CN Tower

Since its completion in 1976, the 553.3-metre CN Tower in Toronto has enjoyed the distinction of being the world's tallest free-standing structure. It was built at a time when Toronto's skyline was exploding with tall buildings that interfered with reception of communications signals within the city. The tower's innovative design provided not only the tallest building in the world, but also a telecommunications hub that ensures clear reception for one of the fastest-growing cities in the world.

Construction of the Tower began on February 6, 1973. The first job was to remove over 56,000 metric tons of earth and shale to create a hole big enough for the 15-metre deep concrete and steel foundation. The Y-shaped foundation, which sits on a base of shale, contains over 7,000 cubic metres of concrete and 450 metric tons of reinforcing steel.

Above-ground operation began four months later. The design called for three concrete legs, with hollow hexagonal cores to house all of the electrical cables and water piping. Twenty-four hours a day, five days a week, concrete was poured into a massive mould or "slipform". As the concrete hardened, the "slipform" was moved upwards, gradually decreasing in size to produce the gracefully tapered contour of the Tower.

The concrete portion of the Tower was completed in February, 1974. The next step, which began the following August, was to build a seven-storey structure, called the SkyPod. This part of the tower would ultimately house two observation decks, the 360-degree revolving restaurant, a bar, and various other entertainment and technical areas. At its base, the SkyPod has a donut-shaped collar technically known as the radome, which protects the sensitive microwave communication dishes from the elements. Made of teflon-coated, fibreglass-rayon fabric, its balloon-like shape is the result of inflating the skin to five times its normal size, then maintaining constant air pressure.

Situated at the top of the SkyPod is the world's highest public observation gallery, called the Space Deck. The last piece that had to be put in place was the hollow, 102-metre steel antenna that extends above the Space Deck. Erecting the enormous antenna presented a special challenge to the designers of the Tower. The solution was a giant Sikorsky helicopter, "Olga", which lifted into place, with remarkable precision, the 39 pieces of the antenna - the heaviest of which weighed more than seven metric tons.

The 36th piece of the antenna mast was placed on March 31, 1975. On that day, the CN Tower surpassed Moscow's Ostankino Tower, to become the world's tallest free-standing tower. On hand to record the milestone was Ross McWhirter, then editor of The Guinness Book of World Records of London, England. In December of 1995, the CN Tower was acknowledged as one of the Seven Wonders of the Modern World by the American Society of Civil Engineers.

Today, the CN Tower welcomes approximately 1.7 million Canadian and international visitors annually.