Skyscraper

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Skyscraper

The ten-story Home Insurance Company Building, built in Chicago, Illinois, in 1885, is considered to be the first modern skyscraper.

A skyscraper is a very tall building with many stories. Skyscrapers usually refer to structures that serve as residences or work places for thousands of people. The term "skyscraper" was first used in the United States in the 1880s, where the form of the structure originated. Originally used to describe a building with at least ten stories, today it refers to buildings with forty to more than one hundred floors. Its height is measured from the street level where the main entrance is located to the top of the structure, which includes spires (the tapered portion on top of a building's roof). However flagpoles, television antennas, and radio antennas are not included.

The tallest skyscrapers in the world are a pair of office buildings in Kuala Lumpur, Malaysia. The Petronas Twin Towers stand 1,483 feet (452 meters) tall, including spires. Completed in 1997, each of the towers holds eighty-eight stories. The Sears Tower in Chicago, Illinois, at 1,454 feet (443 meters), is the second tallest skyscraper. It opened in 1974. (The television antenna on top of the 110-story building adds an additional 253 feet [77 meters] to its height.) The third tallest skyscraper, the Jin Mao Tower in Shanghai, China, rises to 1,381 feet (421 meters). It was constructed in 1998 and has eighty-eight stories.

The Empire State Building in New York, New York, completed in 1931, held the title of the world's tallest skyscraper for more than forty years. In 1972, the twin towers of the World Trade Center surpassed the Empire State Building in height. The twin towers, which collapsed due to damage sustained from a terrorist attack on September 11, 2001, each had 110 floors and rose to 1,362 feet (415 meters) and 1,368 (417 meters), respectively.

Building up instead of out

During the 1850s, the need for more office spaces in big cities, where land was expensive or scarce, gave rise to the construction of buildings that could hold several stories. The walls of a building typically support its structure; therefore, a tall building would require very thick stone or brick walls on the lower stories to hold up the higher levels. The earliest tall buildings in the United States consisted of about five stories, with the lower floors losing plenty of space to the thick walls. In later years, architects used cast iron (a metal composed of a mixture of iron, carbon, and silicon) frames to bear the weight of the upper floors. The walls remained of stone and brick construction.

Rising to new heights

In the late 1880s, major technological advances made possible the design and construction of buildings with more stories. The invention of steel-frame construction, in which a rigid steel skeleton supported the building's weight, eliminated the need for very thick walls. The outer walls, made of bricks or stones, held only their own weight and were then supported by the steel frame. The first skyscraper to use this design was the ten-story Home Insurance Company Building, which was built in Chicago, Illinois, in 1885 by William Le Baron Jenney (1832–1907). This was considered the first skyscraper.

Steel (an alloy of iron and carbon), being stronger and weighing less than iron, was the ideal material for the framework because it allowed for additional stories. The ability to mass-produce steel further increased the construction of skyscrapers.

The invention of the safety elevator in 1852 by Elisha Otis (1811–1861) created the ability to reach the upper stories of tall buildings with ease and safety. This steam-powered rope elevator had an automatic safety device that kept it from falling if the lifting rope broke. Nearly forty years later, in 1889, the high-speed, electric-powered, roped elevator enabled the construction of higher structures.

Raw Materials

Reinforced concrete is an important component (part) of skyscrapers. It is made from concrete poured around a framework of steel rods, strengthening the resulting dried concrete against bending movement caused by the wind. A type of high-strength concrete has been developed by adding very fine particles to the regular concrete ingredients. The increased surface area of these tiny particles produces a stronger bond. This type of concrete was used in Petronas Twin Towers.

The other important material for skyscraper construction is steel. Different sizes of steel beams (long pieces of steel) are delivered to the construction site as they are needed. Before delivery, the beams are coated with a substance that protects them from rust and heat. After each beam is welded into place, the same coating substance is used to cover the fresh joints. Another layer of insulation, such as fiberglass batting covered with aluminum foil, may then be wrapped around the beams.

The exterior walls of a skyscraper are called curtain walls because they hang from the frames like curtains. A variety of materials, called claddings, make up these exterior walls. They may be glass; metal, such as aluminum or stainless steel; or masonry materials, such as granite, marble, or limestone.

Design

The architect designs the skyscraper, determining the shape and height as well as its inside and outside appearances. Some architects use a computer program that helps them see how the skyscraper will look when completed, or how the building will fit into its surroundings.

