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EVOLUTION OF SUPER TALL AND SUPER SLENDER

SKYSCRAPERS STRUCTURAL SYSTEMS IN

CONJUNCTION WITH ARCHITECTURAL FORMS &

AESTHETICS Prof. Jayesh Pitroda, Ar. Ruma Singh

Professor, Civil Department, BVM Engg. College, Vallabh Vidyanagar, Gujarat, India 1

Assistant Prof., Architecture & Interior Design, SMAID, New V. V. Nagar, Gujarat, India2

Abstract: Tall building developments have been rapidly increasing worldwide. The

continuing economic prosperity and population increase in the urban areas point toward a

future with increased activity in high- rise construction of residential, commercial and

office buildings. However, construction of high- rise buildings can be economically

attractive only if the structural engineers can have comprehensive understanding of the

structural behaviors of various systems on one hand and the practical sense of the

construction problems on the other. This paper gives a brief reviews of the advancement of

Buildings, a with simple forms of construction toward modern innovative construction, and

evolution of tall building’s structural systems and the technological driving force behind

tall building developments. Further, contemporary “out-of-the-box” architectural design

trends, such as aerodynamic and twisted forms, shifting of central to uncommon core

which directly or indirectly affect the structural performance of tall buildings, are

reviewed. Lastly, case studies of buildings are mentioned, which represent a new

generation of sustainable high-rise buildings that are challenging conventional high-rise

building practices and setting trends for such future projects incorporating innovations in

materials and smart building systems. These buildings are seemingly well-tuned to their

climate; and they provide a major portion of their own energy requirements through

integrated passive design, daylighting, and intelligent control systems. The purpose of this

paper is also to briefly discus the evolution of tall building’s structural systems and the

technological driving force behind tall building developments.

Keywords: Architectural Expression, Built Form, Building Materials, Building

Structural Systems, Construction Techniques, Dampers, Earthquake-Resistant Design

Skyscrapers, moment-resisting frames, Sustainable Supertall, High-Tech Glass, Tall

Structural Evolution, wind and weight.

I. INTRODUCTION

A lot of buildings in the world today, that are considered representatives of Modern

Architecture, seem to have exceeded all natural limitations when it comes to height. The

typically monolithic shape of the popular (from the looks of it) skyscrapers continue to

increase in height. The tallest building in the world in the year of 2018 will be over 1

kilometer high. This “mine is bigger than yours” contest has been going on between

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countries and continents ever since the industrial revolution. The United States held the

title for over forty years with the Empire State Building (picture below in the middle),

with its 102 floors and 380 meters, but now it is Little League in comparison with the

newest super tall building structures, many of them located in Asia (the picture below to

the left is the Burj Khalifa Building in Dubai).

The phenomena of Skyscrapers probably stems from our desire to reach skyward, which we

can see examples of all through our history. From the time of the ancient pyramids of Egypt

and the great cathedrals and towers of Europe, humankind have sought toward heaven in our

buildings. The pyramids were built to guide the deceased kings and pharaohs towards their

afterlife, whereas the cathedrals were built to inspire fear of God, piety and awe of the

congregation. But these buildings also represent a more mundane quest for symbols of power

and prosperity. In the Modern version of tall buildings this last factor has become

predominant, and skyscrapers first and foremost represent the power of money.

A huge building like the Modern skyscraper can house a lot of people, almost like a little

micro-city with vertical main streets in the form of stairs and elevators. The skyscraper—

enabled cities to add vast amounts of floor space using the same amount of ground area.

Given the rising demand for center-city real estate, the skyscraper seemed like a godsend. We

do not need skyscrapers, yet people choose to build them for several reasons. They are built

so that many living spaces or office spaces can exist on a smaller piece of land. Since land is

expensive, it may be cheaper to build up rather than outward on the ground. They may also

be built for aesthetic reasons: to beautify an area, to attract tourists, to improve the

appearance of big cities, or to compete with other skyscraper designs.

