CN Tower in Toronto

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The CN Tower is a communications and observation tower in Downtown Toronto, Ontario, Canada. Standing 553.33 m tall, it was completed in 1976, becoming the world's tallest free-standing structure and world's tallest tower at the time. It held both records for 34 years until the completion of the Burj Khalifa and Canton Tower. It remains the tallest free-standing structure in the Western Hemisphere, a signature icon of Toronto's skyline, and a symbol of Canada, attracting more than two million international visitors annually.

Its name "CN" originally referred to Canadian National, the railway company that built the tower. Following the railway's decision to divest non-core freight railway assets, prior to the company's privatization in 1995, it transferred the tower to the Canada Lands Company, a federal Crown corporation responsible for real estate development. Since the name CN Tower became common in daily usage, the abbreviation was eventually expanded to Canadian National Tower or Canada's National Tower. However, neither of these names is commonly used.

In 1995, the CN Tower was declared one of the modern Seven Wonders of the World by the American Society of Civil Engineers. It also belongs to the World Federation of Great Towers, where it holds second-place ranking.

History


The idea of the CN Tower originated when the 1968 Canadian National Railway had a desire to build a large TV and radio communication platform to serve the Toronto area, as well as demonstrate the strength of Canadian industry and CN in particular. These plans evolved over the next few years, and the project became official in 1972. The tower would have been part of Metro Centre (see CityPlace), a large development south of Front Street on the Railway Lands, a large railway switching yard that was being made redundant by newer yards outside the city. Key project team members were NCK Engineering as structural engineer; John Andrews Architects; Webb, Zerafa, Menkes, Housden Architects; Foundation Building Construction; and Canron (Eastern Structural Division).

At the time, Toronto was a boom town, and the late 1960s and early 1970s had seen the construction of numerous large skyscrapers in the downtown core, most notably First Canadian Place. This made broadcasting into the downtown area very difficult due to reflections off the buildings. The only solution was to raise the antennas above the buildings, demanding a tower over 300 m tall. Additionally, at the time, most data communications took place over point-to-point microwave links, whose dish antennae covered the roofs of large buildings. As each new skyscraper was added to the downtown, former line-of-sight links were no longer possible. CN intended to rent "hub" space for microwave links, visible from almost any building in the Toronto area. The CN Tower can be seen from at least as far away as Kennedy Street in Aurora, Ontario, approximately 40 km to the north, 60 km east of Toronto, in Oshawa, and from several points on the south shore of Lake Ontario, 48 km to the south in New York state in the United States.

The original plan for the tower envisioned a tripod consisting of three independent cylindrical "pillars" linked at various heights by structural bridges. Had it been built, this design would have been considerably shorter, with the metal antenna located roughly where the concrete section between the main level and the Sky Pod lies today. As the design effort continued, it evolved into the current design with a single continuous hexagonal core to the Sky Pod, with three support legs blended into the hexagon below the main level, forming a large Y-shape structure at the ground level.

The idea for the main level in its current form evolved around this time, but the Sky Pod was not part of the plans until some time later. One engineer in particular felt that visitors would feel the higher observation deck would be worth paying extra for, and the costs in terms of construction were not prohibitive. It was also some time around this point that it was realized that the tower could become the world's tallest structure, and plans were changed to incorporate subtle modifications throughout the structure to this end.

On June 26, 1986, the ten-year anniversary of the tower's opening, high-rise firefighting and rescue advocate Dan Goodwin, in a sponsored publicity event, used his hands and feet to climb the outside of the tower, a feat he performed twice on the same day. Following both ascents, he used multiple rappels to descend to the ground.

On September 12, 2007, Burj Khalifa surpassed the CN Tower as the world's tallest free-standing structure.

Construction

Construction on the CN Tower began on February 6, 1973 with massive excavations at the tower base for the foundation. By the time the foundation was complete, 56000 t of dirt and shale were removed to a depth of 15 m in the centre, and a base incorporating 7000 m3 of concrete with 450 t of rebar and 36 t of steel cable had been built to a thickness of 6.7 m. This portion of the construction was fairly rapid, with only four months needed between the start and the foundation being ready for construction on top.

To build the main support pillar, a hydraulically-raised slipform was built at the base. This was a fairly impressive engineering feat on its own, consisting of a large metal platform that raised itself on jacks at about 6 m per day as the concrete below set. Concrete was poured continuously by a team of 1,532 people until February 22, 1974, during which it had already become the tallest structure in Canada, surpassing the recently built Inco Superstack, which was built using similar methods. In total, the tower contains 40500 m3 of concrete, all of which was mixed on-site in order to ensure batch consistency. Through the pour, the vertical accuracy of the tower was maintained by comparing the slip form's location to massive plumb-bobs hanging from it, observed by small telescopes from the ground. Over the height of the tower, it varies from true vertical accuracy by only 29 mm.