Next, structural engineers put the architect's ideas into a detailed plan. The engineers make sure the structure will support not only the weight of the skyscraper, but also the weight of the people and furniture the skyscraper will contain.

THE TALLEST STRUCTURES MAY NOT BE SKYSCRAPERS

Some of the world's tallest structures are not necessarily skyscrapers because they do not house offices or living quarters. The world's tallest freestanding (standing unsupported or with attachment) structure is not a skyscraper. The CN tower in Toronto, Canada, was erected in 1975 to support a huge television antenna. It measures 1,815 feet (553 meters), with the Skypod (comprised of a restaurant, nightclub, and an observation viewing deck) at around 1,100 feet (335 meters) from the street. The world's second largest freestanding structure is also a television tower that was built in 1967. The Ostankino Tower in Moscow, Russia, has an observation deck located almost two-thirds up the tower's total height of 1,771 feet (540 meters). A restaurant called the Seventh Heaven, with three dining areas, is located below the observation deck.

Tall buildings, especially their top levels, can sway anywhere from two inches to more than two feet in strong winds. The engineers have to make sure the structure is sound enough that it will not be toppled by the sideways force of wind, or that it will not sway too much to cause the occupants physical or emotional discomfort. Models of the planned building are tested in wind tunnels to determine the effects of high winds.

If tests show the building will sway excessively in strong winds, designers may add a central core. They may also incorporate devices to counteract the motion or restrict motion. One such device is called the tuned mass damper, which is a heavy weight in the top level of the building. When the building starts to sway, the computer system moves this heavy weight in the opposite direction, thereby reducing the amount of sway.

Tall buildings also affect the wind patterns of surrounding areas. The wind between skyscrapers that are built near one another is known to be stronger.

Each skyscraper design is different. Major structural designs that may be used alone or combined with others include a steel skeleton hidden behind curtain walls that do not support the structure's weight and a reinforced concrete skeleton that is in-filled with cladding panels to form exterior walls. The design may also include a central concrete core to hold the elevators, as well as the air conditioning, water pipes, and ducts for electrical wirings. Still another design includes support columns around the perimeter of the building that are connected by horizontal beams to one another and to the core.

The Construction Process

Each skyscraper is designed to suit the needs of its future occupants, whether they are apartment dwellers or office workers. The owner and the architect have to approve what the final structure will look like. The structure of the building also has to take into consideration the layout of the land and the type of climate in the area. For example, in Japan, designers have to allow for the occurrence of earthquakes when they design the structure. The construction process for each building differs. However, all skyscrapers follow basic methods.

The substructure

1 Construction usually begins with digging a hole that will hold the foundation. The depth of the hole depends on how far down the bedrock (solid rock deep underground) is and how many basement levels will be built.

Digging a deep hole can cause movement of the surrounding soil. To prevent this from happening and to seal out water from around the foundation site, a diaphragm wall is constructed. This is done by digging a deep narrow ditch around the boundary of the planned hole. As the ditch is dug, it is filled with slurry (watery clay) to keep its sides from collapsing. As each section of the ditch reaches the desired depth, a cage of reinforced steel is lowered into it. Concrete is pumped into the ditch, displacing the slurry. The slurry is reused in other sections of the ditch. The concrete hardens, forming the wall.

2 If the bedrock lies close to the surface, the soil on top of it is removed. The bedrock surface is smoothed to form a leveled surface for the foundation. Footings (holes into which the building's support columns will be held in place) are drilled in the bedrock. Then the support columns made of steel or reinforced concrete are placed in the footings.

3 If the bedrock lies very deep, long steel columns or reinforced concrete columns called piles are sunk through the soil until they are embedded in the bedrock. This can be done using one of two methods. Steel piles may be embedded in the bedrock by repeatedly dropping a heavy weight on their tops. The second method involves drilling shafts (large tubes) through the soil and into the bedrock. Steel rods are inserted through the shafts and concrete is poured around them, resulting in reinforced concrete columns.

4 Finally, a foundation platform of reinforced concrete is poured on top of the support columns.

The superstructure and core

Once construction of a skyscraper has started, work on several phases of the structure generally takes place at the same time. For example, when the support columns are several stories high, workers begin building floors for the lower stories. As the columns are built higher, the flooring crews move to higher stories. In the meantime, the finishing crews start work on the lowest level. This process not only saves time but also ensures that the structure stays safe during construction.