Since the beginning of the industrial age in 1830, building technology has advanced from

monolithic structures with marginally controlled passive environments to glass-enclosed

skeletal frames with intelligent robotic servicing. Much of this change occurred after 1940

with the proliferation of mechanical, electrical, and plumbing systems (Bachman, 2003). The

most obvious influence of industrialization has been first, the progression of advanced

materials that performed better and lasted longer; and second, the standardization of building

components that could efficiently be produced by machines. Modern technical solutions now

may come as well-ordered or totally preconfigured systems designed by other professionals.

BUILDINGS IN THE BEGINNING

The history of the built form and the building

enclosure is more than just a curiosity:

understanding the history helps explain many of

the buildings types, construction techniques and

building materials that we use today. Building

probably began with simple forms of construction

being used for shelter from the wind, sun and rain.

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Gradually, as the desire for better shelter grew, suitable materials were identified and

construction skills were developed. Vernacular architecture throughout the world is usually

characterized by the judicious and advantageous use of readily available local materials and

an experiential understanding of climate and site [Fitch 1960]. These forms of building

evolved over generations and, since the requirements were relatively simple and change was

usually very slow, the design, the building materials and the construction techniques evolved

at a pace dictated by matching need and available resources. As society flourished,

construction materials and techniques developed from reeds and mud into manufactured

baked mud and burnt clay brick [Sandstroem 1970]. For several thousand years, walls in

Europe and elsewhere were built of masonry, wood, or clay material. Because of their size or

massiveness, these walls were exceptionally strong and durable and provided a thermal

protection through their natural heat storage and thermal insulation capabilities. Where the

climate was colder and masonry not as readily available, houses of logs or half-timber with

clay infill or earth toppings were built for their greater insulating properties.

An Irish Church from 1000 AD composed of just dry-stacked rock. Simple and durable,

but hardly comfortable

Mesa Verde, an adobe community built on a

superbly located site protected from summer sun,

but able to capture solar heat in winter, it was

abandoned around 1200, likely because climate

change brought on by unsustainable agricultural

practices.

The Industrial Revolution dramatically changed the

situation, leading to the rapid development of new

materials, products and techniques. The building structure,

its form, assembly techniques and materials underwent

radical change in the relatively short period between the

19th century and the present time. Specialization and mass

production, the hallmarks of the Industrial Revolution,

were slowly introduced into the building industry. The

superstructure, and to a much lesser degree, the enclosure

began to be considered separately as specialized

components. In the West, the traditional massive wall

systems gave way to skeletal structural systems, often with non-loadbearing enclosures.

Solid masonry office with generous window area, and fine detailing. Durable, strong, and

functional, with moderate R-value, this type of building was common after the Industrial

Revolution

In Europe, cast-iron structural frameworks with load-bearing infill were being used in

English mills and warehouses by the turn of the 19th century. In the mid-1800s truly flexural

frameworks of cast iron and, later, the much stronger and more ductile wrought iron, first

appeared in the form of train sheds in England. While they may have been atypical or

temporary structures, the Palm House at Kew Gardens (1845), and the Crystal Palace (1851)

were the first examples of mass-produced, pre-fabricated buildings with thin enclosures

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separate from the structure [Fitch 1961]. Hippolyte Fontaine's St. Ouen Docks warehouse in

Paris (completed 1865) was the first multistorey building to carry all structural loads,

including that of the exterior masonry walls, on an iron frame [Condit 1968, p. 124].

Buildings with framed structural systems and specialized enclosures were developed to their

modern state in commercial high-rise construction in the US, primarily in Chicago. The first

iron-framed skyscraper (Home Life Insurance, 1884) and the first all-steel frame building

(Rand McNally Building, Burnham and Root Architects, 1890) were both built in Chicago

[Randall, p. 105-107 and pp. 118-120].

LIGHT-WEIGHT BUILDING SYSTEMS

The birth of the modern skyscraper occurred with the start of construction, in 1884, of the

Home Life Insurance Building designed by William Le Baron Jenney. In this building there

were no load-bearing walls since the entire building weight was carried by the metal skeleton.

