CN Tower

About

The CN Tower is a 553.3-metre-high (1,815.3 ft) communications and observation tower in Toronto, Ontario, Canada. Completed in 1976, it is located in downtown Toronto, built on the former Railway Lands. Its name "CN" refers 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 the government's real estate portfolio. The CN Tower held the record for the world's tallest free-standing structure for 32 years, from 1975 until 2007, when it was surpassed by the Burj Khalifa, and was the world's tallest tower until 2009 when it was surpassed by the Canton Tower. It is currently the tenth-tallest free-standing structure in the world and remains the tallest free-standing structure on land in North America, the Western Hemisphere, and the entire Western world. 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. It is a signature icon of Toronto...

The original concept of the CN Tower was first conceived in 1968 when the Canadian National Railway wanted to build a large television and radio communication platform to serve the Toronto area, and to 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 after the opening of the MacMillan Yard north of the city in 1965 (then known as Toronto Yard). 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).

As Toronto grew rapidly during the late 1960s and early 1970s, multiple skyscrapers were constructed in the downtown core, most notably First Canadian Place, which houses Bank of Montreal 's head offices. The reflective nature of the new buildings reduced the quality of broadcast signals, requiring new, higher antennas that were at least 300 m (980 ft) tall. The radio wire is estimated to be 102 metres (335 ft) long in 44 pieces, the heaviest of which weighs around 8 tonnes (8.8 short tons; 7.9 long tons).

At the time, most data communications took place over point-to-point microwave links, whose dish antennas 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 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 Top lies today. As the design effort continued, it evolved into the current design with a single continuous hexagonal core to The Top, 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 Space Deck (currently named The Top, and originally dubbed 'Bud's Bubble' named after Bud Andrews) was not part of the plans until later. Bud Andrews (president of the Metro Centre Developments Group) 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. Also around this time, it was realized that the tower could become the world's tallest free-standing structure to improve signal quality and attract tourists, and plans were changed to incorporate subtle modifications throughout the structure to this end.

The CN Tower was built by Foundation Company of Canada with Canron of Etobicoke handling the steel and antenna fabrication work.

Construction 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 (62,000 short tons ; 55,000 long tons ) of earth and shale were removed to a depth of 15 m (49.2 ft) in the centre, and a base incorporating 7,000 m 3 (9,200 cu yd) of concrete with 450 t (496 short tons; 443 long tons) of rebar and 36 t (40 short tons; 35 long tons) of steel cable had been built to a thickness of 6.7 m (22 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 create the main support pillar, workers constructed a hydraulically raised slipform at the base. This was a fairly unprecedented engineering feat on its own, consisting of a large metal platform that raised itself on jacks at about 6 m (20 ft) per day as the concrete below set. Concrete was poured Monday to Friday (not continuously) by a small team of people until February 22, 1974, at which time it had already become the tallest structure in Canada, surpassing the recently built 381-metre (1,250 ft) tall Inco Superstack in Sudbury, built using similar methods.

The tower contains 40,500 m 3 (53,000 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 mm (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 Top 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.

While still under construction, the CN Tower officially became the world's tallest free-standing structure on March 31, 1975.

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 one 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 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 t (130,000 short tons; 116,000 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."

Helicopter lifting part of antenna, March 1975

The CN Tower opened on June 26, 1976. The construction costs of approximately CA$ 63 million ($333 million in 2025 dollars) were repaid in fifteen years.

From the mid-1970s to the mid-1980s, the CN Tower was practically the only development along Front Street West; it was still possible to see Lake Ontario from the foot of the CN Tower due to the expansive parking lots and lack of development in the area at the time. As the area around the tower was developed, particularly with the completion of the Metro Toronto Convention Centre (north building) in 1984 and 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 Union Station railway and subway 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 early 21st century, as well as the 2013 opening of the Ripley's Aquarium by the base of the tower.

When the CN Tower opened in 1976, there were three public observation points: The Top (then known as the Space Deck) that stands at 447 m (1,467 ft), the Indoor Observation Level (later named Main Observation Level) at 346 m (1,135 ft), and the Outdoor Observation Terrace (at the same level as the Glass Floor) at 342 m (1,122 ft). One floor above the Indoor Observation Level was the Top of Toronto Restaurant (now named "360 The Restaurant at the CN Tower"), which completed a revolution once every 72 minutes.