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Skyscraper


Published on Jan 10, 2017

Abstract

The word "SKYSCRAPER" originally was a nautical term referring to a tall mast or its main sail on a sailing ship. The term was first applied to buildings in the late 19th century as a result of public amazement at the tall buildings being built in Chicago and New York City. The traditional definition of a skyscraper began with the "first skyscraper", a steel-framed ten-storey building. Chicago's now demolished ten-storey steel-framed Home Insurance Building (1885) is generally accepted as the "first skyscraper".

The structural definition of the word skyscraper was refined later by architectural historians, based on engineering developments of the 1880s that had enabled construction of tall multi-storey buildings.

This definition was based on the steel skeleton—-as opposed to constructions of load-bearing masonry, which passed their practical limit in 1891 with Chicago's Monadnock Building. Philadelphia's City Hall, completed in 1901, still holds claim as the world's tallest load-bearing masonry structure at 167 m (548 ft). The steel frame developed in stages of increasing self-sufficiency, with several buildings in Chicago and New York advancing the technology that allowed the steel frame to carry a building on its own. Today, however, many of the tallest skyscrapers are built almost entirely with reinforced concrete. Pumps and storage tanks maintain water pressure at the top of skyscrapers.

A loose convention in the United States and Europe now draws the lower limit of a skyscraper at 150 meters (500 ft). A skyscraper taller than 300 meters (984 ft) may be referred to as supertall. Shorter buildings are still sometimes referred to as skyscrapers if they appear to dominate their surroundings. The somewhat arbitrary term skyscraper should not be confused with the slightly less arbitrary term high-rise, defined by the Emporis Standards Committee as "...a multi-storey structure with at least 12 floors or 35 meters (115 feet) in height."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.

INTRODUCTION

The skyscraper as a concept is a product of the industrialized age, made possible by cheap energy and raw materials. The amount of steel, concrete and glass needed to construct a skyscraper is vast, and these materials represent a great deal of embodied energy. Tall skyscrapers are very heavy, which means that they must be built on a sturdier foundation than would be required for shorter, lighter buildings. Building materials must also be lifted to the top of a skyscraper during construction, requiring more energy than would be necessary at lower heights.

Furthermore, a skyscraper consumes a lot of electricity because potable and non-potable water must be pumped to the highest occupied floors, skyscrapers are usually designed to be mechanically ventilated, elevators are generally used instead of stairs, and natural lighting cannot be utilized in rooms far from the windows and the windowless spaces such as elevators, bathrooms and stairwells. Despite these costs, the size of skyscrapers allows for high-density work and living spaces, reducing the amount of land given over to human development. Mass transit and commercial transport are economically and environmentally more efficient when serving high-density development than suburban or rural development. Also, the total energy expended towards waste disposal and climate control is relatively lower for a given number of people occupying a skyscraper than that same number of people occupying modern housing.

CONCRETE FOUNDATIONS IN SKYSCRAPERS

Due to the great height of skyscrapers, huge foundations are needed to support these structures. First, a large hole is dug into the ground to reach a point of stable soil (often bedrock). After some stability is reached, large steal footings are placed, and from these, vertical steal beams are placed along with a network of rebar. This is an image in actual proportion; the foundation of the CN Tower is 50 ft (15m) deep, to support the 1,815 ft (553m) structure. The foundation itself is almost five floors deep! Most house foundations are only one to two feet deep. Compare that to a skyscraper!

Skyscraper

But this is nothing compared to the worlds deepest foundation! The Petronas Towers has a foundation that is 394 ft (120 m) deep! The unusually deep foundation needed for the Petronas Towers was because of the areas bedrock. The bedrock in Kuala Lumpur, Malaysia is quite deep, causing many headaches for construction companies.

SKYSCRAPERS DESIGN AND CONSTRUCTION:

The design and construction of skyscrapers involves creating safe, habitable spaces in very tall buildings. The buildings must support their weight, resist wind and earthquakes, and protect occupants from fire. Yet they must also be conveniently accessible, even on the upper floors, and provide utilities and a comfortable climate for the occupants. The problems posed in skyscraper design are considered among the most complex encountered given the balances required between economics, engineering, and construction management.