The thin masonry facade was hung from the frame like a curtain on shelf angles fastened to

spandrel beams [Condit 1964 pp.83-85]. By 1895, the D.H. Burnham and Co. design for the

Reliance Building in Chicago contained many of the technical components of the modern

high-rise office tower: curtain walls, elevators, and a wind-braced frame of steel. Louis

Sullivan, who has been credited with the statement that ―form follows function,‖ brought the

aesthetic statement of the skyscraper to maturity with the Carson Pirie Scott Department

Store in 1899 [Schueller 1990].

The advent of self-supporting building endoskeletons of steel and, later, reinforced concrete

led to the development of thin building ―skins‖ consisting of lightweight façade panels

(usually spandrel panels) and large expanses of window. The combination of low-heat, low-

cost fluorescent lighting, suspended ceilings, and sealed fully air-conditioned HVAC systems

became the standard after 1945. This form of building rapidly became the universal standard

promoted by the architectural vision of architects such as Le Corbusier, Mies Van de Rohe,

and the Bauhaus School. The limited regard for site, climate, and locality were exemplified

by this movement’s ―International Style‖ label. Over the same time period, expectations for

comfort, durability, and utility rose. These relatively recent changes in building structure and

form, in materials, and methods of assembly did not always result in an improvement in

durability or in better control of the interior environment.

Tall buildings are also exposed to greater environmental extremes because of their

exposure; the ability of the enclosure to contribute to the control of the interior environment

has often been inadequate and durability has become a very serious concern. The Equitable

Savings and Loan Building, completed in 1948 in Portland, Oregon, was the first fully sealed

and air conditioned curtain-wall clad office building (at least in the U.S.)[Moore 1993, p.5].

By the early 1950’s, New York’s significant Lever House and the United Nations Building

were both dominated by large areas of heat-absorbing, tinted, double-pane curtainwall

glazing made habitable by powerful air conditioning. If unprotected from the sun, the large

expanses of glass on modern skyscrapers produce solar gains that result in large and rapid

temperature fluctuations and glare.

TOWARDS THE FUTURE

In recent years the field of building science and a growing awareness of the

interrelationship and interaction between the building and both the interior and exterior

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environments has led to improvements in building performance. Because of rapidly changing

materials, building techniques and equipment, the ability to predict the performance of

buildings has become much more important. The need to conserve global material and energy

resources also requires more efficient buildings. It is becoming clear that building designers

must have some knowledge of building science and the performance of the building

enclosure in order to design better building enclosures and better buildings.

NECESSITY OF CONSTRUCTING HIGHRISE STRUCUTRES

Skyscrapers are a result of the belief that only sky is the limit of ambitions. The

skyscrapers are a wonder of modern innovative construction methods and they demonstrate

the power of technology and human capability. The skyscrapers are not just high and mighty

in their outward appearance. They have the most modern facilities till the very peak. People

aspire and plan to buy apartments in skyscrapers or book office rooms in one of them because

they give a strange sort of thrill and feeling of power. Looking down at the world from a high

rise apartment house gives us an indefinable pleasure.

The world is short of space and the population is always increasing. With the turn of the

century, the child mortality rates decreased and the population of young people started

growing steadily. Spread of education and accessibility to higher education has given the

youth of developing and developed nation’s better purchasing power. The family structure

has also changed. People are living apart from their parents and forming their own family

unit. The personal space and privacy has become more valuable. All these have increased the

need of making skyscrapers so that more living space can be made out of little ground space.

It can be observed how people tried to build high buildings for deities. This shows that

people have always associated tall buildings with power and respect. The Egyptian kings

would build high tombs or pyramids for themselves and fill those with their treasure.

Skyscrapers have a certain charm and opulence about them which attracts the human fantasy.

Though the tall skyscrapers have all the modern amenities inside them still there is a high risk

involved in living inside them. The skyscrapers are vulnerable in front of natural calamities

like earthquake and there can be several casualties during a fire breakout.

