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	The CN Tower

        the CN division to build a structure that has been a great help to communications. It was the CN Hotels that built the CN Tower in Toronto.

About The CN Tower
When the CN Tower project was first announced, it seemed almost impossible. At the time, Toronto was reeling from the effects of a major building boom, which had dotted its skyline with monolith buildings. The highest soared a staggering 750 feet. Now CN boasted that it would build a-structure more than twice that height-possibly as high as the world-leading Ostankino Tower in Moscow, maybe even higher. Toronto's ascending skyline had introduced a problem for existing communications systems. Pre-skyscraper transmission towers were simply not high enough to broadcast over the new obstacles. As a result, signal-bounce, or ghosting, was becoming increasifigly severe.

Fortunately, the Tower designers had the foresight to realize that the new buildings were only the beginning. In the future, they predicted downtown buildings would grow to 1000 feet and more. As a result, the proportions for the Tower were set, with its microwave receivers at 1 1 00 feet and its antenna topping off at the 1815 feet mark.

Its location, at the front door of the city, also determined the need for the Tower to be more than a communications structure. It would have to be attractive. In addition, as an integral part of the populated downtown landscape, the incorporation of public facilities was as sumed from the onset.

While several individuals figured prominently, no one person can be given the credit for the total design and construction of the Tower. From the initial proposal in 1968, to the final model in 1972 and eventually the Tower itself, which was completed in 1976, the project involved a consortium of experts from around the world. Initially, the plan called for three towers linked with structural bridges. Prohibited by construction and financial limitations, the design gradually evolved into a single Tower.

Because it is the first of its size and type, the CN Tower broke new ground in many of its answers to engineering challenges. No one had ever built a structure which would rise as high off the ground. Consequently, never had a foundation been devised which would bore so deeply into the ground. This in itself represented many engineering breakthroughs. First, beginning in 1972, an elaborate soil testing operation was put into effect to assess the condition of the bedrock and its reaction to changes in hydros tatic pressure. Equipped with this information, the Above: Inside the CN Tower's 'Sparkles' nightclub. At right: The CN Tower is the apex, so to speak, of a downtown Toronto CN complex. At the base of the Tower can he seen the CN Convention Center. and CN Hotels' semi-circular L'H6tel. The Tower's microwave equipment is housed in the 'doughnut' collar under its rotunda, which is below the Tower's long needle antenna. designers developed foundation Specifics which would assure the Tower's stability. On 6 February 1973, construction crews moved in. Over 62,000 tons of earth and shale were removed by giant backhoes, digging 50 feet into the ground. Next, the 22-foot thick concrete and steel foundation was erected on a base of handand machine-smoothed shale. When finished, the Y-shaped foundation contained 9200 cubic yards of concrete, 500 tons of reinforcing steel and 40 tons of thick, tensioning cable.

Barely four months after construction began, all was in readiness for the above-ground operation. First, 12 steel and wood brackets, weighing 350 tons had to be systematically lifted, inch by inch, up the sides of the Tower. The task required 45 hydraulic jacks and miles of steel cable. Once in place, one-fifth of a mile above the ground, workmen bolted the brackets to tensioned steel bars and placed concrete in the wooden frames. Next, a three-foot high concrete compression ring was placed around the outside edge of the brackets to make one strong unit.

Twenty-four hours a day, five days a week, concrete was poured into a massive mold or 'slipform.' As the concrete hardened, the 'slipform,' supported by a ring of climbing jacks that were powered by hydraulic pressure, moved upwards, gradually decreasing in size to produce the gracefully tapered contour. To assure its strength and quality, the concrete was mixed on the site, continuously checked and tested, then reinforced with a unique system of post-tensioning which produced a strength of 6000 pounds per square inch. The legs of the Tower are hollow, as is its hexagonal core which houses all of the electrical cables and water piping. The concrete portion of the Tower was completed in February 1974. Situated at the top of the concrete is the 'World's Highest Public Observation Gallery,' the Space Deck. All that extends above it is the 335 foot steel antenna. A two-story structure, the Space Deck was created by cantilevering a concrete platform around the top edge of the Tower. Next, from the overhang of its roof, a glass wall was suspended, banking inwards at the bottom and completely enclosing the upper story in glass. An enclosed elevator transports visitors to the glass-surrounded viewing balcony. In August 1974, work began on the seven-story Skypod which would ultimately house two observation decks.

Intrinsic to the construction of the CN Tower was the mighty crane, which over its four years of round-the-clock service lifted some 50,000 tons of material and machinery. With the completion of the Skypod, however, its usefulness also ended. Fortunately, the answer to dismantling the crane and erecting the enormous antenna came in one solution-'Oiga', a giant Sikorsky helicopter. Reduced to eight sections, the crane was brought swiftly down to earth and the 39 pieces of the antenna (the heaviest weighing eight tons) were lifted into place with remarkable precision.

For many, the purpose behind the familiar donutshaped collar at the base of the Skypod is a mystery. Technically, it is known as a radorne and without its protection, the sensitive microwave dishes which receive the transmissions would be ravaged by the elements. Made of teflon-coated, fiberglass-rayon fabric, it is strong enough to hold the weight of an average adult male, yet measures a mere one-thirty-second of an inch thick. Its balloon-like shape is the result of inflating the skin to five times its normal size then maintaining constant air pressure. A second radome protects the full length of the antenna. Designed to prevent ice buildup, it's made of 1 1/2-inch thick glass-reinforced plastic.