BASIC DESIGN CONSIDERATIONS:

Good structural design is of importance in most building design, but especially among skyscrapers since even a small likelihood of catastrophic failure is unacceptable given the number of individuals served by skyscrapers and the resulting price of failure. This presents a paradox to civil engineers: the only way to assure a lack of failure is to test for all modes of failure, in both the laboratory and the real world. The only way to know of all modes of failure is to learn from previous failures. In this way, no engineer can be absolutely sure that a given structure will resist all loadings that could cause failure, but can only be sure, that given large enough margins of safety, that a sufficiently small percentage of the time will a failure ever occur. When buildings do fail, engineers question if the failure was due to some lack of foresight on their part or some unknowable factor that would have never been expected to have been designed for.

LOADING AND VIBRATION :

The load a skyscraper experiences is largely from the force of the building material itself. In most building designs, the weight of the structure is much larger than the weight of the material that it will support beyond its own weight. In technical terms, the dead load, the load of the structure, is larger than the live load, the weight of things in the structure (people, furniture, vehicles, etc). As such, the amount of structural material required within the lower levels of a skyscraper will be much larger than the material required within higher levels. This is not always visually apparent, or borne out visually.

The wind loading on a skyscraper is also considerable. In fact, the lateral wind load imposed on super-tall structures is generally the governing factor in the structural design. Wind pressure increases with height, so for very tall buildings, the loads associated with wind are larger than dead or live loads.

STEEL FRAME:

When one thinks of a skyscraper, the steel frame design comes to mind. This design is characterized by a large steel box, containing smaller steel boxes inside. This 3D grid is simple and efficient for most low-rises, but has its’ drawbacks for high-rise structures. As the building's height increases, the space between steel beams must decrease to compensate for the extra weight, resulting in less office space and the need for more material.

DESIGN

Designing a low-rise building involves creating a structure that will support its own weight (called the dead load) and the weight of the people and furniture that it will contain (the live load). For a skyscraper, the sideways force of wind affects the structure more than the weight of the building and its contents. The designer must ensure that the building will not be toppled by a strong wind, and also that it will not sway enough to cause the occupants physical or emotional discomfort.

Skyscraper

Each skyscraper design is unique. Major structural elements that may be used alone or in combination include a steel skeleton hidden behind non-load-bearing curtain walls, a reinforced concrete skeleton that is in-filled with cladding panels to form the exterior walls, a central concrete core (open column) large enough to contain elevator shafts and other mechanical components, and an array of support columns around the perimeter of the building that are connected by horizontal beams to one another and to the core.

Because each design is innovative, models of proposed super tall buildings are tested in wind tunnels to determine the effect of high wind on them, and also the effect on surrounding buildings of wind patterns caused by the new building. If tests show the building will sway excessively in strong winds, An example of a skyscraper ground floor design and building frame designers may add mechanical devices that counteract or restrict motion.

In addition to the superstructure, designers must also plan appropriate mechanical systems such as elevators that move people quickly and comfortably, air circulation systems, and plumbing.

The superstructure and core:

Once construction of a skyscraper is underway, work on several phases of the structure proceeds simultaneously. For example, by the time the support columns are several stories high, workers begin building floors for the lower stories. As the columns reach higher, the flooring crews move to higher stories, as well, and finishing crews begin working on the lowest levels.

Overlapping these phases not only makes the most efficient use of time, but it also ensures that the structure remains stable during construction.

 If steel columns and cross-bracing are used in the building, each beam is lifted into place by a crane. Initially, the crane sits on the ground; later it may be positioned on the highest existing level of the steel skeleton itself. Skilled workers either bolt or weld the end of the beam into place (rivets have not been used since the 1950s). The beam is then wrapped with an insulating jacket to keep it from overheating and being weakened in the event of a fire. As an alternative heat-protection measure in some buildings, the steel beams consist of hollow tubes; when the superstructure is completed, the tubes are filled with water, which is circulated continuously throughout the lifetime of the building.

 Concrete is often used for constructing a building's core, and it may also be used to construct support columns. A technique called "slip forming" is commonly used. Wooden forms of the desired shape are attached to a steel frame, which is connected to a climbing jack that grips a vertical rod. Workers prepare a section of reinforcing steel that is taller than the wooden forms. Then they begin pouring concrete into the forms. As the concrete is poured, the climbing jack slowly and continuously raises the formwork.

The composition of the concrete mixture and the rate of climbing are coordinated so that the concrete at the lower range of the form has set before the form rises above it. As the process continues, workers extend the reinforcing steel grid that extends above the formwork and add extensions to the vertical rod that the climbing jack grips. In this way, the entire concrete column is built as a continuous vertical element without joints.

