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8/16/2019 Ch-4 Wind Effects
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D!"#$ C%$!"D&A'"%$!D!"#$ C%$!"D&A'"%$!
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 ;.