Download Wind Induced Motion Of Tall Buildings

International Conference on Science & Technology for Sustainable Development (ICSTSD) - 2016
Wind Induced Motion Of Tall Buildings
Author : PRANAV S. BALKI
Civil Engineering
Prof. Ram Meghe Institute Of Technology & Research
Badnera, Maharashtra ( India )
Abstract- Tall and super-tall buildings are going up all over the
world, notably in east and south Asia and middle east. Advances
in materials, structural design, and wind engineering ensure that
these buildings meet strength and safety requirements. Modern
buildings designed such that their lateral drifts under statically
applied wind loads are less than some fraction of building height ,
may vibrate excessively during winds and cause occupant
discomfort. Methods are presented for evaluation the vibration
characteristics of buildings using random vibration theory to
relate the fluctuating wind forces to structural response. These
methods can be used to evaluate serviceability or to plan wind
tunnel tests of building. In order to reduce the effect of vibration
or motion, various design concepts are adopted.
I. INTRODUCTION
As today population in the metro cities is increasing
day by day and space required for their residence and work
place is less i.e. why we have to increase the space not
horizontally but vertically by constructing this type of sky
scrapers as in the case of Dubai, New York, Mumbai,
Hyderabad, and Delhi
The past few decades have witnessed a tremendous
growth of tall buildings all over the world, particularly in east
and south Asia and Middle East, which is evidently shown in
the recent statistics published by the Council on Tall Buildings
and Urban Habitat. In the years 2011, 2012 and 2013 recorded
as many as 81, 69, and 73 high rise buildings taller than 200m
built around the world. The projected number is expected to
escalate in 2014 and 2015 (Safarik and Wood, 2014). This
increasing trend can be attributed to population growth,
shortage of land, and the consequent increase in land prices
especially in metropolitan areas. Besides that, many
developing countries boost their countries’ reputations as
financial powerhouses and tourist destinations by building
high rise structures such as the Burj Khalifa in Dubai,
Shanghai World Financial Center in China, Taipei 101 in
Taiwan, and the Petronas Twin Towers in Malaysia (Council
on Tall Buildings and Urban Habitat, 2014). With the
increasing trend of tall buildings, it is important to continually
develop more accurate methods for structural design of tall
buildings as the collapse of such buildings can lead to
catastrophic consequences with a loss of many human lives.
The strength and serviceability of tall buildings are governed
by lateral loads either due to either wind or earthquake loads.
The focus of this research will be on dynamic wind action as it
is commonly the governing load case for tall and slender
buildings. The performance of tall buildings under the action
of wind is a relatively new area of study in the history of
structural engineering and is becoming a growing concern.
Although advances in engineering materials, structural design
and knowledge of wind structure interaction ensure that these
buildings meet strength and safety requirements under wind
action and response to wind induced motion of buildings.
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Wind design normally governs the design of tall buildings
around the world. These new structures are less dense than
their predecessors, with greater flexibility and less inherent
damping. As a result, these buildings are more
susceptible to wind-induced motion and designing to resist
wind is becoming an area of great consequence.
An important characteristic of the wind is that it is
unsteady or fluctuating in Nature; the wind can be considered
to consist of a mean and a fluctuating component. The
unsteady wind velocity gives rise to pressures on the structure,
a mean pressure which varies with height and a localized
fluctuating pressure. The unsteady wind pressures transmit
fluctuating forces or loads into the structure.
II. WIND AND CAUSES OF WIND
Wind is air in motion. It is produced by the
uneven heating of the earth’s surface by the sun. Since the
earth’s surface is made of various land and water formations,
it absorbs the sun’s radiation unevenly. Two factors are
necessary to specify wind i.e. speed and direction. As the sun
warms the Earth's surface, the atmosphere warms too. Some
parts of the Earth receive direct rays from the sun all year and
are always warm. Other places receive indirect rays, so the
climate is colder. Warm air, which weighs less than cold air,
rises. Then cool air moves in and replaces the rising warm air.
This movement of air is what makes the wind blow. Wind is
cause by air flowing from high pressure to low pressure. Since
the earth is rotating, however the air does not flow directly
from high to low pressure , but it is deflected to the right (
from northern to the left in the southern hemisphere ), so that
the wind flows mostly around the high and low pressure areas.