In August 1974, construction of the main level commenced. Using 45 hydraulic jacks attached to cables strung from a temporary steel crown anchored to the top of the tower, twelve giant steel and wooden bracket forms were slowly raised, ultimately taking about a week to crawl up to their final position. These forms were used to create the brackets that support the main level, as well as a base for the construction of the main level itself. The Sky Pod was built of concrete poured into a wooden frame attached to rebar at the lower level deck, and then reinforced with a large steel compression band around the outside.

The antenna was originally to be raised by crane as well, but during construction the Sikorsky S-64 Skycrane helicopter became available when the United States Army sold off theirs to civilian operators. The helicopter, named "Olga", was first used to remove the crane, and then flew the antenna up in 36 sections. The flights of the antenna pieces were a minor tourist attraction of their own, and the schedule was printed in the local newspapers. Use of the helicopter saved months of construction time, with this phase taking only three and a half weeks instead of the planned six months. The tower was topped off on April 2, 1975 after 26 months of construction, officially capturing the height record from Moscow's Ostankino Tower, and bringing the total mass to 118000 t.

Two years into the construction, plans for Metro Centre were scrapped, leaving the tower isolated on the Railway Lands in what was then a largely abandoned light-industrial space. This caused serious problems for tourists to access the tower. Ned Baldwin, project architect with John Andrews, wrote at the time that "All of the logic which dictated the design of the lower accommodation has been upset," and that "Under such ludicrous circumstances Canadian National would hardly have chosen this location to build."

Opening

The CN Tower opened to the public on June 26, 1976, although the official opening date was October 1, 1976. The construction costs of approximately CDN$63 million ($ in 2011 dollars) were repaid in fifteen years. Canadian National Railway sold the tower prior to taking the company private in 1995, when they decided to divest themselves of all operations not directly related to their core freight shipping businesses.

As the area around the tower was developed, particularly with the introduction of the Metro Toronto Convention Centre in 1984 and the SkyDome in 1989, the former Railway Lands were redeveloped and the tower became the centre of a newly developing entertainment area. Access was greatly improved with the construction of the SkyWalk in 1989, which connected the tower and SkyDome to the nearby railway and subway station, Union Station and, in turn, to the city's PATH underground pedestrian system. By the mid-1990s, it was the centre of a thriving tourist district. The entire area continues to be an area of intense building, notably a boom in condominium construction in the 2000s and the 2010s.

From 1997 to January 2004, TrizecHahn Corporation managed the tower and instituted several expansion projects including a $26 million entertainment expansion and revitalization that included the addition of two new elevators (to a total of six) and the relocation of the staircase from the north side leg to inside the core of the building, a conversion that also added nine stairs to the climb.

The CN Tower was closed during the G-20 summit on June 26–27, 2010, for security reasons, given its proximity to the Metro Toronto Convention Centre.

Structure

The CN Tower consists of several substructures. The main portion of the tower is a hollow concrete hexagonal pillar containing the six elevators, stairwells, and power and plumbing connections. On top of this is a 102 m tall metal broadcast antenna, carrying TV and radio signals. There are two visitor areas: the main deck level (formerly known as SkyPod) located at 346 m, and the higher Sky Pod (formerly known as "Space Deck") at 446.5 m, just below the metal antenna. The hexagonal shape can be seen between the two areas; however, below the main deck, three large supporting legs give the tower the appearance of a large tripod.

The main deck level is seven storeys, some of which are open to the public. Below the public areas — at 338 m — is a large white donut-shaped radome containing the structure's microwave receivers. The glass floor and outdoor observation deck are at 342 m. The glass floor has an area of 24 m2 and can withstand a pressure of 4100 kPa. The floor's thermal glass units are 64 mm thick, consisting of a pane of 25 mm laminated glass, 25 mm airspace and a pane of 13 mm laminated glass. Some people experience acrophobia when standing on the glass floor and looking down at the ground 342 m below. In 2008, one elevator was upgraded to add a glass floor panel, believed to have the highest vertical rise of any elevator equipped with this feature. The Horizons Cafe and the lookout level are at 346 m. The 360 Restaurant, a revolving restaurant that completes a full rotation once every 72 minutes, is at 351 m. When the tower first opened, it also featured a disco named Sparkles, billed as the highest disco and dance floor in the world. On August 1, 2011, the CN Tower opened the EdgeWalk, in which thrill-seekers can walk on and around the roof of the main pod of the tower at 356 m, which is directly above the 360 Restaurant.

The Sky Pod is the second-highest public observation deck in the world, surpassed only by the Shanghai World Financial Center. On a clear day, it is possible to see 100 to 120 kilometres (62–75 mi) away, to the city of Rochester across Lake Ontario in the United States, the mist rising from Niagara Falls, or the shores of Lake Simcoe.

