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CN Tower

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Projects > Buildings
CN Tower breaking through the clouds
The CN Tower is perhaps Canada’s most iconic architectural structure, built in the 1970s by Canadian National Railways. Currently owned by crown corporation, the Canada Lands Company (CLC), the 1,815-foot, five-inch[1] (553-m, 13-cm) concrete tower looms over the Toronto skyline and stood as the "World’s Tallest Building" for close to 31 years until the Taipei 101 skyscraper in Taiwan surpassed it in height in 2004. Today, the Burj Dubai in the United Arab Emirates, still currently under construction, holds the title of the world’s tallest building, its actual height remaining a closely guarded secret. The CN Tower still holds the designation of the being "World’s Tallest Freestanding Structure" or tower on land according to the Guinness World Records though. In fact, the tower is not really considered a building in the traditional sense because it doesn’t occupy full floors that run from the ground to the top of the structure. It does does have 147 stories, but this is a result of an internal emergency staircase that has earned the tower the distinction for having the world’s longest metal staircase.[2]

At a weight of 130,000 tons,[3] the CN Tower cost approximately $63 million dollars[4] to build and is used as the city’s primary communication tower serving over 16 different television and FM radio stations.[5] In addition, it also houses a 360-degree revolving restaurant, a nightclub, and two observation decks, making it one of Toronto’s most popular entertainment and tourism destinations. About two million people[6] visit the CN Tower on an annual basis to take a ride on one of six of the structure’s 58-second glass elevator rides that travel at a speed of 15 miles (22 km) per hour[7] from the ground up to the structure’s two observation decks. At 1,465 feet (446.5 m), the tower’s observatory decks are currently the highest in the world.[8] Another first of its kind is the glass floor located in one of the tower’s observation decks. The glass floor, 256 square feet (24 m2)[9], provides an interesting perspective of a view down to the bottom, about a third of a mile (0.5 km), to the tower’s base.[10]

The CN Tower is currently a member of the “World Federation of Great Towers.”[11] In 1994 the American Society of Civil Engineers also classified it as one of the Seven Wonders of the Modern World.[12]

Contents

[edit] Construction History

The building of the CN Tower was based more on necessity than anything else. Throughout the 1960s and 1970s, Toronto’s downtown core experienced a large construction boom and skyscrapers with steel framing were being built at an unprecedented rate. The high rises began to compromise the city’s already weak television and radio reception and created problems for existing transmission towers that were no match in height for the new superstructures. Signals were deflecting off the buildings leading to rather poor television and radio reception for residents. The solution presented was to erect a communication tower over 1,000 feet (305 m) high.[13]

[edit] Laying the Ground Work

The primary architect who was instrumental in the design concept of the CN Tower was John Andrews. A native Australian, Andrews rose to local prominence when he became a finalist in a design competition for Toronto’s new city hall.[14] His firm, John Andrews Architects, in collaboration with Webb Zerafa Menkes Housden Partnership, came up with the final design of the CN Tower. Planning started in 1968 with the original design consisting of three independent pillars linked at various heights by structural bridges. The tower featured in the first design was also shorter than the present CN Tower.[15] Other ideas were not factored into the design at the onset, such as the addition of the space deck and increasing the actual height of the tower. The current design was finalized by 1972.

For his role, Andrews was noted as an architect whose designs were more concerned with overcoming specific engineering obstacles rather than exuding a specific architectural style.[16] Christopher Hume, an urban affairs columnist with the Toronto Star remarked in regards to Andrew’s CN Tower design, “[Andrews] came up with a masterpiece, a structure that is somehow quintessential, not compromised by design trends of architectural fashion. It is elemental; this, we feel, is what a tower should look like.”[17]

Eventually, the design merged the three independent towers into a single tower incorporating three hollow legs into a hexagonal shaped base resembling somewhat of a Y-shaped pattern. The final design style of the tower has been described as exhibitionist modern and was considered quite futuristic for the time.[18]

[edit] Forming a Foundation

When engineers began to break ground to build the foundation that would support such a massive tower, they were essentially breaking ground in more ways than one. No one had ever attempted to construct a base so far into the ground and build a structure of such towering height. Preliminary testing was conducted on the soil to assess the condition of the bedrock and how it would react to hydrostaticpressure.[19]

Construction on the foundation for the CN Tower commenced on February 6, 1973. Backhoes were used to remove a total of 62,000 tons of earth and shale to a depth of 50 feet (15 m). Afterwards, a Y-shaped hexagonal foundation of pre-pressed concrete and reinforced steel 22 feet (6.7 m) thick[20] was laid in a continuous pour. Workers even promised not to strike during this time as not to interrupt progress. By the time the foundation was actually completed, 9,200 yards (8,412 m) of concrete, 500 tons of reinforced steel, and 40 tons of thick tensioning cables had been used in its construction.[21] As a result, this phase was completed relatively quickly taking only four months.[22]