In September 1973, the Tower became the tallest structure in Toronto and by January 1974, it was unrivalled in Canada. The top of the 'slipform' concrete, 1464 feet, was reached on 22 February 1974 and to that was added a special 16-foot concrete extension which would serve as a base for the transmission mast. With the placing of the 36th piece of the antenna mast, at 9:52 am on 31 March 1975, the CN Tower surpassed Moscow's Ostankino Tower to become the 'World's Tallest Freestanding Structure.' On hand to record the milestone was Ross McWhirter, then editor of the Guinness Book of World Records, of London, England.

During the Dedication Ceremony on 1 October 1976, presided over by Prime Minister Pierre E Trudeau, a time capsule was placed in the wall on the Indoor Observation Deck. It will be opened on 1 October 2076. Only 17 other self-supporting structures in world history have held the title tallest. At 1815 feet 5 inches, the CN Tower brought the title to Canada for the first time.

The tower has been the site of many height-related records. As a promotion for the Egg Marketing Board, Patrick Ballie, 17, of Toronto, beat the Guinness Book record for egg dropping by over 500 feet. He dropped a Grade A egg, unscathed, from 1120 feet into a specially-designed net, on 27 July 1979. The first person to parachute off the Tower was a member of the construction crew, Bill Eustace, aka 'Sweet William', on 9 November 1975: for his daring, he was discharged. Hollywood stuntman, Dar Robinson, made two jumps from the Level 5 of the Skypod. The first was made with a parachute on 21 Septernber 1979, while shooting a scene for the movie 'Highpoint.' The second, which was conducted for the television show 'That's Incredible,' involved only wire cables to break the fall. To mark the Tower's fourth birthday' two mountain climbers, David Smart, 17, and Gerald Banting, 30, attempted an unauthorized climb up the outside of the Tower on 23 June 1980. They made it halfway before backing down. Once a year, the Tower's staircase are opened to the public, to raise funds for the United Way. As a rule, this is an autumn event, with climbers starting their ascent just above the main lobby and concluding at the Skypod. Many records have been set and broken at this event.

Not only is the CN Tower the tallest freestanding structure in the world but it is also probably the safest. Everything from the forces of nature to man-made disasters were considered in the formulation of its design.

The upper reaches of the Tower are continually bombarded by turbulent winds. To test the Tower's design against this and other extreme wind forces, a special 'wind tunnel,' the first of its kind in North America, was created at the University of Western Ontario. Within the tunnel, construction models of the Tower were tested under conditions which simulated the maximum wind strength based on the analysis of a 1000-year time frame. The wind speed reached in the tunnel measured 130 mph.

When plans for the Tower were concluded, their wind resistance factor was established at 260 mph. In addition to the structure, the windows of the Tower, which are armor-plated, were carefully designed for extreme wind tolerance. The outside panes are three-eighths of an inch thick and the inside panes are one fourth of an inch thick. They can withstand internal or external pressure in excess of 120 pounds per square inch. The ability of the CN Tower to wobble under extreme wind conditions was not overlooked either. Two 10-ton swinging counterweights, mounted on the antenna, ensure that the intentional sway of the Tower never exceeds acceptable levels. There's cer tainly not a more exceptional or perhaps safer place in the city from which to view the awesome spectacle of a raging thunderstorm.

Though the tower is struck by lightning about 60 times a year, there is never any danger. Every surface which could possibly attract lightning is attached to three copper strips running down the Tower, connected to 42 22-foot grounding rods buried 20 feet below the surface.

Despite the Tower's extreme height (at left), it is within 1. 1 inches of absolute plurnb-1e, perfectly vertical. One danger designed against in the Tower's construction was 'torsional oscillation'-the tendency of tall structures to twist with the rotational motion of the Earth, just as water twists down a sink drain. Such 'tvAsting,' if unprepared for, would no doubt cause a catastrophe.

To ensure against the possibility of ice or snow building up and falling to earth, every place where ice is likely to collect (such as changes in the roof contour) is ice proofed, either with de-icing tracers or sheathed with a cling-proof plaster surface.

Fire prevention has always been a Tower priority. Smoking is not permitted on most of the public areas. All of the construction materials used were either fire-proof or fire resistant. Even the interior furnishings selected are as fireproof as possible. In the unlikely event of fire, emergency generators, located in the basement, are capable of supplying power for the elevators and other systems, including the emergency fire pumps which can pump water to the top of the Tower at a rate of 500 gallons a minute. A reservoir of water is also maintained in the Skypod itself.

Since its opening, the Tower has boasted a perfect record thanks to its carefully conceived design, interior monitoring system, and diligent security force.

Next to the Tower itself, the 'Universal Man,' a 10-foot high by 18-foot wide bronze sculpture, is one of the popular CN Tower subjects for photographers. Created by Canadian artist Geraid Gladstone, it is one of the largest bronze castings ever made and was air-freighted from the Singer Foundry in England in nine sections.

In addition to this impressive artwork (which signifies earthbound human energies reaching towards a higher knowledge through communications), the outdoor plaza also features a mounted 1000-pound piece of the great Matterhorn mountain, which was presented to the Tower in April 1981 in conunction with the Salute to Switzerland exhibition.

Today, both public and experts from the field of construction come to marvel at the Tower which stands as a testament to human ingenuity and achievement.

Its official owner is CN Tower Limited, a subsidiary of Canadian National and part of the CN Hotels, but few will argue when one says it really belongs to Canada. While some day, some place, someone might build a Tower higher, the significance of the CN structure and its tremendous contributions will never be diminished.

The CN Tower

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

		© Ragnar Torfason 2006 March 28