5 If steel columns and beams are used in the building, each piece is lifted into place by tall cranes (machines that lift and move heavy materials). While the lowest stories are under construction, the cranes stay on the ground. As the structure rises higher, the cranes may be placed on the highest completed level of the steel skeleton.

Workers either bolt or weld the ends of the beams into place.

6 Many skyscrapers have a core (middle section) made of concrete. The core serves to keep the structure from swaying too much from strong winds. The core usually contains the elevator shafts, as well as the pipes for transporting water and the ducts (large pipes) for electrical wirings. Concrete may also be used to construct support columns.

Concrete cores and support columns are constructed using a technique called slip-forming. Steel rods are put inside a metal or wooden form called a formwork, which is made to the desired shape. The formwork is constructed so that it moves upward. As concrete is poured around the rods inside the formwork, the formwork rises. The speed of the climbing formwork is timed so that the concrete at the lower part has set before the formwork moves upward. The concrete mixture has to be such that it has hardened by the time the formwork is raised. In this way, the entire concrete core or column is made as one continuous piece.

7 In some skyscrapers, the floors are made of reinforced concrete. In others, the floors are supported by horizontal steel beams that are attached to the core and/or support columns. Steel decks (panels of thin, corrugated steel) are laid on the beams and welded in place. Then, concrete is poured over the steel decks.

The exterior

8 In most skyscrapers, the core and support columns bear the weight of the structure and its contents. The outer walls simply enclose the structure. The walls are constructed by attaching panels of materials, such as glass, metal, and stone, to the building's framework. These materials are called claddings.

Finishing

9 After a story has been enclosed by exterior walls, it is ready for interior finishing. This includes installation of electrical wires, telephone wires, plumbing pipes, interior walls, ceiling panels, bathroom fixtures, lighting fixtures, water tanks, and sprinkler systems for fire control. Mechanical components, such as elevators, air conditioners, heating systems, and electricity generators are also put in place.

10 After completion of the superstructure, a roof is installed. This may be constructed like a floor and then waterproofed with a layer of rubber or plastic. Finally, the roof is covered with attractive weather-resistant tiles or metal.

The Future

The race is always on for constructing the highest skyscraper. The building at 7 South Dearborn Street in Chicago, Illinois, due for completion by 2004, will surpass the Petronas Twin Towers. It will rise 1,550 feet (471 meters) and contain 108 stories. Two television antennas will make the total height 2,000 feet.

Plans for other skyscrapers are being developed. In India, two pyramid-shaped buildings are being proposed. The taller of the two would reach 2,222 feet (677 meters). Japan, faced with scarce land to house its growing population, has proposed the X-Seed 4000, its name designating its height in meters. The 12,000-foot building is designed to accommodate about one million people. Another idea involves a skyscraper called the Millennium Tower (2,438 feet, or 800 meters) to be constructed in the Bay of Tokyo.

While building these skyscrapers is possible, they are just ideas. Such projects would cost a lot of money because extremely tall structures require very strong foundations and materials. The task of getting materials up to the highest stories would also require more expenses. Additional stories would require more elevators that would take up more spaces in the building core. One solution is to group passengers according to a common destination. Another solution is to have two sets of elevators—one elevator to take passengers partway up the building and another elevator to take these passengers the rest of the way.

alloy:: A mixture of a metal and a nonmetal or a mixture of two or more metals. For example, steel is an alloy made of the metal iron and the nonmetal carbon. Brass is an alloy made of two metals, copper and zinc.
architect:: A person who designs a building, determining the shape and height as well as its inside and outside appearances.
beam:: Horizontal piece of a frame.
bolt:: A threaded metal pin with a head. It is inserted through a hole in a building piece and is secured with a nut. The nut, a small piece of metal, has a threaded hole that fits around the bolt to keep it in place.
cladding:: Material that makes up the exterior wall of a skyscraper.
column:: Vertical piece of a frame.
concrete:: A mixture of cement powder, water, gravel, and sand.
corrugated steel:: A steel sheet, shaped into folds for rigidity.
masonry:: Stonework or brickwork.
reinforced concrete:: Concrete that is made stronger by having steel rods embedded in it.
spire:: The tapered portion on top of a building's roof.
weld:: To unite metal pieces by applying heat, which melts the edges of the pieces, joining them together.