The CN Tower

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CraigBaird

It is one of the most recognizable structures not only in Canada, but worldwide. The CN Tower is something many Canadians feel pride over, but what was involved in its construction? Today, I am looking at the building at the structure that was once the tallest in the world.

In the 1960s, Toronto began to see skyscrapers pop up across the city. While it showed the city moving into the modern era, it also caused issues with communications for existing transmission towers. They were not high enough to broadcast over existing buildings and the signals bounced off buildings, creating poor television and radio reception. The city had been expanding heavily, reaching a population of one million in the 1950s, and 2.5 million by 1970.

The original idea for the CN Tower came about in 1968 when the Canadian National Railway was looking to build a television and radio platform that would serve the Toronto area. Another goal of the tower would be to demonstrate the strength of not only the Canadian National Railway, but also Canadian industry itself. The entire project was going to be part of the Metro Centre proposal, a $1 billion joint plan between Canadian Pacific and Canadian National to convert the surplus railway lands of that area into the largest revitalization scheme ever conducted in North America. The entire site covered 190 acres, made up of storage buildings and roundhouses. The plan was to have apartment buildings that would house 20,000 people, as well as terraced homes, 418,000 square metres of new office space and 55,750 square metres of commercial property. The Metro Centre would also separate automobile, pedestrian and transit traffic through various levels. The most important piece of the entire development was a free standing structure that would rise above everything.

In an article from the time, the Toronto Star wrote, quote:

“This would be Canada’s tallest structure of any kind and one of the tallest self-supporting structures in the world.”

Work on Metro Centre was scheduled to begin in 1969, but even with approval from the city, the planning stalled, and delays would begin until in 1975, when the plan was scrapped. One item would remain from the plan, the CN Tower. By this point, Canadian Pacific had dropped out of the project, leaving only Canadian National.

There was worry from aviation experts that the tower would be a hazard for aircraft using the Hamilton Airport and the Toronto International Airport. The Canadian Owners and Pilots Association would say in a press release, quote:

“Sooner or later an aircraft is bound to strike it, possibly killing people in the tower, and on the ground, as well as those in the aircraft.”

Another concern was migratory birds, since Toronto is part of a vital travel route for birds. David Crombie, a city alderman and future mayor, tried unsuccessfully to have the height of the tower cut by two-thirds in response to the concerns.

For the next few years, plans for the tower would change and evolve until it received the official go-ahead in 1972. The final design came from John Andrews, Webb Zerafa, E.R. Baldwin and Menkes Housen.

The first step was to analyze and test the soil at the site of construction to assess the condition of the bedrock and determine how it would react to changes in hydrostatic pressure. Upon testing the bedrock, the engineers soon realized that they would be able to make the CN Tower the tallest building in the world.

Contracted to the Canada Cement Company, construction began on Feb. 6, 1973 with a massive excavation of the tower base for the purpose of constructing the foundation. In order to accomplish this, 56,000 tons of earth and shale were removed to a depth of 49.2 feet at the centre.

Once the base was excavated, the base was constructed using 7,000 cubic metres of concrete, with 450 tons of rebar and 3 tons of steel cable, built with a thickness of 22 feet. Construction of this base was extremely fast, taking only four months in total.

The next step was the construction of the main support pillar for the tower. This was done using an engineering feat that had never been attempted before. Using a raised slipform at the base, this large metal platform would raise itself on jacks 20 feet per day as the concrete below began to set. Concrete was poured from Monday to Friday, with a small team of people supervising. To verify the vertical accuracy of the tower, massive plumb bobs were hung from the tower and they were observed using telescopes on the ground. This allowed the accuracy of the tower to vary by only 29 millimeters over the height of the tower. As the slipform rose, with the hardening of the concrete, it would slowly decrease in size, to produce the tapered contour of the finished tower.

On Feb. 22, 1974, the structure had become the tallest structure in Canada, passing the Inco Superstack that was built in Sudbury.

To construct the concrete portion of the tower, 40,500 cubic metres of concrete was used.

The next step was to construct the main level. This began in August of 1974. Through the use of 45 hydraulic jacks that were attached to a temporary steel crown on top of the tower by cables, 12 steel brackets were raised. It took a week for the jacks to crawl up to the top of the tower to their final potion. These brackets would support the main level, which was built of concrete poured over a wooden frame, attached to rebar at the lower level of the deck, then reinforced with large steel compression. The construction of this main level was described as building a seven-storey building 1,100 feet in the air.

At the base of the main level is a donut shaped structured called the radome. It protects the sensitive transmission equipment inside from the elements. It is a Teflon-coated fiberglass fabric that only measures one-32nd of an inch but is strong enough for an adult male to stand on. It balloons out to a size five times its normal size to maintain constant pressure.