Winds that are of interest in the design of
buildings can be classified into three major types :
1) Prevailing Winds (Trade Winds)
Prevailing winds are winds that blow
predominantly from a single general direction over a particular
point on the Earth's surface. The dominant winds are the trends in
direction of wind with the highest speed over a particular point on
the Earth's surface. A region's prevailing and dominant winds are
often affected by global patterns of movement in the Earth's
atmosphere
2) Seasonal Winds
Seasonal winds are movements of air
repetitively and predictably driven by changes in largescale weather patterns. Seasonal winds occur in many
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locations throughout the world. The name assigned to a
particular seasonal wind and the underlying physical forces
that drive the winds depend upon the unique geographic
location. One of the most commonly recognized seasonal
winds are the monsoon winds. Although monsoons are often
identified as rainstorms, they are actually a seasonal wind. A
monsoon is a wind in low-latitude climates that seasonally
changes direction between winter and summer.
3) Local Winds
Local winds are small scale convective winds
of local origin caused by temperature differences. Local
terrain has a very strong influence on local winds, and the
more varied the terrain, the greater the influence. Some ways
in which local winds develop are convection from daytime
heating, unequal heating and cooling of the surface, gravity,
including downdrafts.
III. WIND SPEED
Wind speed is affected by a number of
factors and situations, operating on varying scales. These
include the pressure gradient. There are also links to be found
between wind speed and wind direction, notably with the
pressure gradient and surfaces over which the air is found.
Pressure gradient is a term to describe the difference in air
pressure between two points in the atmosphere or on the
surface of the Earth. It is vital to wind speed, because the
greater the difference in pressure, the faster the wind flows
(from the high to low pressure) to balance out the variation.
The pressure gradient, when combined with the Coriolis
effect and friction, also influences wind direction. At great
heights above the surface of the earth, where frictional effects
are negligible, air movements are driven by pressure gradients
in the atmosphere, which in turn are the thermodynamic
consequences of variable solar heating of the earth. This upper
level wind speed is known as the Gradient Wind Velocity.
IV. WHAT IS HIGH RISE BUILDING
The Empiric Standards Committee defines
a high-rise building as a multi-story structure between 35–100
meters tall, or a building of unknown height from 12–39
floors and a skyscraper as a multi-story building whose
architectural height is at least 100 m or 330 ft. Some structural
engineers define a high rise as any vertical construction for which
wind is a more significant load factor than earthquake or weight.
Note that this criterion fits not only high-rises but some other tall
structures, such as towers. A building whose height creates
different conditions in the design, construction, and use than
those that exist in common buildings of a certain region and
period. Part of a building or anything affixed thereto or any wall
enclosing or intended to enclose any land or space and signs and
outdoor display structure whose architectural height is between
35 & 100 meters is known as a High Rise Building. According to
the building code of Hyderabad, India, a high-rise building is one
with four floors or more, or one 15 meters or more in height.
V. RELATION BETWEEN WIND SPEED AND HEIGHT
OF BUILDING
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In common usage, wind gradient, more
specifically wind speed gradient or wind velocity gradient, or
alternatively shear wind, is the vertical gradient of the mean
horizontal wind speed
in
the
lower atmosphere. It
is the rate of increase
of wind strength with
unit increase in height
above ground level.
Surface friction forces
the
surface wind to
slow and turn near the
surface of the Earth,
blowing
directly
towards
the
low
pressure,
when
compared
to
the
winds in the nearly
frictionless flow well above the Earth's surface. This layer, where
surface friction slows the wind and changes the wind direction,
Typically, due to aerodynamic drag, there is a wind gradient in
the wind flow just a few hundred meters above the Earth's
surface—the surface layer of the planetary boundary layer.
Wind speed increases with increasing height above the
ground, starting from zero due to the no-slip condition. Flow near
the surface encounters obstacles that reduce the wind speed, and
introduce random vertical and horizontal velocity components at
right angles to the main direction of flow. This turbulence causes
vertical mixing between the air moving horizontally at one level,
and the air at those levels immediately above and below it. The
reduction in velocity near the surface is a function of surface
roughness, so wind velocity profiles are quite different for
different terrain types. Rough, irregular ground, and man-made
obstructions on the ground, retard movement of the air near the
surface, reducing wind velocity. Because of low surface
roughness on the relatively smooth water surface, wind speeds do
not increase as much with height above sea level as they do on
land. Over a city or rough terrain, the wind gradient effect could
cause a reduction of 40% to 50% of the geotropic wind speed
aloft; while over open water or ice, the reduction may be only
20% to 30%. The design of buildings must account for wind
loads, and these are affected by wind gradient. Near the earth’s
surface, the motion is opposed, and the wind speed reduced, by
the surface friction. At the surface, the wind speed reduces to zero
and then begins to increase with the height and at some height
known as Gradient Height, the motion may be considered to be
free of the earth’s frictional influence and will attain its Gradient
Velocity. As shown in the figure, due to negligible obstruction
elevation of wind is low on flat, unobstructed area and water
surface than that of urban and suburban terrain which has more
elevation of wind due to high obstruction.