CHALLENGES AND SOLUTIONS BEHIND THE WORLD'S SUPER TALL AND

SUPER SLENDER SKYSCRAPERS

1. EARTHQUAKE-RESISTANT DESIGN CONCEPTS

(Reference: The NEHRP Recommended Seismic Provisions)

Generally, a building can be defined as an enclosed

structure intended for human occupancy. However, a

building includes the structure itself and non-structural

components (e.g., cladding, roofing, interior walls and

ceilings, HVAC systems, electrical systems) permanently

attached to and supported by the structure. Building frames

are a common structural system for buildings constructed

of structural steel and concrete. In building frame

structures, the building’s weight is typically carried by

vertical elements called columns and horizontal elements

called beams. Lateral resistance is provided either by

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diagonal steel members (termed braces) that extend between the beams and columns to

provide horizontal rigidity or by concrete, masonry, or timber shear walls that provide lateral

resistance but do not carry the structure’s weight. In some building frame structures, the

diagonal braces or walls form an inherent and evident part of the building design as is the

case for the high-rise building in San Francisco shown in Figure 01. In most buildings, the

braces or walls may be hidden behind exterior cladding or interior partitions.

Figure 01 A high-rise braced frame building in San Francisco, California.

Moment-resisting frame systems are commonly used for both

structural steel and reinforced concrete construction. In this

form of construction, the horizontal beams and vertical

columns provide both support for the structure’s weight and

the strength and stiffness needed to resist lateral forces.

Stiffness and strength are achieved through the use of rigid

connections between the beams and columns that prevent

these elements from rotating relative to one other. Although

somewhat more expensive to construct than bearing wall and

braced frame structural systems, moment-resisting frame systems are popular because they do

not require braced frames or structural walls, therefore permitting large open spaces and

facades with many unobstructed window openings.

Figure 02 A tall steel moment- resistant frame structure under construction.

Dual systems, an economical alternative to moment-resisting frames, are commonly used for

tall buildings. Dual system structures feature a combination of moment-resisting frames and

concrete, masonry, or steel walls or steel braced frames. The moment-resisting frames

provide vertical support for the structure’s weight and a portion of the structure’s lateral

resistance while most of the lateral resistance is provided either by concrete, masonry, or steel

walls or by steel braced frames. Some dual systems are also called frame-shear wall

interactive systems.

2. NONSTRUCTURAL COMPONENTS

In addition to the structural framing and the floor and roof systems, buildings include many

components and systems that are not structural in nature but that can be damaged by

earthquake effects. The types of nonstructural components include:

• Architectural features such as exterior cladding and glazing, ornamentation, ceilings,

interior partitions, and stairs;

• Mechanical components and systems including air conditioning equipment, ducts, elevators,

escalators, pumps, and emergency generators;

• Electrical components including transformers, switchgear, motor control centers, lighting,

and raceways;

• Fire protection systems including piping and tanks; and

• Plumbing systems and components including piping, fixtures, and equipment.

The design and construction requirements are intended to ensure that most of these

components are adequately attached to the supporting structure so that earthquake shaking

does not cause them to topple or fall, injuring building occupants or obstructing exit paths.

For those pieces of equipment and components that must function to provide for the safety of

building occupants (e.g., emergency lighting and fire suppression systems), the Provisions

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provides design criteria intended to ensure that these systems and components will function

after an earthquake.

Energy dissipation systems are composed of structural elements capable of dissipating large

amounts of earthquake energy without experiencing damage, much like the shock absorbers

placed in the suspensions of automobiles. Energy dissipation systems usually are placed in a

structure as part of a diagonal bracing system. Several types of energy dissipation system are

available today including hydraulic dampers, friction dampers, wall dampers, tuned mass

dampers, and hysteretic dampers.

Hydraulic dampers are very similar to automotive shock absorbers. They consist of a double

acting hydraulic cylinder that dissipates energy by moving a piston device through a viscous

fluid that is contained within an enclosed cylinder.

Friction dampers are essentially structural braces that are spliced to the structure using

slotted holes and high-strength bolts with a tactile material on the mating surfaces of the

connection. When the braces are subjected to tension or compression forces, they slip at the

splice connection and dissipate energy through friction. When the structure displaces laterally

in response to earthquake shaking, the plates shear the viscous material and dissipate energy.

