Ch-4 Wind Effects

8/16/2019 Ch-4 Wind Effects

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In designing for wind, a building cannot be considered

independent of its surroundings because configuration of

nearby buildings and natural terrain has substantial

influence on the design loads, and hence on the sway

response of the building. Sway is defined as the horizontal

displacement at the top of a building.

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The sway at the top of a tall building caused by wind may not be

seen by a passerby, but may be of concern to those experiencing

wind-motion problems at the top floors.

There is scant evidence that winds, except those due to a

tornado or hurricane, have caused major structural damage to

buildings.

evertheless, it is prudent to investigate wind-related behaviour

of modern s!yscrapers, typically built using lightweight curtain

walls, dry partitions, and high-strength materials, because theyare more prone to wind-motion problems than the early

s!yscrapers, which had the weight advantage of heavy masonry

partitions, stone facades, and massive structural members.

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"ll buildings sway during windstorms, but the motion in old tall

buildings with heavy full-height partitions has usually been

imperceptible and, therefore, has not been a cause for concern.

Structural innovations coupled with lightweight construction

have reduced the stiffness, mass, and damping characteristics of

modern buildings.

In these buildings, objects may vibrate, doors and chandeliers

may swing, pictures may lean, and boo!s may fall off shelves.

"dditionally if the building has a twisting action, its occupants

may get an illusory sense that the world outside is moving,

creating symptoms of vertigo and disorientation.

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$A'*&A+ "$D$A'*&A+ "$Dind is not constant either with height or time, is not

uniform over the windward side of the building, and does

not always cause positive pressure.

In fact, wind is a complicated phenomenon it is air in

turbulent flow, which means that motion of individual

particles is so erratic that in studying wind, one ought to

be concerned with statistical distributions of speeds and

directions rather than with simple averages.

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ind is the term used for air in motion and is usually

applied to the natural horizontal motion of the

atmosphere.

/otion in a vertical or nearly vertical direction is called

current.

/ovement of air near the surface of the earth is 01, with

horizontal motion much greater than the vertical motion.

The wind-tunnel testing provides information regarding

the response of buildings subject to different wind speed

and direction.

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'P! % "$D 'P! % "$Dinds that are of interest in the design of buildings can be

classified into three major types2

3revailing winds.

Seasonal winds.

4ocal winds.

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P&A"+"$# "$D!P&A"+"$# "$D!Surface air moving towards the low-pressure e5uatorial

belt is called prevailing wind or trade wind.

In the northern hemisphere, the northerly wind blowing

toward the e5uator is deflected by the rotation of the earth

to a northeasterly direction, and hence commonly !nown

as the northeast trade wind.

The corresponding wind in the southern hemisphere is the

southeast trade wind.

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!A!%$A+ "$D!!A!%$A+ "$D!"ir over the land is warmer in summer and colder in

winter than the air adjacent to oceans during the same

seasons.

1uring summer, the continents become seats of low

pressure, with wind blowing in from the colder oceans.

In winter, the continents experience high pressure with

winds directed winds directed toward the warmer oceans.

These moments of air caused by variations in pressure

difference are called seasonal winds.

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The monsoons of the +hina Sea and the Indian *cean

are example of these movements of air.

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+%CA+ "$D!+%CA+ "$D!These are associated with the regional weather patterns

and include whirlwind and thunderstorms.

They are caused by daily changes in temperature and

pressure, generating local effects in winds.

The daily variations in temperature and pressure may

occur over irregular terrain, causing valley and mountain

breezes.

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CA&AC'&"!'"C! %CA&AC'&"!'"C! %

"$D"$Dind flow is complex because numerous flow situations

arise from the interaction of wind with structures.

+haracteristics of wind as following2

6ariation of wind velocity with height &velocity profile(

ind turbulence

Statistical probability

6ortex shedding

1ynamics nature of wind-structure interaction

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"$D '*$$+!"$D '*$$+!ind tunnels such as those shown in figures ) and fig. 0

are used, among other things, to provide accurate

distributions of wind pressure on buildings as well as

investigate aeroelastic behavior of slender and light

weight structures.

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Ser!i"e# $ro!ided b% a &ind t'nnel "on#'ltant

t%$i"all% offer the follo&ing benefit#

3rovides an accurate distribution of wind loads, especially

for structures in a built-up environment by determining

directly the impact of surrounding structures.

3rovides predictions of wind-induced building motions

li!ely to be experienced by occupants of the top floors,

and compares the test results to available serviceability

criteria.

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3retest estimate of cladding pressures and overall loads by

a wind engineer, based on a review of similar buildings,

with appropriate consideration of the local meteorologicaldata can help the engineer, the architect, and the faced

engineer to develop a preliminary foundation design and

initial cost estimate for the curtain wall.

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The projected area of the modeled building and its

surroundings is less than 7 % of test section cross-

sectional area unless correction is made for bloc!age.

The longitudinal pressure gradient in the wind-tunnel test

section is accounted for.

8eynolds number effects on pressure and forces are

minimized.

8esponse characteristics of the wind-tunnel

instrumentation are consistent with the re5uired

measurements.

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&"#"D 4%D+ !'*D"!&"#"D 4%D+ !'*D"!The primary purpose of the rigid model test is for

obtaining cladding design pressure, the data ac5uired from

the wind-tunnel tests may be integrated to provide floor

by floor shear forces for design of the overall /98S

&/ain ind 9orce 8esisting System(, provided there is

sufficient distribution of pressure taps.

/ost commonly, pressure study models are made from

methyl methacrylate commonly !nown as 3lexiglas,

4ucite and 3erspex.

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This material has several advantages over wooden or

aluminium alloy models because it can be easily and

accurately machined and drilled and is transparent,

facilitating observation of the instrumentation inside the

model .

It can also be formed into curved shapes by heating the

material to about $'':+.

The model is typically instrumented with as many as ;''-

<'' pressure taps. It includes detailed topography of

nearby surroundings within a radius of =;< m.

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The wind tunnel test is run for a duration of about >'s

which corresponds to approximately )h in real time.

Typically measurements are ta!en for wind direction of

)': increments, sufficient numbers of readings are

gathered from each port to offset the effect of time

dependent fluctuations. The measured pressures are

divided by a reference pressure measured in the wind

tunnel.

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&"#"D 4%D+ '!'-%&A++&"#"D 4%D+ '!'-%&A++

B*"+D"$# +%AD!B*"+D"$# +%AD!8igid model test results are primarily used to predict wind

loads for design of glass and other cladding elements, they

can nevertheless be integrated to provide lateral loads for

the design of the /98S.

The procedure entails combining wind load information

with the building response characteristics using random

vibration theory.

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In spit of the fact that rigid-model wind study does not

ta!e into account may of the factors typically considered

is an aeroelastic study, it is still considered ade5uate to

provide design data for buildings with height-to-width

ratio of less than ;.

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Ch-4 Wind Effects