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The idea of the CN Tower originated from the 1968 Canadian National Railway 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 would be 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 antenna 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, 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
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.
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 ST; 55,116 LT) 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 ST; 443 LT) of rebar and 36 tonnes (40 ST; 35 LT) 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).
Brackets being raised, August 1974.
The sky pod
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 ST; 116,136 LT).
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-metre (334.6 ft) 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 metres (1,135 ft), and the higher Sky Pod (formerly known as “Space Deck”) at 446.5 metres (1,465 ft), 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 level is seven stories, some of which are open to the public. Below the public areas — at 338 metres (1,108.9 ft) — is a large white donut-shaped radome containing the structure’s microwave receivers. The glass floor and outdoor observation deck are at 342 metres (1,122.0 ft). The glass floor has an area of 24 square metres (258 sq ft) and can withstand a pressure of 4,100 kilopascals (595 psi). The floor’s thermal glass units are 64 millimetres (2.5 in) thick, consisting of a pane of 25-millimetre (1.0 in) laminated glass, 25 millimetres (1.0 in) airspace and a pane of 13-millimetre (0.5 in) laminated glass. Some people experience acrophobia when standing on the glass floor and looking down at the ground 342 metres (1,122.0 ft) 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 metres (1,135.2 ft). The 360 Restaurant, a revolving restaurant that completes a full rotation once every 72 minutes, is at 351 metres (1,151.6 ft). When the tower first opened, it also featured a disco named Sparkles, billed as the highest disco and dance floor in the world.
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 three 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.
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