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. 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. 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 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 SkyPod lies today. As the design effort continued, it evolved into the current design with a single continuous hexagonal core to the SkyPod, 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 (later renamed SkyPod) 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 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 Canada Cement Company (also known as the Cement Foundation Company of Canada at the time),[19] a subsidiary of Sweden's Skanska, a global project-development and construction group. 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 m3 (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.[18] 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 Inco Superstack in Sudbury, which was built using similar methods.
| 1 | Builder | : | Canada Cement Company | ||
|---|---|---|---|---|---|
| 2 | construction | : | February 6, 1973 | ||
| 3 | old name | : | Canadian National | ||
| 4 | Length | : | 1,815.3 ft | ||
| 5 | Height | : | 553 m | ||
| 6 | Width | : | 66.6m (218.4 ft.) |
Construction of Cologne Cathedral began in 1248 but was halted in the years around 1560, unfinished. Work did not restart until the 1840s, and the edifice was completed to its original Medieval plan in 1880.
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