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CN Tower: Canada Rising

Jeffrey Syken

Toronto went through its first building boom in the late 1920s / early 1930s, during which time the number of high-rise buildings in the city significantly increased. There was a lull in construction between 1932 and 1964, with only a single building over 300-feet-tall built. Toronto then experienced a second, much larger building boom, which was at its peak between 1967 and 1976. In fact, when it was topped-off in 1967, the TD Bank Tower was the fourteenth tallest building in the world. However, the high-rise city Toronto was fast becoming was highly problematic for FM radio and TV signals. Typically, there were about a dozen tall buildings en route to the average Toronto home, resulting in a dozen or so “bounced” signals arriving nearly simultaneously; producing static and loss of stereo effect for FM listeners and “ghost” pictures on TV screens. Something had to be done.

Allowing broadcast signals to start from a point so high they could pass over neighboring buildings for a “clear shot” at rooftop antennas was the key. Thus, the idea for a tower, 1,815-feet tall (553m) with broadcast facilities, was born out of necessity. What would become the CN Tower was actually part of a much larger project called “Metro Centre” - a massive office and entertainment development planned for the Toronto railway yards. A co-development between Canadian National (CN) and Canadian Pacific (CP), Metro Centre was originally planned in the early 1960s as a $1.5 billion residential-commercial-transportation complex on 190 acres southwest of Union Station. It was to be the largest single downtown redevelopment ever undertaken in North America. Alas, political opposition hindered the grand plans for Metro Centre, leaving the broadcast tower as the only viable element.

A joint-venture between CN, CP and the Canadian Broadcasting Company (CBC) was formed to build the tower, but CP and the CBC backed out, leaving a determined CN to build it. Construction began in December 1972, before a building permit was even issued. The shape of an inverted golf-tee, the tower’s Y-configuration and tapering legs would help offset the dynamic forces acting against it (i.e. wind). To give the reinforced concrete structure tensile strength, post-tensioning would be used to good effect, allowing the tower to bend, but not break. The steel antenna at the pinnacle would be assembled in thirty-nine pieces, all of which were airlifted into place dramatically by a Sikorsky Skycrane helicopter named “Olga.” Two “Hoola-Hoop” tuned dampers would help stabilize the antenna. To construct the superstructure of the tower, an innovative “slip-form” was used. The result was the tallest free-standing structure in the world from 1976 to 2007, when the Dubai’s Burj Khalifa took the title. Even so, the CN Tower is still the tallest freestanding structure in the Western Hemisphere and it’s still the exclamation point on Toronto’s skyline.

This course includes a multiple-choice quiz at the end, which is designed to enhance the understanding of the course materials.

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