VI. WIND INDUCED BUILDING VIBRATION
On especially tall buildings such as, where height is
significantly greater than width or breadth of the building,
the effect of the wind on the structure is an important source
of the building vibration.
Vibration Susceptibility
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Buildings can be classified into two categories with regards
to wind-induced vibration, vibration sensitive (flexible) and
vibration insensitive (rigid). Classification of buildings into
these two types is governed by the relationship between
height , width and stiffness-generally most buildings above
50m in height are deemed to be vibration sensitive. Aero
elastic phenomena are very complex to model, but their
effects can be described quite simply. The following is a list
of effects likely to be found in regards to tall buildings.
Wind excitation can cause vibration through several
different mechanisms.
A. Gust actions in the wind direction
The pseudo-periodic nature of gusting wind can cause
turbulence at the bluff face of a structure. This in turn causes
vibration of the building. Wind forces or pressure is one of
the most important loading condition regarding tall
buildings. The motion of tall buildings due to wind consists
of two components.
1) Static : Static wind effect primarily causes elastic bending
and twisting of structure.
2) Dynamic : For tall , long span and slender structures a
dynamic analysis of the structure is essential .Wind gusts
causes fluctuating forces on the structure which induce large
dynamic motions, including oscillations.
fluctuating along wind load, which consists of turbulent
velocity in the along wind direction, could be used to estimate
the along wind response of a tall building. The characteristics
of fluctuating across wind loads differ from those of
fluctuating along wind loads. Across wind loads consists of a
complicated interaction mechanism between the fluctuating
atmospheric flow and the building sides. Therefore estimating
the across wind response
theoretically is impossible
and across wind load
formulation depends upon
wind tunnel experiment
using a scaled model.
Not only is the wind
approaching a building a
complex phenomenon, but
the flow pattern generated
around
a
building
is
equally
complicated by
the distortion of
the mean flow,
flow separation,
the formation of
vortices,
and
development of
the wake. Large wind pressure fluctuations due to these effects
can occur on the surface of a building. As a result, large
aerodynamic loads are imposed on the structural system and
intense localized fluctuating forces act on the facade of such
structures.
B. Buffeting in wind direction
D. VORTEX SHEDDING
A high-frequency instability caused by airflow
separation around a building. A random forced vibration
capable of causing
stress fractures on
building facades and,
in extreme cases,
damage to internal
supporting structures.
These
vibrations
depends upon the
shape of the building.
If the building has
more surface area in
contact with wind
then vibration will be more as it is in square or rectangle but if
surface area is less with contact of wind which is possible in
oval or circular it will give less area to oppose to the wind.
C. ALONG AND ACROSS-WIND LOADING
Most modern tall buildings using lighter
construction materials that have high strength and less
stiffness are more flexible and thus excessive wind induced
vibrations can occur. Such dynamic responses cause
discomfort to the building residents and structural un-safety.
Therefore many studies on estimating wind induced vibration
of tall buildings have been conducted over recent decades. The
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Vortex shedding is an oscillating flow that takes
place when a fluid such as air or water flows past a bluff (as
opposed to streamlined) body at certain velocities, depending
on the size and shape of the body. In this flow, vortices are
created at the back of the body and detach periodically from
either side of the body. The fluid flow past the object creates
alternating low-pressure vortices on the downstream side of
the object. The object will tend to move toward the lowpressure zone. If the bluff structure is not mounted rigidly and
the frequency of vortex shedding matches the resonance
frequency of the structure, the structure can begin to resonate,
vibrating with harmonic oscillations driven by the energy of
the flow. Tall buildings are bluff bodies which cause the flow
to detach from the structure instead of the contour of the
building. When this happens vortices are created which cause
a periodically alternating force perpendicular to the wind
direction as shown in figure. Vortex excitation is one of the
critical phenomena that affect tall slender towers. Because
vortex shedding is a serious problem the structural engineer
should try to mitigate the vortex shedding process.
Whether vortex shedding becomes a problem for the building
is dependent on two frequencies, namely:
1) The fundamental frequency of vibration the building.