 In a steel-skeleton building, floors are constructed on the layers of horizontal bracing. In other building designs, floors are supported by horizontal steel beams attached to the building's core and/or support columns. Steel decking (panels of thin, corrugated steel) is laid on the beams and welded in place. A layer of concrete, about 2-4 in (5-10 cm) thick, is poured on the decking to complete the floor.

The exterior:

 In most tall buildings, the weight of the structure and its contents is borne by the support columns and the building's core. The exterior walls themselves merely enclose the structure. They are constructed by attaching panels of such materials as glass, metal, and stone to the building's framework. A common technique is to bolt them to angle brackets secured to floor slabs or support columns.

Finishing:

 When a story of the building has been enclosed by exterior walls, it is ready for interior finishing. This includes installation of such elements as electrical wires, telephone wires, plumbing pipes, interior walls, ceiling panels, bathroom fixtures, lighting fixtures, and sprinkler systems for fire control. It also includes installation of mechanical components like elevators and systems for air circulation, cooling, and heating.

 When the entire superstructure has been completed, the top of the building is finished by installing a roof. This may be built much like a floor, and then waterproofed with a layer of rubber or plastic before being covered with an attractive, weather—resistant layer of tiles or metal.

WIND RESISTANCE

In addition to the vertical force of gravity, skyscrapers also have to deal with the horizontal force of wind. Most skyscrapers can easily move several feet in either direction, like a swaying tree, without damaging their structural integrity. The main problem with this horizontal movement is how it affects the people inside. If the building moves a substantial horizontal distance, the occupants will definitely feel it. The most basic method for controlling horizontal sway is to simply tighten up the structure. At the point where the horizontal girders attach to the vertical column, the construction crew bolts and welds them on the top and bottom, as well as the side. This makes the entire steel super structure move more as one unit, like a pole, as opposed to a flexible skeleton.

For taller skyscrapers, tighter connections don't really do the trick. To keep these buildings from swaying heavily, engineers have to construct especially strong cores through the center of the building. In the Empire State Building, the Chrysler Building and other skyscrapers from that era, the area around the central elevator shafts is fortified by a sturdy steel truss, braced with diagonal beams. Most recent buildings have one or more concrete cores built into the center of the building. Making buildings more rigid also braces them against earthquake damage.

Basically, the entire building moves with the horizontal vibrations of the earth, so the steel skeleton isn't twisted and strained. While this helps protect the structure of the skyscraper, it can be pretty rough on the occupants, and it can also cause a lot of damage to loose furniture and equipment. Several companies are developing new technology that will counteract the horizontal movement to dampen the force of vibration. Some buildings already use advanced wind-compensating dampers. The Citicorp Center in New York, for example, uses a tuned mass damper. In this complex system, oil hydraulic systems push a 400-ton concrete weight back and forth on one of the top floors, shifting the weight of the entire building from side to side. A sophisticated computer system carefully monitors how the wind is shifting the building and moves the weight accordingly. Some similar systems shift the building's weight based on the movement of giant pendulums.

CONCLUSION

Newer and improved methods of construction materials especially iron and steel and innovative methods in construction catalyzed the skyscrapers construction. Building more and more skyscrapers would add beauty and enchanting view to the city .But meanwhile utmost care should be taken while constructing and after it, otherwise it may result in huge loss of both human lives and property. In future skyscrapers would become the part of every metropolitan city. Although height may not be the problem the, but still sky is the limit.

REFERENCES

[I] Tina Lai "Structural behavior of Skyscraper and their applications to lightweight bridge decks" ,M.Tech thesis, MIT, 2009.

[2] Sergiu Cal in, Ciprian Asavoaie and N. Florea, "Issues for achieving an experimental model" Bul. Inst. Polit. la��i, t. LV (LIX), f. 3, 2009.

[3] Martina Schnellenbach-Held and Karsten Pfeffer,"Punching behavior of biaxial hollow slabs" Cement and Concrete Composites, Volume 24, Issue 6, Pages 551-556, December 2002.

[4] Sergiu Calin, Roxana Glntu and Gabriela Dascalu, "Summary of tests and studies done abroad on the Skyscraper system", The Buletinul Institutului Politehnic din la��i, t. LV (LIX), f. 3, 2009.

[5] Sergiu Calin and Ciprian Asavoaie, "Method for Skyscraper concrete slab with gaps", The Buletinul Institutului Politehnic din la��i, LV (LTX), f. 2,2009.














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