Reaching For The Sky

Debates about the significance, efficacy and even the morality of tall structures, especially skyscrapers have been raging since the building type was invented in Chicago more than a hundred years ago. There were fear of heights (ironically, the architect of the WTC, Minoru Yamasaki, was slightly acrophobic one reason why the windows were so narrow) and fear of fire (that many people legitimately experience in high structures). But the recent terrorist attacks on WTC, renewed the debate that these types of structures make ideal targets for terrorists stable, highly visible and big [Subramanian (2002)]. In spite of the debate many tall structures are under construction and many countries are vying to have the world’s tallest structure in their own soil.

Skyscrapers are a product of the industrial revolution, which began in England in the eighteenth century.

They provided a solution to the problems of overcrowding of urban spaces, since they save

space on the ground. The first skyscraper was built in 1884 in the city of Chicago, Illinois. It was only ten-storeys high. Construction of skyscrapers was aided by the high-speed elevator invented by Otis in 1857. In 1913, the Woolworth Building in New York dared to reach fifty-five storeys, soaring up 241 m. Eighteen years later, the Empire State Building, reached 102 storeys with a height of 381m.

The skyscraper has become the preferred symbol of belonging to the modern, global world. According to the list compiled by the Council on Tall Buildings and Urban Habitat (CTBUH) at Lehigh University, the United States has lost its competitive edge in building tall structures and most of the recent tall structures are in other countries. A distinction has also been made between tall towers (non-habitat structures) and tall buildings. Currently, the tallest tower in the world is the CN Tower in Canada (553m high) and the tallest skyscraper is Taipei 101 in Taiwan (509.2m up to the tip of Antenna/Spire; Roof-449.2m). They are shown in Figure 1 and a comparison of a few completed tall structures is shown in Figure 2.

Both these structures will lose their record this year, due to the opening of Burj Dubai Tower and the construction of the Tokyo Sky Tree. Some details of these structures are presented.

Tall Towers

A tower differs from a building in that the latter has floors, and is designed for residential, business, or manufacturing use. Table 1 provides information about the tallest towers in the world, which are principally telecommunications towers. They may have observation decks or restaurants, but do not have floors all the way up.

Tokyo Sky Tree

The Japanese are in the process of constructing the world’s new tallest free-standing structure, with a 610m tall Tokyo Sky Tree tower (Figure 3 and 4).

The Tokyo Sky Tree (originally referred to as New Tokyo Tower) is a broadcasting tower currently under construction in Sumida, Tokyo, Japan. If completed as planned, it will be the tallest artificial structure in Japan at 610.58 m tall. The present Tokyo Tower (333 m) is not tall enough for complete digital terrestrial television broadcasting coverage since the construction of many highrise buildings in the central part of the metropolis. The tower will break the record currently held by the CN Tower in Toronto by 57m, and is expected to hold the record for only a few months. Burj Dubai in the United Arab Emirates will in turn shatter the Sumida Tower record when it opens in 2009.

The planning is being led by a group of six terrestrial broadcasters, led by public broadcaster NHK, who plans to complete construction of the tower by December 2011, planned to open to public in spring 2012. The completed structure will be the highlight of a massive commercial development around Oshiage Station.

The Tokyo Sky Tree is designed to have graceful curves similar to samurai swords and traditional Japanese buildings so that it does not detract from the surrounding scenery (Figure 4). It has a cross-section of an equilateral triangle at the base, gradually forming circle with graceful curves similar to samurai swords. At a height of about 350m and above, the tower’s structure is cylindrical to withstand very strong winds. An observation platform, which is expected to attract 3 million tourists a year, made the tower viable for construction. With the lattice-work that makes up the main neck of the tower and the base with arches forming legs on each of the four corners, this tower has some resemblance to the Eiffel tower of Paris. However, with the light glass and steel it shows a distinctly futurist architecture.

The tower also has state-of-the-art seismic proofing including a central shaft (made of reinforced concrete) developed from five-tier pagodas that have withstood earthquakes for hundreds of years. Six different names were considered for this tower, viz.,Tokyo Edo Tower, Tokyo Sky Tree, Mirai Tree, Yume Miyagura, Rising East Tower, and Rising Tower. After a nationwide vote, it was officially named as Tokyo Sky Tree on June 10, 2008. Construction of the tower started on 14 July, 2008.