Hysteretic dampers dissipate energy by yielding specially shaped structural elements that

are placed in series with conventional wall or brace elements. Tuned mass dampers consist of

a large mass on a spring-like device. When they are mounted on a structure, the lateral

displacement of the structure excites the mass, which then begins to move and dissipate

significant portions of the earthquake’s energy, protecting the structure in the process.

FACTORS CONTROLING DESIGN OF HIGH-RISE BUILDINGS

View of One World Trade Center from New Jersey. Standard concrete has a compressive

strength of 3,000-5,000 psi (pounds per square inch). All the concrete used during the

construction of One World Trade Center exceed that strength, with slabs rangin

Our tallest buildings elicit all manner of flowery descriptions and grandiose statement, owing

to both their scale and symbolism. In the entryway to Dubai's Burj Khalifa, currently the

world's highest, quotes such as "the word impossible is not in the leaders' dictionaries" are

prominently displayed, a series of Successors for skyscrapers. But it's numbers, not words,

that make these structures so inspiring, specifically

complicated engineering calculations. While it's dizzying

to think of what's required to construct these massive

buildings, for many new construction projects, the math

has gotten a even more complicated. Consider New York's

Empire State Building, a model of classic skyscraper

construction from the early 20th century, and 432 Park

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Avenue, a recently built, slender 96-story luxury tower overlooking Central Park. It's not

surprising the newer building is more than 100 feet taller. What's arguably more impressive is

the relative footprints at ground level; at its widest point, the Empire State building stretches

424 feet across. 432 Park Avenue was constructed on a 90-foot square lot.

According to Bill Baker, the structural engineer for Skidmore, Owings & Merrill responsible

for the Burj Khalifa, the trend towards taller, thinner buildings has presented new spins on

old engineering challenges. When the ratio between the height and width of a building goes

beyond 8:1 or 9:1, it becomes increasingly more expensive to construct, since it requires

thicker walls and more sophisticated technology to reduce the amount of swaying and

shaking caused by the wind (Baker compared today's thinner supertalls to a fishing rod, and

making one stand up straight requires much more reinforcement). The height-to-width ratio

for 432 Park Avenue is 15:1; to put that in perspective, if you place a standard ruler on its

end, it has a ratio of 12:1.

According to Baker and Stephen DeSimone, Chief Executive of DeSimone Consulting

Engineers, a company that has worked on a string of supertalls with thin footprints, it's a

matter of wind and weight.

A photo of the facade of China Merchants Tower, located in Shenzhen, China, and a

graphic explaining how designers shaped the building, in part, to mitigate wind forces. Photo

courtesy SOM / © Tim Griffith. Graphic courtesy Skidmore, Owings and Merrill (SOM).

WIND TUNNEL "CONFUSING THE WIND"

Wind is the "dominant force" in tall buildings, says Baker. Over time, engineers and

architects have become more and more sophisticated when it comes to shaping a building to

account for gusts that can, on very rare days, reach 100 miles-per-hour at the crown of a 90-

or 100-story skyscraper. Early in the design process, different shapes for a proposed tower are

workshopped and run through wind tunnel testing to determine which one is most efficient.

Computer simulations for complex wind patterns still take a long time, so model testing often

works best to determine factors such as lift and cross-breezes. Baker says, "the wind tunnel is

a giant calculator."

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Skyscraper designers want to "confuse the wind," says Baker. Air pushing against the

surface of a tall tower creates vortices, concentrated pockets of force that can shake and

vibrate buildings (the technical term is vortex shedding). The aim of any skyscraper design is

to break up these vortices. Facades often have rounded, chamfered or notched corners to help

break up the wind, and sometimes, open slots are grooves will be added to let wind pass

through and vent, in effect disrupting the air flow. "It's interesting that the aerodynamics of

the building are almost counterintuitive," says DeSimone. "We don't want smooth shapes, we

want shapes that break up the air flow."

50 West Street, designed by Helmut Jahn and scheduled for

completion sometime next year. Set to be one of New

York's tallest residential structures, the building will use

tuned mass dampers to reduce swaying. Photo by Field

Condition.