[edit] Towering to New Heights

The construction of the actual tower presented a very unique challenge for engineers. The technique of poured concrete had never been undertaken on a structure of such height. To overcome this problem a huge mold commonly referred to as a slip form had to be used. A slip form is a “continuously moving form, moving at such a speed that the concrete when exposed has already achieved enough strength to support the vertical pressure from the concrete still in the form so as to withstand lateral pressure caused by wind, inclination of walls and so on.”[23]Developed by Swedish technologists, slip form technology was used to construct chimneys and cooling towers. The technology has also been used more recently in the construction of lighthouses.[24] In the construction of the CN Tower, a ring of climbing hydraulic jacks that moved upward as the concrete hardened supported the slip form. As the slip form ascended the tower, it virtually cinched inwards, giving the tower its present shape.

[edit] The Sky’s the Limit

At 1,100 feet (335 m), builders made preparations to start construction on the Skypod. The Skypod, a seven-story structure, encompassed the tower’s two observation decks, revolving restaurant, nightclub and broadcasting unit. Construction involved using 45 hydraulic jacks and a lot of steel cable to lift about 318 metric tons of steel and wood brackets[25] to the top of the tower to be used to anchor the structure into place. The Skypod’s walls were comprised of concrete and a donut shaped ring called a radome. The radome, added to the Skypod’s base, protected microwave dishes receiving radio and television signals.[26]

[edit] Finishing Touches

The concrete tower was constructed to extend out from the Skypod, ending at a space deck at about 1,465 feet (447 m), supported by cantilevers that extended out from underneath the concrete. The very last phase of the construction process was perhaps the most exciting. In 1975, the finishing touch of the tower, a 335-foot (102-m) communications mast, was installed using an industrial Sikorsky Skycrane helicopter nicknamed "Olga."[27] The lifting of 40 seven-ton sections of mast to the top of the CN Tower took a little over three weeks.[28] Without Olga, the process could have taken more than six months to complete. Once airlifted to the top of the tower, workers secured the sections into place using a total of 40,000 bolts; the mast was then covered with a fiberglass-reinforced sheathing to protect it from ice.[29] The raising of the communication mast starting on April 2, 1975 by helicopter drew the interest of people and the local media alike.[30] Olga became an object of fascination with flight schedules being printed in daily newspapers and any changes being announced as breaking news on radio and television. Torontonians also embraced the raising of the antenna by signing their names on the pieces of the antenna before it was erected.[31] The tower opened to the public on June 26, 1976.[32]

[edit] Elements of Nature

Taking into consideration the CN Tower’s height, specific structural elements had to be factored in to the design and construction of the building. For instance, engineers developed the tower to withstand winds up to 260 miles (418 km) per hour. Counterweights were also fastened to the antenna to prevent it from wobbling in the wind.[33]

The CN Tower is also an easy target for lightning. In fact, according to a comprehensive study conducted by Ryerson University, the tower is the struck by lightning more than any other structure in Toronto, at a rate of 40 to 50 times annually.[34] To compensate for this, long copper rods were installed to run down the sides of the tower. Connected to grounding rods buried below ground, the copper rods assist in maintaining the tower’s structural integrity.

“Tall structures naturally attract lighting so engineers and lightning production experts need to understand the behavior of lightning to design better protection for such structures,” notes Ali Hussein who spearheaded the study and is a professor in the Department of Electrical and Computer Engineering at Ryerson.[35]

In addition, the CN Tower was also built to withstand earthquakes measuring 8.5 on the Richter scale.[36]

[edit] Equipment Used

[edit] Refurbishment/Recent Projects/Renovations

In 1995, the 360 Restaurant underwent major renovations to bring it in line with current fine dining trends.

