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Seven Wonders of the Modern World:
CN Tower

The CN Tower, located in downtown Toronto, Ontario, Canada, is a communications and observation tower standing 1,815 ft. 5 inches tall. It surpassed the height of the Ostankino Tower while still under construction in 1975, becoming the tallest free-standing structure on land in the world. On September 12, 2007, after holding the record for 31 years, the CN Tower was surpassed in height by the still-under-construction Burj Dubai. It remains the tallest free-standing structure in the Americas and the signature icon of Toronto's skyline, attracting more than two million international visitors annually.

Local residents wished to retain the name CN Tower, the abbreviation is now said to expand to Canada's National Tower rather than the original Canadian National Tower; however, neither of these are commonly used.

History and Construction

The concept of the CN Tower originated from a 1968 Canadian National Railway desire to build a large TV and radio communication platform to serve the Toronto area, as well as demonstrate the strength of Canadian industry and CN in particular. These plans evolved over the next few years, and the project became official in 1972.

The tower would have been part of Metro Centre, a large development south of Front Street on the Railway Lands, a large railway switching yard that was being made redundant by newer yards outside the city.

Construction on the CN Tower began on February 6, 1973 with massive excavations at the tower base for the foundation. By the time the foundation was complete, 56,000 tonnes of dirt and shale were removed to a depth of 15 metres in the centre, and a base incorporating 7,000 cubic metres of concrete with 450 tonnes of rebar and 36 tonnes of steel cable had been built to a thickness of 6.7 metres.

Structure

The CN Tower consists of several substructures. The main portion of the tower is a hollow concrete hexagonal pillar containing the six elevators, stairwells, and power and plumbing connections. On top of this is a 102-metre tall metal broadcast antenna, carrying TV and radio signals. There are two visitor areas: the main deck level located at 346 metres, and the higher Sky Pod at 446.5 metres, just below the metal antenna. The hexagonal shape can be seen between the two areas; however, below the main deck, three large supporting legs give the tower the appearance of a large tripod.

Lighting

The CN Tower was once lit at night with incandescent lights, but they were removed in 1997 because they were expensive and inefficient to repair. In June 2007, the tower was outfitted with 1,330 super-bright LED lights inside the elevator shafts, shooting up over the "bubble" and upward to the top of the tower's mast to light the tower from dusk until 2 a.m. The tower changes its lighting scheme accordingly to holidays and major events.

Programmed from a desktop computer with a wireless network interface card, the LEDs use less energy to light than the previously used incandescent lights (10 per cent less energy than the dimly lit version and 60 per cent less than the brightly lit version). The estimated cost to use the LEDs is $1,000 per month.

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