wind loading to

8/7/2019 WIND LOADING TO

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WIND LOADING TO

ASCE 7-05


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What is Wind?

Wind is the result of horizontal differencesin air pressure

These pressure differences are due touneven heating of the earths surface


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Hurricanes

Features of hurricanes:

High speed sustained winds up to 74 mi/h Typical diameter of 311 miles

Cyclonic effect with direction of spin controlled

by Coriolis effect


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The ingredientsThe ingredients

a hurricanea hurricane

needsneeds

Warm water above 27C

Converging winds

Unstable air

Humid air being pulled intothe storm(up to about18,000 ft)

Pre-existing winds comingfrom nearly the samedirection

An upper atmospherehigh-pressure area helpspump away air rising in the

storm


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SaffirSaffir--Simpson scaleSimpson scale

forhurricane categoriesforhurricane categories

Category Velocity

1 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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Wind loading

In general the pressures derived from wind come

from the dynamic part of Bernoullis equation:

2

21

Vq V!


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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

Different internationalDifferent international

design standardsdesign standards


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Different calculations for designDifferent calculations for design

wind speeds and dynamicwind speeds and dynamic

pressurespressures

AS1170.2-89

BNSCP28

AIJ

DRBC-03

ENV 1991-2-4

CUBiC

ISO 4354

BuildingPressure/Force

PressureSpeedStandard

2ref21

refq V!

V

V

gusts3V

V

fi

rf

q!

zplae,pe qKKKCP !

peqCP !

221

ref Vq V!

221

ref Vq V!

2dztz2

1z IVKKKq V!

2H2

1h Uq V!

2z2

1h Vq V!

232121 SSVSq V!

0,refalttemdirref CCCCV !

REEUU gfgH !

itscat,zz MMMVV !

pihpz GCqGCqp !

peeexprefe CZCqW !

dynCfigexpref CCqW !

AGCqW ffhf !


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General Wind Load Definition

From the Wind Loads for Structural

Design the definition reads:

"The basic wind speed V is the 3-second

gust speed estimated to be exceeded on

the average only once in 50 years ..... at a

height of 10 m above the ground in an

open situation ....."


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Basic Wind Speed (V) to ASCE 7-

05

The basic wind speed corresponds to a 3-s gust speed at 33 ft (10 m) above ground

in exposure Category C


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Wind load considerations

Features of all wind codes, wind loading

affected by:

Averaging period,

Return period,

Height above ground,

Topography and ground roughness.


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Features of ASCE basic wind

speed (V)

Averaging time based on 3 second gust

Return period based on 50 yr return period

H

eight 10m

Exposure Category C


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Vg Z

FOREST, SUBURB

5010

100

300

OPEN SOIL

gZ

Zg = Height wind gradient

400

V

500

1/7

ZZ

1/9.6

EXPOSURE B - EXPOSURE C -

Effects of terrainEffects of terrainroughness androughness andheight on windheight on wind

speedsspeeds


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Sketch showing effects ofSketch showing effects oftopography on wind velocity ontopography on wind velocity ona hilly islanda 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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Procedures used in ASCE 7-05

Basic Procedure Analytical Procedure

Wind Tunnel Procedure


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Limitations of Basic & Analytical

P

rocedures Factors not taken into consideration:

Across wind loading as a result of vortex

shedding or instability due to galloping and

fluttering

Does not have a site location for which

channeling effects or buffeting in the wake of

upwind obstructions warrant specialconsideration


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Across Wind Loading

Vortex shedding


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Turbulent flow on highTurbulent flow on high--riserisebuildings due to upwindbuildings due to upwindobstructionsobstructions


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Other limitations of Basic

P

rocedure

Simple diaphragm building Low Rise


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Enclosure Classification of

Buildings to ASCE 7-05

Open

Partially enclosed

Enclosed- That which does not fall into the

first 2 classifications


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Features of both Methods

The wind loads are calculated for:

1) MWFRS- Mean Wind Force ResistingSystem a main feature of which receives

wind loading from more than one surface

2) Components and cladding (C&C):elements that do not meet MWFRS

definition


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Basic ProcedureBasic Procedure


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NotationNotation FactorFactor What does it mean?What does it mean?

Directionality

Takes into account the probability that themaximum wind has the same direction asthat of the maximum pressure

Importance IFactor based on type of structure andbuilding use

ExposureRepresents the wind velocity at a z heightabove the ground

Topography

Takes into account the fact that thestructure may be located on top of a hill oron an escarpment, increasing the windvelocity

dK

zK

ztK

Meaning of factors inMeaning of factors in

ASCEASCE--77


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NotationNotation FactorFactor What does it mean?What does it mean?

Gust effect factor GRepresents the turbulence-structureinteraction and the dynamicamplification of the wind

External pressurecoefficient

Estimates the wind pressure on thebuilding, external walls

Internal pressurecoefficient

Reflects the internal pressure due towall opening uantity and sizes

Design pressure p Represents the design pressure

Design force FRepresents the net force on openstructures

pC

piC

Meaning of factors inMeaning of factors in

ASCEASCE--77


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2dztz2

1 IVKKKq V!

Modified basic pressureModified basic pressure--

ASCEASCE--77

Modified basic pressure inModified basic pressure in

ASCEASCE--7 to accommodate local7 to accommodate local

parametersparameters


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Analytical ProcedureAnalytical Procedure

Rigid primary systemsRigid primary systems

Flexible primary systemsFlexible primary systems

p = q GCp - qi (GCpi)

p = qGfCp - qi (GCpi)


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Analytical Procedure contdAnalytical Procedure contd

Components and claddingComponents and cladding

p = qh [(GCp) - (GCpi) ]


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Design pressure diagramDesign pressure diagram

on gabled roof buildingon gabled roof building

U

h

L

z

GCpqz

qGCph

q GCphq GCph

Wind

direction


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THE END

wind loading to

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