DAMPERS: SHOCK ABSORBERS FOR

SUPERTALLS

To help counter the shifting and swaying of building,

engineers also utilize dampers, massive devices that shift

and help stabilize tall structures like counterweights. Think

of them like the weights in a grandfather clock; engineers

attach 300-800 ton pieces of steel or concrete on a floor

near the top of a tower, tuning and adjusting chains to

balance them so they move out of phase with local wind

patterns, steadying the tower. Two main types of dampers

are used today; tuned mass dampers, which function like

swinging pendulums, and slosh dampers, or slosh tanks,

large pools of water that help absorb vibrations. The

technology isn't new; it's been used on buildings such as the Seagram Tower, completed in

1958. But it's become more common and more sophisticated. Some tuned mass dampers even

use actuators, or small motors, to shift and move in opposition to the wind. The engineers of

the Shanghai Tower even devised a damper system with powerful magnets.

According to DeSimone, all this effort to limit the swaying of a building, which can cost

upwards of $5 million per project, pays off. Top floors of buildings with these types of

systems will only shift two-and-a-half feet during rare, incredibly strong, once a century gusts

of wind, an amount that's imperceptible to the naked eye (though it can make people feel

seasick).

AMAZING CONCRETE

Even with carefully engineered facades and vibration-canceling technology, supertalls still

need to support massive amounts of weight. While we haven't moved past concrete and steel,

technological advances means the elemental ingredients of skyscrapers can support much

larger loads with much less material. "Concrete is amazing these days," says Baker. "We

should call it something new, since it's so different than concrete from a few decades ago."

More workable and up to five times stronger, concrete today has gained these powers due to a

more complex chemical composition. In many cases, industrial by-products, such as fly ash,

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slag from steel mills and micro silica left over from silicon manufacturing, are added to

strengthen the mix, allowing it to be stiffer and support heavier loads.

Baker says that many building engineers are experimenting with composite structures that

combine high-strength steel and concrete in different ways (concrete-filled steel tubes, for

instance) to find the right balance of strength and flexibility. Where builders may have been

limited in the past, stronger materials means they can build taller while maintaining the same

size structural elements, according to DeSimone.

The most exciting part about these technical advances is that they promote unique designs.

To explain, Baker compares the design process of buildings against that of cars. Since

vehicles are all trying to solve a similar engineering issue in regards to wind and

aerodynamics, car shapes have tended to move towards a uniform middle, and bear a much

closer resemblance than they did decades ago. The opposite is happening with tall buildings;

the combination of site-specific environmental factors, and the desire to make each supertall a

signature part of a city's skyline, means towers will continue to evolve in different and

creative ways.

GLASS ISN’T JUST GLASS

It’s not wholly an aesthetic choice: Glass is cheaper than masonry and steel, and it weighs

significantly less. ―Glass is one of the cheaper building components,‖ said Brian Dawley, a

senior technical services representative for Viracon, another major glass supplier for

supertalls. ―It not only looks good, but it performs

well, too.‖ The problem is, panes that large have to be

incredibly strong to withstand high wind forces, and

they have to be designed in a way to compensate for

the enormous amount of light they let in. Two

decades ago, many of these challenges would have

been insurmountable – the technology simply wasn’t

there. But a lot has happened in the intervening years.

The first advance is in the manufacturing of the

glass itself. Until recently, architects were limited by

the size of the glass panes that companies like

Guardian Industries could produce. But thanks to

advances in so-called flat-glass production, in which

molten glass is poured onto a bed of liquid tin and

then slowly cooled, manufacturers can now produce thick glass panes several meters wide.

―In just the last ten years, we’ve seen that manufacturers can do larger and larger pieces of

glass,‖ said Steven Ball, a structural engineer John A. Martin and Associates, which

specializes in part in supertall design. ―Nowadays they can do six-meter-wide glass, if not

more.‖

Kingdom Tower - 1,000 meters

Kingdom Tower, in Jeddah, Saudi Arabia, is currently under construction with a planned

completion date of 2018. It will be about 3,280 feet (1,000 meters, or one kilometer) tall,

making it the tallest building in the world.

