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The CN Tower

CN Tower, Toronto, Ontario, Canada

It is fitting that television, the technological wonder that profoundly changed life in the 20th century, spurred the building of the era's tallest freestanding structure. In the late 1960's, Toronto's soaring skyline began to play havoc with signals from conventional transmission towers. Signals bouncing off the city's skyscrapers produced a number of problems, including the annoying phenomenon of "ghosting" on television sets. Weaker signals competed with stronger ones, giving viewers the effects of watching two programs at once. To improve the situation, Canadian National Railways, or CN, proposed building a transmission tower that would stand head and shoulders - and then some - above Toronto's tallest buildings.

A Toronto firm prepared the initial design, enlisting the aid of engineering experts the world over. Their original plan showed three towers linked by structural bridges. Gradually the design evolved into a single 1,815.5-foot-tall tower comprised of three hollow "legs."

Foundation work began in 1973. Giant backhoes excavated more than 62,000 tons of earth and shale to a depth of 50 feet from a along the shore of Lake Ontario in Toronto harbour. Next, prestressed concrete and reinforced steel were arranged in a Y-shaped pattern 22 feet thick. Each hollow leg of the Y would carry its fair share of the tower's 130,000-ton burden.

The foundation took only four months to complete. The tower itself presented a challenge of height never before met by the technique of poured concrete. To meet that challenge, engineers designed a huge mold known as a slip form. Concrete was poured 24 hours a day, five days a week, and as it hardened, the mold moved upward by means of a ring of hydraulic jacks. The ascending slip form gradually decreased in girth to give the tower its tapering shape.

When the tower reached the 1,100-foot mark, the builders made preparations for the SkyPod, a seven-story structure housing two observation decks, a revolving restaurant, a nightclub, and broadcasting equipment. The SkyPod is anchored by 12 steel-and-wooden brackets that were slowly pushed up the tower by 45 hydraulic jacks. Concrete formed the SkyPod's "walls," and a doughnut-shaped ring, called a radome, was added to its base to protect the delicate microwave dishes receiving radio and television transmissions. The SkyPod is reached by four high-speed, glass-fronted elevators whose rapid rise simulates a jetliner's takeoff, unless weather conditions call for a much slower ascent.

The concrete tower continues above the SkyPod, ending at the Space Deck 1,465 feet up. The Space Deck receives support from cantilevers extending out of the concrete section beneath it. After a 58-second elevator ride from the SkyPod below, visitors can enjoy breathtaking vistas from a glass-enclosed balcony. On a clear day they might be able to glimpse sites 75 miles away.

For the last phase of construction, a Sikorsky Skycrane helicopter arrived to install the tower's 335-foot communications mast. One by one the helicopter lifted about 40 seven-ton sections of the mast to the top of the tower, where workers braved blustery March winds to receive them. When the sections were in place, they were secured by a total of 40,000 bolts. Afterward, the entire mast was covered by a fiberglass-reinforced sheathing to prevent icing.

Of interest to Torontonians since construction began, the CN tower gained additional fans with the arrival of the helicopter. Nicknamed Olga, its daily schedule was printed in newspapers, and changes were announced as breaking news on radio and television. With Olga, the mast assembly took a little longer than three weeks; without Olga, the job would have lasted six months.

Completed in 1975, the tower had cost $57 million to build, a bargain compared with other modern wonders. It also boasted incredible statistics of precision and safety. During construction, surveyors' transits up to a thousand feet away focused on optical plumbs mounted on the slip-form base. The constant surveillance kept the structure an incredible 1.1 inches within plumb.

Engineers established a wind-tolerance standard for the tower of 260 miles an hour, a level well above nature's most extreme demands. Counterweights on the antenna correct for wobble in high winds. Because the tower is an easy target for lightning, copper grounding wires were installed. As a result, visitors can safely view some 75 spectacular strikes a year.

The CN Tower is a work in progress. In recent years the tower gained two new elevators to accommodate an increase in visitors. To accomplish this, the 2,579-step metal staircase was moved to the interior of the structure. In addition, a glass floor was added to the SkyPod's observation deck. Brave visitors, the majority not surprisingly children, inch out over the visual void. More often than not the experience is pronounced, "Awesome!"

Almost twice as tall as the Eiffel Tower and more than three times the height of the Washington Monument, the CN Tower has taken proud ownership of Toronto's skyline, while exorcising the ghosts from its TV sets.

Source:  The Wonders of the World, National Geographic Society

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