The most recognizable part of the structure is the large antenna that rises above the main platform. At first, the plan was to raise this by crane but the United States sold a Sikorsky S-64 Skycrane to civilian operators, and the plan was changed to use that helicopter. The helicopter, which gained the nickname of Olga, removed the crane and then flew the antenna to the top of the structure in 36 sections. During the removal of the crane used during assembly, the helicopter became tethered to the tower when a section of boom seized in place. Workers scrambled to free the aircraft before it ran out of fuel. With only 14 minutes to spare, workers managed to melt the metalwork that was causing the problem.

The flights to put the antenna on top of the tower were a tourist attraction and the schedule was printed in the newspaper so people could watch. Thanks to the use of the helicopter, months of construction time was saved. This allowed the construction of the antenna to take only three and a half weeks, rather than six months. The heaviest piece of the antenna that had to be moved into place was eight tons.

On April 2, 1975, 26 months after construction started, the tower was topped off. At this point, it had captured the height record that it would hold for decades. It had officially become the world’s tallest free standing structure on March 31, 1975, rising to 1,815.3 feet. If it was a typical building with floors, it would have 147-storeys. Ross McWhirter, the editor of the Guinness Book of World Records was on hand for the event. Paul Mitchell, a foreman on the project, had the honour of topping off the tower.

When construction had finished, the total mass of the structure was an astounding 118,000 tons. Construction cost $63 million, or $278 million in today’s funds. This was repaid in only 15 years. In all, 1,537 people worked on the job, 24 hours a day, five days a week. For the workers, pay varied depending on where they worked. Iron worker Larry Porter, made $8.01 an hour, $1 more than what the individuals 1,000 feet below made. Today, that hazard pay would be $42 per hour. At the time, there was little safety in terms of working at these types of heights. Most contractors had a safety harness that hooked around the chest and arms. If a contractor slipped, they would have most likely slipped out of the harness. Even with this, only one person died during construction of the CN Tower. Jack Ashton, a consultant with the concrete inspection company, was hit in the head by a falling piece of plywood, which broke his neck and killed him instantly. No one died from working at the tall heights due to falling.

On June 26, 1976, the structure was opened to the public in a ceremony that including Finance Minister Donald Macdonald, stilt walkers, and several others. Roger Tickner, a seven-foot-tall kitchen equipment manager and 6-foot-three Paula Lishman, activated the exterior lights as the clock hit midnight.

Paul King would write for the Toronto Star, quote:

“The only place higher that man’s ever stood on a stationary base, except for a mountain peak, is the moon. On Earth, man can climb no higher in any enclosed structure.”

In 1979, Dar Robinson, a stuntman, jumped off the CN Tower for a scene in the movie Highpoint, for which he was paid $250,000, or $850,000 today. He then jumped again in 1980 as part of a personal documentary.

On June 26, 1986, to mark the ten year anniversary of the tower’s opening, Dan Goodwin, a high-rise firefighter, in a sponsored publicity event, used his hands and feet to climb the outside of the tower. He performed the feat twice in one day.

On Sept. 12, 2007, three decades after it took the record, the Burj Khalifa surpassed the CN Tower as the world’s tallest free-standing structure. Holding the title for 34 years, only the Eiffel Tower, the Empire State Building and the Great Pyramid of Giza spent more time as the world’s tallest.

The CN Tower remains the tallest freestanding structure in the Western Hemisphere, almost 100 feet taller than the Willis Tower in Chicago and 12 feet taller than One World Trade Center in New York. The tower continues to hold the record for the world’s highest public observation gallery, the world’s highest glass floor paneled elevator, the world’s longest metal staircase, the world’s highest and largest revolving restaurant, the world’s highest bar and the world’s highest wine cellar.

The tower is also one of the safest in the world for its height. One example is the elevator control. In the event of a power failure, five diesel generators supply emergency power within 10 seconds. If an elevator exceeds a certain speed or starts to fall, the most distance it can fall is 1.83 metres due to devices that jam the elevator into the elevator shaft. Wind resistance is another important feature of the building. While the CN Tower will sway in extreme winds, it can handle up to 418 kilometres per hour. Inside the antenna, two ten-ton swinging counterparts, ensure that the tower never exceeds acceptable conditions. Armour plated windows also prevent the windows from breaking in extreme wind.

In 1995, the CN Tower was listed as one of the seven wonders of the modern world by the American Society of Civil Engineers.

In non-COVID times, 1.5 million people a year visit the tower. It also serves as a workplace for 500 people throughout the year, and its telecommunications antennas serve 16 Canadian television and FM radio stations. Each year, the tower earns roughly $72 million in revenue.

Information comes from Canadian Encyclopedia, Wikipedia, CN Tower, TDotShots.com, Dozr Hub, IEEE Canada, Spacing.ca, CBC,

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