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The raw, or annealed, glass is placed in an oven and then rapidly cooled, a process called

heat strengthening, after which manufacturers or fabricators apply layers of metal coatings to

achieve various performance qualities. Such coatings are applied in nanometer-thick layers,

as thin as a one-thousandth of a human hair, often several on top of each other in a custom

―recipe‖ designed to meet a building’s unique needs. For example, a client might want a glass

that allows in maximum light but minimum heat.

Finally, for supertalls, the glass pieces will be cut and either laminated together or

configured with a fraction of an inch of air between them, known as double glazing (for

maximum strength, many supertall towers require triple glazing, with a laminated pair of

glass panes and another thinner, single pane). The result is a high-tech glass barrier that can

be almost an inch thick: strong enough to withstand hurricane-force winds and reflect all but

a fraction of the heat pouring across the Arabian Desert, yet flawlessly clear enough to appear

invisible.

CLASSIFICATION OF TALL BUILDING STRUCTURAL SYSTEMS

In 1969 Fazlur Khan classified structural systems for tall buildings relating to their heights

with considerations for efficiency in the form of ―Heights for Structural Systems‖ diagrams

(Khan, 1969). This marked the beginning

of a new era of skyscraper revolution in

terms of multiple structural systems. Later,

he upgraded these diagrams by way of

modifications (Khan, 1972, 1973). He

developed these schemes for both steel and

concrete as can be seen from Figure 3 (Ali,

2001; Ali & Armstrong, 1995; Schueller,

1986). Khan argued that the rigid frame

that had dominated tall building design and

construction so long was not the only

system fitting for tall buildings. Because of

a better understanding of the mechanics of

material and member behavior, he reasoned

that the structure could be treated in a

holistic manner, that is, the building could

be analyzed in three dimensions, supported

by computer simulations, rather than as a

series of planar systems in each principal

direction. Possible structural systems, according to him, are rigid frames, shear walls,

interactive frame-shear wall combinations, belt trusses, and the various other tubular systems.

Classification of tall building structural system by Fazlur Khan (Above: steel; Below:

concrete)

The other classification of structural systems is based on lateral load-resisting

capabilities. Structural systems of tall buildings can be divided into two broad categories:

interior structures and exterior structures. A system is categorized as an interior structure

when the major part of the lateral load resisting system is located within the interior of the

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building. Likewise, if the major part of the lateral load-resisting system is located at the

building perimeter, a system is categorized as an exterior structure.

RECENT DEVELOPMENTS IN THE FORM OF TALL BUILDINGS

The direction of evolution of the tall building’s structural systems has been toward

efficiently increasing the lateral stiffness against lateral loads – primarily wind loads. In order

to obtain the necessary lateral stiffness, introduced first were braced frames and MRFs

followed by tubular structures, core-supported outrigger structures, and more recently diagrid

structures. Evolution of structural systems in conjunction with architectural forms and

aesthetics, from the conventional rigid frame to the more recent re-formed ―out-of- the-box‖

such as aerodynamic, twisted, and other forms has been traced. the tall building phenomenon

will continue in a greater scale to meet the needs of the growing population in future large

cities. This paper demonstrates that structural systems have come a long way since the late

nineteenth century when they were conceived as framed systems. 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.

STRUCTURAL EVOLUTION AND ARCHITECTURAL EXPRESSION

The inherent monumentality of skyscrapers resulting from their scale makes their

architectural expression very significant in any urban

context where they soar. Thus, constructing any tall

building requires careful studies on aesthetic adequacy of

the new structure within the existing urban context. Some

structural systems for tall buildings have had major impacts

on the building aesthetics, while others have had only

minor impacts.

In the traditional braced frames, the braces – the main

lateral stiffness provider – were generally constrained

within the interior cores, and serve only for structural

performance. Consequently, no aesthetic expressions had

been sought from these bracings until the emergence of the

exterior-braced tubular structures such as the John Hancock

Center in Chicago.

