wind Load

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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.

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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 persaffir-Simpson scale

Category Velocity1 minute

(km/hr)

Pressure(mb)

Damages

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

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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.

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• 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

- 0.7

- 0.6

- 0.6

- 0.6

0.4

0.40.40.30.3

0.3

0.6

0.5

0.7

- 0.5

- 0.5

- 0.6

- 0.5

0.8

0.9

- 0.6

- 0.6

- 0.5

- 0.6

SIDE FRONT BACK WIN

D

WIND

ROOF

Pressure varies

withheight

(Wind ward)

Pressurekeeps constantwith height(Leeward)

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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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1.0

0.5

0 10 20 30 40 50 60 70 80 90

Angle of attack - degrees

x

x

x

N-S

E-W

x

10U

3

2 x

h

Y(N-S)

X(E-W)

wind

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

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

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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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Static Analyses of wind force

• General steps

• Design wind speed at height z (Vz)

• Wind pressure at height (pz)=0.6 Vz2

• Design wind pressure (pd)

• Design wind force (F)

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Comparisons

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28

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

10 m

80Vs

100Vg

60

gV100

g

120sV

Vs

V

100gV

40

100

Open sea Wind ward Speed up over Sheltered leeward

coast

Speed up

Coast hill crest

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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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39

40

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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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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

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Different calculations for design wind speeds and dynamic pressures

AS1170.2-89

BNSCP28

AIJ

DRBC-03

ENV 1991-2-4

CUBiC

ISO 4354

Building Pressure/Force

PressureSpeedStandard

2ref2

1ref Vq

V

V

gusts3V

V

dynfigexpref CCCqW

zplae,pe qKKKCP

peqCP

221

ref Vq

221

ref Vq

2dztz2

1z IVKKKq

2H2

1h Uq

2z2

1h Vq

232121 SSVSq

0,refalttemdirref CCCCV

REEUU gfgH

itscat,zz MMMVV

pihpz GCqGCqp

peeexprefe CZCqW

dynCfigexpref CCqW

AGCqW ffhf

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Building types in seven international wind standards

Building Shape/Type

ISO 4354

CUBiC ENV 1991

DRBC2003

AIJ AS1170.2

BNS 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

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Protection effect of building

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

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Unfavorable location of buildings adjacent to a hospital

A bad location of nearby buildings might induce increase of wind loads

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

hospitals should be avoided

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Thank you