wind standards Building Shape/Type

ISO 4354

CUBiC

ENV 1991

DRBC 2003

AIJ AS1170. BNS 2 CP28

Stepped Roofs

no

no

no

yes

no

no

yes

Free-standing walls

yes

yes

yes

yes

no

yes

no

Multispan canopies

no

no

yes

yes

no

no

no

Arched roofs

yes

yes

yes

yes

yes

yes

yes

Domes

no

no

yes

no

yes

no

no

Silos and tanks

yes

yes

yes

yes

no

yes

no

Circular sections

yes

yes

yes

yes

yes

yes

yes

Polygonal sections

no

no

yes

no

no

yes

yes

Lattice towers

yes

yes

yes

yes

no

yes

yes

Spheres

no

yes

yes

no

no

no

yes

Signs

yes

yes

yes

yes

yes

yes

yes 55

Protection effect of building A favorable location of adjacent buildings can decrease the hurricane effects by reducing the wind loads

56

Unfavorable location of buildings adjacent to a hospital A bad location of nearby buildings might induce increase of wind loads

57

Flat-slab systems without capitals present little resistance against lateral forces. Their use on hospitals should be avoided

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Thank you

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