JOHN HANCOCK CENTER IN CHICAGO

In the outrigger structures, a lateral load-resisting system is extended from the

conventional core to the building perimeter columns through the outriggers that connect

them. This basic configuration often requires perimeter super columns and/or belt trusses at

the outrigger levels, and these elements of the outrigger system are sometimes incorporated

with building aesthetics. For example, the First Wisconsin Center in Milwaukee clearly

expresses the belt trusses on the façade at the outrigger levels as a building aesthetic element.

WISCONSIN CENTER IN MILWAUKEE

More research is needed for exterior structural systems which are technically more

efficient, However, placing structural frames on the perimeter has some drawbacks from an

architectural point of view. Structural solutions to overcome these problems are very much

needed. Efficient structural systems in seismic zones also need to be further investigated.

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All rights reserved by www.ijaresm.net ISSN : 2394-1766 13

Finally, the newly evolving ―out-of-the-box‖ systems should be seriously investigated in

terms of their structural efficiency and economy. Cost analysis of such irregular systems can

be performed to determine the relative economic efficiency of these systems considering

various geometric parameters.

According to project manager Grant Uhlir of Gensler, it's the sustainable features of

mixed-use structure—the "sky garden" concept that will introduce green living more than a

thousand feet above the city streets—that may change how we construct and conceive of

cities. "With these gardens, you don't have to go down to street level," says Uhlir. "You can

grab a cup of coffee and walk among the trees in the sky." Landscaped with plants and trees

as well as paths and small cafes, these parks and atriums will provide an escape for workers

and residents. "Think of that double skin space like a thermos, " says Uhlir. "On a cold day, it

will insulate and warm the structure, while it'll cool the building in the summer. Plus, since

you can just walk out to a garden, you reduce elevator usage. You don't have to always go

down to the ground floor."

"There's always going to be a taller building," Uhlir says. "We tried to make the best

building, the most sustainable and efficient. This concept of vertical urbanism here is unique.

It sets a precedent that I think will be part of the legacy."

CONCLUSIONS

This paper has presented a general review of structural systems for tall buildings. Unlike the

height-based classifications in the past, a system-based broad classification (i.e., exterior

versus interior structures) has been proposed. Various structural systems within each category

of the new classification have been described with emphasis on innovations. Evolution of

structural systems in conjunction with architectural forms and aesthetics, from the

conventional rigid frame to the more recent re-formed ―out-of- the-box‖ systems, has been

traced. Speculations on the future possibilities of tall buildings from a structural viewpoint

have been made. It is concluded that the tall building phenomenon will continue in a greater

scale to meet the needs of the growing population in future large cities.

This paper demonstrates that structural systems have come a long way since the late

nineteenth century when they were conceived as framed systems. There is a need for creating

a comprehensive database of structural systems for tall buildings throughout the globe. The

innovative and emerging systems can be placed within the classification scheme presented in

this paper and can be continuously updated for the benefit of the practicing professionals and

researchers.

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. The damping systems discussed in this paper can be very helpful in this

regard. Future innovations in passive and cost-effective active damping systems and

associated technologies are highly desirable.

Innovative structural systems for the next generation of sustainable, ultra-high tall buildings

and megastructures should be developed. A major challenge for multi-use tall structures is to

make them adaptive to possible changes in occupancy at different floor levels responding to

the demands of the prevailing real estate market.

Finally, the newly evolving ―out-of-the-box‖ systems should be seriously investigated in

terms of their structural efficiency and economy. Cost analysis of such irregular systems can

IJARESM

All rights reserved by www.ijaresm.net ISSN : 2394-1766 14

be performed to determine the relative economic efficiency of these systems considering

various geometric parameters. Such studies will suggest if the complexities involved in these

buildings justify their continued construction within the constraint of limited resources.

ACKNOWLEDGMENT

The authors are thankfully acknowledge to Mr. J. N. Patel, Chairmain Vidyabharti Trust, Mr.

K. N. Patel, Hon. Secretary, Vidyabharti Trust, Dr. H. R. Patel, Director,

S.N.P.I.T.&R.C.,Umrakh, Bardoli, Gujarat,India for their motivational & infrastructural

supports to carry out this research.

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