2) The frequency at with which the vortices are shed.
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When these two frequencies are equal resonance occurs. The
forces due to the shedding of vortices then shake the building
at its most vulnerable frequency which results in large across
wind vibrations.
speed and turbulence profile acting on the structure. Wind
tunnel tests provide the necessary design pressure
measurements in use of the dynamic analysis and control of
tall buildings.
VII. STUDYING THE WIND INDUCED MOTION
There are many situations where analytical methods cannot
be used to estimate certain types of wind loads and associated
structural response. For example, when the aerodynamic shape
of the building is rather uncommon or the building is very
flexible so that its motion affects the aerodynamic forces
acting on it. In such situations, more accurate estimates of
wind effects on buildings can be obtained through aero elastic
model
testing
in
a
boundary-layer
wind
tunnel. Wind tunnel testing
is a powerful tool that
allows
engineers
to
determine the nature and
intensity of wind forces
acting
on
complex
structures. Wind tunnel
testing
is
particularly
useful
when
the
complexity of the structure
and
the
surrounding
terrain,
resulting
in
complex wind flows.
In order to avoid damage to the structure due
to wind, we should study the flow pattern and behaviour of the
structure against it. Wind engineering is the branch which
analyse the effect of
wind in the natural and
the build environment
and studies the possible
damage, inconvenience
causes by wind. Wind
engineering
also
involves
study
of
impact of wind on
structures(buildings,
bridges, towers), wind
comfort near buildings.
Wind
engineering
draws
upon
fluid
dynamics, geographic
information system and
specialist engineering
systems
such
as
aerodynamics
and
structural dynamics.
To study the wind various tools are used,
which includes atmospheric models, open jet facilities,
computational fluid dynamic models and wind tunnel testing.
Studying the structural behaviour against wind, wind tunnel
testing is reliable and easy to perform.
VIII. WIND TUNNEL TESTS
The earliest wind tunnels were invented
towards the end of the 19th century, in the early days of
aeronautic research, when many attempted to develop
successful heavier-than-air flying machines. The development
of wind tunnels accompanied the development of the airplane.
Large wind tunnels were built during the Second World War.
Wind tunnel testing was considered of strategic importance
during the Cold War development of supersonic aircraft and
missiles. Later on, wind tunnel study came into its own: the
effects of wind on manmade structures or objects needed to be
studied when buildings became tall enough to present large
surfaces to the wind, and the resulting forces had to be resisted
by the building's internal structure. Determining such forces
was required before building codes could specify the required
strength of such buildings and such tests continue to be used
for large or unusual buildings. In Wind Engineering, wind
tunnel tests are used to measure the velocity around, and
forces or pressures upon structures. Very tall buildings,
buildings with unusual or complicated shapes (such as a tall
building with a parabolic or a hyperbolic shape), cable
suspension bridges or cable stayed bridges are analyzed in
specialized atmospheric boundary layer wind tunnels. These
feature a long upwind section to accurately represent the wind
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Wind
tunnel testing involves
blowing air on the building
model under consideration
and its surroundings at various angles relative to the building
orientation representing the wind directions. This is typically
achieved by placing the complete model on a rotating platform
within the wind tunnel. Once testing is completed for a
selected direction, the platform is simply rotated by a chosen
increment to represent a new wind direction.
IX. TECHNIQUES TO OVERCOME WIND INDUCED
MOTION OF TALL BUILDINGS:
A. Wide openings in the structure
Pearl River Tower , China
The building consists of four large openings,
approximately 3x4 meters wide. The façades are shaped to
decrease the drag forces and optimize the wind velocity passing
through the four openings. These openings functions as pressure
relief valves for the building. This strategy maximizes the wind
power potential at these four locations as the power potential
from the wind speed is a cube function of wind velocity, therefore
a small increase in velocity can translate to a larger increase in
power potential. If the wind strikes the building perpendicular to
the opening there is a drop in portal velocity. However, from
almost all other angles, the wind velocity increase and exceeds
the ambient wind speeds. In most cases the velocity increases are
more than twice the ambient wind speeds. Figure shows the result
from wind tunnel testing from one the portals.
