Editing CN Tower (section)

=== Construction ===
[[File:Ripleys Aquarium Toronto map.png|thumb|Map of the area immediately surrounding CN Tower shortly before the opening of [[Ripley's Aquarium of Canada]] in 2013]]

The CN Tower was built by Canada Cement Company (also known as the Cement Foundation Company of Canada at the time),<ref>{{cite book|last=Barr|first=Elinor|date=2015|page=173|title=Swedes in Canada: Invisible Immigrants|publisher=[[University of Toronto Press]]|isbn=978-1442613744}}</ref> 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, {{convert|56000|t|ST LT|abbr=on|lk=on}} of [[soil|earth]] and [[shale]] were removed to a depth of {{convert|15|m|ft|1|abbr=on}} in the centre, and a base incorporating {{convert|7000|m3|cuyd|abbr=on}} of concrete with {{convert|450|t|ST LT|0|abbr=on}} of [[rebar]] and {{convert|36|t|ST LT|abbr=on}} of [[wire rope|steel cable]] had been built to a thickness of {{convert|6.7|m|ft|abbr=on}}. 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.<ref name="ieee"/>

To create the main support pillar, workers constructed a hydraulically raised [[Slip forming|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 {{convert|6|m|ft|abbr=on}} 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 {{convert|381|m|ft|adj=on}} tall [[Inco Superstack]] in [[Greater Sudbury|Sudbury]], built using similar methods.

The tower contains {{convert|40500|m3|cuyd|abbr=on}} 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 bob]]s hanging from it, observed by small telescopes from the ground. Over the height of the tower, it varies from true vertical accuracy by only {{convert|29|mm|in|1|abbr=on}}.<ref name="CNTower" /><ref name="ieee" />

[[File:CN Tower40 construction skycrane March 1975 01c.jpg|thumb|right|upright|Skycrane "Olga" lifting antenna segment]]

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.<ref name="ieee"/>

While still under construction, the CN Tower officially became the world's tallest free-standing structure on March 31, 1975.<ref name=records/>

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 {{convert|118000|t|ST LT|abbr=on}}.

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."<ref>{{cite book| last=Fulford| first=Robert| date=March 13, 1996| title=Accidental city: the transformation of Toronto| publisher=MacFarlane, Walter & Ross| page=32| isbn=978-0395773079}}</ref>

==== Phases of construction ====
<gallery mode="packed" heights="135px" style="text-align:left">
File:CN Tower footings 1973.jpg|Constructing the base, July 1973
File:Cntower1974.jpg|Brackets being raised, August 1974
File:CN Tower50 construction skycrane March 1975 01d.jpg|Helicopter lifting part of antenna, March 1975
File:CN Tower under construction (April 1975).jpg|Main pod construction, April 1975
File:CN Tower under construction.jpg|Nearing completion, December 1975
File:CN Tower 1976.jpg|Two months after opening, August 1976
</gallery>