Title:
CN Tower

Description:
Construction View of downtown Toronto from the CN TowerConstruction on the CN Tower started on February 10, 1973 with massive excavations at the tower base for the foundation. By the time the foundation was complete, 56,000 tonnes (61,729 short 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) of rebar and 36 tonnes (40 short tons) of steel cable had been built to a thickness of 6.7 metres (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 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). The CN Tower as seen from its baseIn August of the same year, 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 not only used to create the brackets which support the main level, but also 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 their examples 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, as 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 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." [edit] Opening The CN Tower opened to the public on June 26, 1976, although the official opening date was October 1. The construction costs of approximately CDN$63 million ($330 million in 2005) were repaid in fifteen years. Canadian National Railway sold the Tower prior to taking the company public 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 and Rogers Centre (known as the SkyDome before 2005), the former railway "wasteland" disappeared 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. 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 recent boom in condominium construction. Canada

Author:
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Tags:
Canada,