More recently, on April 2008, the CN Tower introduced North America’s first, and the world’s highest, glass floor paneled elevator. The glass floor occupies about six square feet (0.6 m2) of the elevator’s 39.25-square foot (3.6-m2) floor.[37]

[edit] Unique Facts

  • The tower was built by Canadian National Railway.
  • It is currently owned by Canada Lands Company (CLC).
  • The tower's construction start date was February 6, 1973.
  • The construction end date was February 24, 1974, with the antenna being added April 2, 1975.
  • The tower's official opening day was June 26, 1976.
  • In total, 1,537 workers worked 24 hours, five days a week for 40 months to complete the project.
  • The total construction cost was $63 million.
  • The tower stands 1,815 feet, five inches (553 m, 13 cm).
  • The tower has 188 stories.
  • It weighs 130,000 tons.
  • The radius of the tower's base is 109.2 feet (33.3 m).
  • The circumference of the tower's base is 358 feet (109 m).
  • The diameter of the tower's base is 218.4 feet (67 m).
  • The tower has one glass floor, on the 113th story, at a height of 1,122 feet (342 m).
  • The tower has a lookout level on the 14th story, at a height of 1,136 feet (346 m).
  • The Skypod is located on the 147th story, at a height of 1,465 feet (447 m).
  • On a clear day, visibility from the observation deck extends 100 miles (60 km).

[edit] Materials Used

  • 53,000 cubic yards (40,521 m3) of concrete
  • 80 miles (129 km) worth of tensioned steel
  • 5,000 tons of reinforced steel
  • 600 tons of structural steel

[edit] References

  1. CNTower.ca The Straight Goods: Facts at a Glance 2008-09-23.
  2. CNTower.ca Celebrating the History of Canada's Architectural, Engineering and Construction Wonder, 2008-09-23.
  3. CNTower.ca The Straight Goods: Facts at a Glance 2008-09-23.
  4. CNTower.ca The Straight Goods: Facts at a Glance 2008-09-23.
  5. CNTower.ca Celebrating the History of Canada's Architectural, Engineering and Construction Wonder, 2008-09-23.
  6. CNTower.ca Celebrating the History of Canada's Architectural, Engineering and Construction Wonder, 2008-09-23.
  7. McLean, Jane. CN Tower: 15 Fascinating Facts. About.com: Canada Travel, 2008-09-23.
  8. SkyscraperPage.com CN Tower, 2008-09-23.
  9. McLean, Jane. CN Tower: 15 Fascinating Facts. About.com: Canada Travel, 2008-09-23.
  10. CNW Group Multi-Media News: CN Tower, 2008-09-23.
  11. SkyscraperPage.com CN Tower, 2008-09-23.
  12. American Society of Civil Engineers CN Tower, 2008-09-23.
  13. CNTower.com CN Tower History, 2008-09-23.
  14. Plummer, Kevin. The CN Tower is Dead. Long Live the CN Tower!. Torontoist, September, 2007. (accessed 2008-09-23)
  15. CNTower.com CN Tower History, 2008-09-23.
  16. GreatBuildings.com John Andrews, 2008-09-23.
  17. Plummer, Kevin. The CN Tower is Dead. Long Live the CN Tower!. Torontoist, September, 2007. (accessed 2008-09-23)
  18. GreatBuildings.com CN Tower, 2008-09-23.
  19. TrizecHahn Corporation Brief Overview of the CN Tower, 2008-09-23.
  20. http://www.justrelaxmobilespa.com/pages/page_32.asp
  21. TrizecHahn Corporation Brief Overview of the CN Tower, 2008-09-23.
  22. http://www.justrelaxmobilespa.com/pages/page_32.asp
  23. ProjectsMonitor.com New Slip Form method for lighthouse construction, 2008-09-23.
  24. ProjectsMonitor.com New Slip Form method for lighthouse construction, 2008-09-23.
  25. TrizecHahn Corporation Brief Overview of the CN Tower, 2008-09-23.
  26. http://www.justrelaxmobilespa.com/pages/page_32.asp
  27. http://www.justrelaxmobilespa.com/pages/page_32.asp
  28. http://www.justrelaxmobilespa.com/pages/page_32.asp
  29. http://www.justrelaxmobilespa.com/pages/page_32.asp
  30. McLean, Jane. CN Tower: 15 Fascinating Facts. About.com: Canada Travel, 2008-09-23.
  31. Plummer, Kevin. The CN Tower is Dead. Long Live the CN Tower!. Torontoist, September, 2007. (accessed 2008-09-23)
  32. CNW Group Multi-Media News: CN Tower, 2008-09-23.
  33. http://www.justrelaxmobilespa.com/pages/page_32.asp
  34. Toye, Suelan. CN Tower struck by lightning more frequently than rest of Toronto. Ryerson University, December 2006. (accessed 2008-09-23)
  35. Toye, Suelan. CN Tower struck by lightning more frequently than rest of Toronto. Ryerson University, December 2006. (accessed 2008-09-23)
  36. McLean, Jane. CN Tower: 15 Fascinating Facts. About.com: Canada Travel, 2008-09-23.
  37. CNTower.ca CN Tower Newly Enhanced Elevator with Glass Floor Panels, 2008-09-23.

[edit] External Links