Tallest Skyscrapers

Skyscrapers often serve as landmarks and reference points to a city. The tallest building symbolizes national prestige in the form of economic status, growth, technological prosperity and modernity. Hence many countries vie to build super-tall skyscrapers. Table 2 shows some of the tallest skyscrapers of the world. As mentioned earlier, Taipei 101 is holding the current record of tallest skyscraper.

Burj Dubai

Burj Dubai is a supertall skyscraper underconstruction in the Business Bay district of Dubai, United Arab Emirates, and will become the tallest man-made structure ever built (Figure 5). Construction began on September 21, 2004 and is expected to be completed by September 2009. The tower is designed by Skidmore, Owings and Merrill (Architect/structural engineers/MEP engineers), who also designed the Sears Tower in Chicago and the Freedom Tower in New York City. The final height of this multi-use skyscraper is a “well-guarded secret”, due to competition from other buildings under construction or proposed. However, its height is expected to be 808m, much higher than the current record holder of 509m tall Taipei 101, Taiwan. Based on this height, the total number of habitable floors is expected to be around 162. The 280,000m2 reinforced concrete tower will be used for retail, an Armani hotel, residences, and offices. The top of the tower has a steel frame structure, and the lower portion is made of high performance reinforced concrete. The goal of Burj Dubai is not simply be the world’s tallest building- it is to embody the world’s highest aspirations.

The building resembles the bundled tube form of the Sears Tower, but is not a tube structure. Structural designers purposely selected the shape of Burj Dubai as Y-shaped in plan to reduce the wind forces as well as to keep the structure simple and foster constructability (Figure 6a and 7). The Y-shaped floor plan also maximized the views of the Persian Gulf. Each wing of the Y-shaped tower, with its own high-performance concrete core and perimeter columns, buttresses the other via a six-sided central core, or hexagonal hub (Figure 7). Hence the tower is extremely stiff torsionally. The design team purposely aligned all the common central core and column elements to form a building with no structural transfers. At the top, the central core emerges and is sculpted to form a finishing spire. The spire—itself over 200m tall—will hold communications equipment. Viewed from above or from the base, the form also evokes the onion domes of Islamic architecture.

Each tier of the building steps back in spiral pattern that causes the tower’s width to change at each setback, decreasing the cross section of the tower as it reaches toward the sky. The main advantage of this stepping and shaping is to “scatter the wind”. The wind vortexes never become organized because at each new tier the wind encounters a different building shape that reduces the overall wind load on the structure. The tower, at its tallest point, sways a total of 1.2m.

An extensive program of wind tunnel testing studies were undertaken at the Rowan, Williams, Davies and Irwin (RWDI) 2.4 x 1.9 m and 4.9 x 2.4 m boundary layer wind tunnels in Guelph, Ontario, Canada (see Figuer 6). The wind tunnel program included rigid model force balance tests, a full aero-elastic model study, measurement of local pressures, and pedestrian wind environment studies. These studies used models of scale 1:500 (Irwin and Baker, 2006). However for the pedestrian wind studies, a larger scale model of scale 1:250 was utilized. These studies were used to refine the architectural shape of the tower in order to reduce the wind forces and accelerations. It was found that the accelerations in the upper residential floors are within the normal comfort criteria without the use of supplementary damping devices.

The exterior cladding of Burj Dubai will consist of 142,000m2of reflective glazing, and aluminium and textured stainless steel spandrel panels with vertical tubular fins. The cladding system is designed to withstand Dubai’s extreme summer temperature. Additionally, at its projected height, the exterior temperature at the top of the building will be noticeably cooler than at its base, by 6°C. Burj Dubai will also feature the world’s fastest elevator, rising and descending at 18m/s. The world’s current fastest elevator (in the Taipei 101) travels at 16.83m/s. A total of 56 elevators will be installed each can carry 42 people at a time.