A metal staircase reaches the main deck level after 1,776 steps, and the Sky Pod 100 metres above after 2,579 steps; it is the tallest metal staircase on Earth. These stairs are intended for emergency use only and are not open to the public, except for two times per year for charity stair-climb events. The average climber takes approximately 30 minutes to climb to the base of the radome, but the fastest climb on record is 7 minutes and 52 seconds in 1989 by Brendan Keenoy, an Ontario Provincial Police Officer. The parallels between the towers led to some concern that the CN Tower could be at risk of a similar tragedy. However, Canadian officials subsequently stated that it is "highly unlikely" that a similar disaster could occur at the CN Tower as it has important safeguards that were not present in the Ostankino Tower. Specifically, officials cited:

  • the fireproof building materials used in the tower's construction,
  • frequent and stringent safety inspections,
  • an extensive sprinkler system,
  • a 24-hour emergency monitoring operation,
  • two 68,160-litre (15,000-imperial gallon) water reservoirs at the top, which are automatically replenished,
  • a fire hose at the base of the structure capable of sending 2725 litres (600 imperial gallons) a minute to any location in the tower,
  • a ban on gas appliances anywhere in the tower (including the restaurant in the main pod),
  • an elevator that can be used during a fire as it runs up the outside of the building and can be powered by three emergency generators at the base of the structure (unlike the elevator at the Ostankino tower, which malfunctioned).

Officials also noted that the CN Tower has an excellent safety record and that there has never been an accidental fire in the tower since it was opened in 1976. On August 27, 2011, the tower was lit in orange from sundown to sunrise on August 28, 2011 to commemorate the death of the federal New Democratic Party leader Jack Layton. Orange is the official colour of the New Democratic Party.

Programmed from a desktop computer with a wireless network interface controller, the LEDs use less energy to light than the previously used incandescent lights (10% less energy than the dimly lit version and 60% less than the brightly lit version). The estimated cost to use the LEDs is $1,000 per month.

During the spring and autumn bird migration seasons, the lights will be turned off to comply with the voluntary Fatal Light Awareness Program, which "encourages buildings to dim unnecessary exterior lighting to mitigate bird mortality during spring and summer migration."

Size comparisons

The CN Tower is the tallest freestanding structure in the Americas. Currently, the only other freestanding structure in the Americas to exceed 500 m in height is the Willis Tower in Chicago, which stands at 527 m when measured to its pinnacle. One World Trade Center, currently under construction in New York City, is expected to have a pinnacle height of 1776 ft, or approximately 12 m shorter than the CN Tower. Due to the symbolism of the number 1776 (the year of the signing of the United States Declaration of Independence), the height of One World Trade Center is unlikely to be increased. The Chicago Spire was expected to exceed the height of the CN Tower, but it was cancelled in early 2010.

Between 2010 and 2013, the rank of the CN Tower in the list of the world tallest structures is expected to drop considerably. At least seven structures currently under construction, all of which are scheduled to be completed by 2013, are expected to exceed the CN Tower's height of 553.33 metres once completed.

Controversy over the "world's tallest" title

Guinness World Records has called the CN Tower "the world's tallest self-supporting tower" and "the world's tallest free-standing tower". Although Guinness did list this description of the CN Tower under the heading "tallest building" at least once, The tower's official web site, however, still claims it is the "world's tallest building" as of 2008.

Although the CN Tower contains a restaurant, a gift shop, and multiple observation levels, it does not have floors continuously from the ground, and therefore it is not considered a building by the Council on Tall Buildings and Urban Habitat (CTBUH) or Emporis. CTBUH defines a building as "a structure that is designed for residential, business, or manufacturing purposes. An essential characteristic of a building is that it has floors."

Bobbie Rosenfeld Park

The open space between Rogers Centre and CN Tower was renamed as Bobbie Rosenfeld Park in 1991 following the completion of the Rogers Centre (known as SkyDome at the time). The city-owned park is mainly an open space covered by paving stone and planters. There are some trees and concession stands selling food and other items to tourists and visitors in the area.

There is a piece of artwork located along the south end facing Bremner Avenue by artist Susan Schelle called Salmon Run. The water fountain piece was not functional for many years, until it was restored in 2006.

EdgeWalk

The EdgeWalk is an amusement that opened on August 1st, 2011 and is Toronto's newest attraction. It is the world's highest full-circle, hands-free walk. Visitors are tethered to an overhead rail system and walk around the edge of the CN Tower's main pod above the 360 Restaurant on a 1.8 m metal floor. The attraction is closed throughout the winter season and during periods of electrical storms and high winds.

See also

  • Architecture of Toronto
  • List of tallest buildings in Toronto
  • List of masts
  • List of tallest buildings in the world
  • List of tallest buildings and structures in the world
  • List of tallest freestanding structures in the world
  • List of tallest structures in Canada
  • List of tallest towers in the world

External links



Source en.wikipedia.org/wiki/CN_Tower