B. Rotate and twisted structure
Shanghai Centre, Shanghai
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The Shanghai Center is one of the largest
and tallest structure in the world and the sustainably advanced
. It is a 121 story building having triangular shape that twists
and tapers . The curved corners of the triangular act to
minimize wind loads . In addition to it’s beauty , the tower’s
shape advances the science and technology of modern supertall buildings. The tower’s taper and asymmetry all combine
to reduce wind load on the building by 24% , offering
considerable saving in both building materials and
construction costs. The shape of
the Shanghai Tower lowers the
wind forces more effectively
because of the twist and tapered
shape of the tower . The tower
consists of rows of wind turbines
to the curtain wall at the tower’s
upper level which utilizes the
wind intensity to create energy.
tubes which form part of the façade design. The three five
bladed wind turbines are anticipated to produce 50MWh of
electricity per year. To put this into context, it is enough
energy to meet the total annual demand of approximately 8%
of Strata SE1’s estimated total energy consumption. The
optimum operating range for the turbines is at a wind speed of
between 8-16 m/s from a southerly direction. It is envisaged
that they will run 24 hours a day in order to maximize the
potential to produce electricity. Noise output has been
considered at great detail throughout the design process and it
is not anticipated that there will be any acoustic impact to the
residents of Strata SE1 or the nearby Draper House from the
operation of the turbines. A venture is a shaped tube that
enables the wind turbines to optimize the pressure differential
that is formed across the building. The venture creates a
pressure differential that results in an acceleration of wind
speed through the tube. Free standing horizontal wind turbines
generate their nominal power output at wind speed around 12
m/s. The reason for Strata SE1 integrating venture into the
facade is to capture lower wind speeds and accelerate them to
greater velocity to increase the annual power output from the
wind turbines.
X.
C.
Stair step corners
Taipei 101,Taiwan
Taipei 101 follows the Chinese Pagoda
from resembling the flexible bamboo that reaches the sky.
Taipei 101 was designed as a mega structural system to with
stand gravity and lateral loads
including typhoon winds. The
original design exhibited some
worrisome patterns when run
through a wind simulator,
severe enough to necessitate a
design change. But the solution
is practically invisible; the
edges were given a double stair
step design, almost like the
fluting of Greek columns. They
may seem humble, but those
redesigned edges reduce the
potentially
dangerous
oscillations
caused by high
winds by about
30-40%,
allowing
the
structure
to
stand,
even
under the force of relentless typhoons. And by now, they’ve
become a recognizable design element of the structure.
The conclusion behind this report is that, as
we are constructing higher and higher we will have to face
many difficulties, wind is the main problem in design of
high rise buildings and skyscrapers. Each high-rise project
is unique and depends on the many conditions which
influence the choices made in the design of a tall building.
Example of such conditions are the wind climate. Vortex
shedding can play the cause of disaster but with proper
analysis and tools such as wind tunnel test it can be avoided.
Innovative structural systems for the next generation of
sustainable, ultra-high tall buildings and megastructures
should be developed. There is a need for creating a
comprehensive database of structural systems for tall
buildings throughout the globe. With the development of
increasingly taller buildings using lighter members,
serviceability issues like lateral sway, floor vibration, and
occupant comfort need to be given more attention by
researchers.
REFERENCE
[1]
Wind
Loading
on
Tall
Buildings by
[2]
P. Mendis, T. Ngo, N.
Haritos, A. Hira
[3]
(
The
University
of
Melbourne, Australia)
[4]
Samali
(University
Technology
of
Sydney,
Australia)
D. Installation of Wind Turbines
Strata SE1 are designed for
sustainable living. The scheme’s environmental strategy
includes three five-bladed, nine-meter diameter integrated
wind turbines. The wind turbines are located at the top of
Strata SE1 installed within three 9 meter diameter venture
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CONCLUSION
[5]
J. Cheung (Monash University, Australia)
[6]
Wind effect on high rise building by Kumar Roshan
[7]
CTBUH Research paper of Pearl River Tower
by Roger Frechette and Russell Gilchrist
[8]
Monitoring of wind-induced motion of tall buildings
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By Paul Carpenter, Peter D. Cenek, Richard G.J. Flay
[9]
Wind and tall building by Y. Tamura Director,
Wind Engineering Research Center,
Tokyo Polytechnic University
[10] The Influence of Wind-Induced Motions on the
Performance of Tall Buildings By John Kilpatrick
[11] CTBUH Research paper “Different approach
to aerodynamic of tall buildings”
[12] Seminar report on Shanghai Tower
by Abhilash Jagtap , PRMIT&R Badnera
[13] Wikipedia.org
[14] nbmcw.com/reports/project-site-report/20080-strata-se1first-residentialbuilding-with-wind-turbines.html
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