The tower is being constructed by a South Korean company, Samsung Engineering & Construction which also built the Petronas Twin Towers and the Taipei 101. Samsung Engineering & Construction is building the tower in a joint venture with Besix from Belgium and Arabtec from UAE. Turner is the Project Manager on the main construction contract.

When completed, Burj Dubai’s construction will have used 330,000m3of concrete, 39,000tonnes of steel rebar, and taken 22 million man hours. The foundation of this tower features 192 numbers of 50m long piles. More details about their design and construction may be found from Ref. 2-5.

Under Construction

The following super-tall skyscrapers are in various stages of proposal, planning, or construction:

• The Pentominium, under construction in Dubai, is expected to be 618m tall and have 120 floors. Construction began in 2007 and completion is expected by 2011.
• The Russia Tower, under construction in Moscow’s International Business Centre, is expected to be 612.2 m tall and have 118 floors. Construction began in September 2007 and completion is expected by 2012.
• Incheon Tower is a 151-floor, 610m tower in Incheon, South Korea. It is estimated to be completed by 2012.
• The Guangzhou TV & Sightseeing Tower, underconstruction in Guangzhou, China, is expected to be 610.0m tall. Construction began in November 2005 and completion is expected by 2009.
• The Chicago Spire (formerly Fordham Spire), under construction in Chicago, is expected to be 609.6m and have 150 floors. Construction began in June 2007 and completion is expected by early 2012.
• The Jakarta Tower (Menara Jakarta) is currently on-hold in Jakarta, Indonesia. It is expected to be 558m tall up to the antenna, thus may be tallest concrete tower. It is expected to be completed by 2011.
• The Federation Tower East, under construction in Moscow’s International Business Centre, is expected to be 506m tall (to the tip of the spire) and have 93 floors. Construction began in 2003 and completion is expected by 2009.

Proposal for Dubai City Tower

A proposal has been drawn for the Dubai City Tower with 400 habitable stories, topped by a 400m tall energy producing spire(see Fig.9), with an overall height of 2400m(1.5 miles)! In this proposal, the tower is broken down into six independent buildings, three rotating clockwise and three counter clockwise about a central core. The overall tower is organized into four 100 storey “neighbourhoods” connected via a vertical bullet train that quickly distributes people between sky plazas that separate the different vertical neighborhoods. More details of this proposal may be found from Ref.9. Though this proposal may seem to be impossible right now, with the quest for reaching for the sky, propelled by the need for housing the millions, it will become a reality in the years to come!

Summary

When WTC towers were attacked, doubts were raised regarding the vulnerability of tall buildings to terrorist attack; in spite of these doubts, the world has witnessed the construction of several super-tall buildings in the past. With limited land, skyrocketing land prices and continuous demographic explosion in big cities worldwide, there is no other possibility than to build high-rise structures.

The current record holders of tall structures may loose their title when new buildings are completed (which are happening are regular intervals). When completed, the Burj Dubai Tower will be the world’s tallest structure. It represents a significant achievement in terms of utilizing the latest design, materials, and construction technology and methods, in order to provide an efficient, rational structure to rise to heights never before seen. We may expect to see more and more steel, concrete and composite high-rise structures shaping the skylines of major cities of the world in the forthcoming years.

References

• Subramanian, N, Collapse of WTC-Its inpact on skyscraper construction, Indian Concrete Journal, V. N.3, Mar. 2002, pp.165-169.
• Baker,W. F., The World’s tallest building, Burj Dubai, U.A.E., CTBUH conference, Seoul, Oct 10-13, 2004, pp.1168-1169
• Baker, W. F. , Korista, D. S., and Novak, L. C., Burj Dubai: Engineering the World’s Tallest Building, CTBUH / Wiley Tal Journal, 2007, pp. 361–375
• Lee, H.J., Kuchma, D.A., Baker, W., and Novak, L.C., Design and analysis of heavily loaded reinforced concrete link beams for Burj Dubai, ACI Structural Journal, V. 105, No., 4, July-Aug.2008, pp.451-459
• Irwin, P.A., and Baker, W.F., The Burj Dubai Tower- Wind engineering, The Structure Magazine, SEI/ASCE, June 2006, pp.28-31