CN Tower – Canada, North America

This slideshow requires JavaScript.

The CN Tower is a communications and observation tower in Downtown Toronto, Ontario, Canada. Standing 553.33 metres (1,815.4 ft) 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.
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 metres (984 ft) 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 kilometres (25 mi) to the north, 60 km east of Toronto, in Oshawa, and from several points on the south shore of Lake Ontario, 48 kilometres (30 mi) 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 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, 56,000 t (61,729 short tons; 55,116 long tons) of dirt and shale were removed to a depth of 15 metres (49.2 ft) in the centre, and a base incorporating 7,000 cubic metres (9,156 cu yd) of concrete with 450 tonnes (496 short tons; 443 long tons) of rebar and 36 tonnes (40 short tons; 35 long tons) of steel cable had been built to a thickness of 6.7 metres (22.0 ft). 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 metres (19.7 ft) 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 40,500 cubic metres (52,972 cu yd) 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 millimetres (1.1 in).
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 118,000 tonnes (130,073 short tons; 116,136 long tons).
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”.
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 ($243 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 (renamed Rogers Centre in 2005), 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.

Advertisements

Posted on December 21, 2011, in North America and tagged , . Bookmark the permalink. 1 Comment.

Comment about this place

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: