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The SkyPod of CN Tower

The CN Tower inspires a sense of pride, inspiration and awe for Canadians and tourists alike. However, its origins are firmly rooted in practicality.

During Toronto's building boom in the early 70's, a serious problem was developing. People were experiencing poor quality television. And it wasn't just the sitcoms. The pre-skyscraper transmission towers of Toronto stations were simply not high enough anymore.

In 1972, Canadian National (CN) set out to build a tower that would solve the communications problems, serve as a world class entertainment destination, and achieve international recognition as the world's tallest tower.

HIGHLY PRACTICAL

The Tower's microwave receivers are located 338 m (1,109 ft) above the ground in the radome (the donut-shaped collar at the base of SkyPod). The important VHF, UHF and television equipment intrinsic to the Tower's purpose as a broadcast transmission facility are located here. Incoming signals are monitored and fed to the antenna for transmitting. Further up at 360 m (1,180 ft) is the centre of FM broadcasting in Toronto. CFNY, CHUM, CHFI, CKFM, Q107, CHIN, CJRT, CJEZ, CBC radio and City TV, CFTO-TV, TVO, CBLT, CFMT, Global, CBLFT, and CICA all use the Tower's superior transmission capabilities.

The transmission equipment, although powerful, is extremely sensitive. The radome, designed to protect it from the elements, is a teflon-coated fibreglass-rayon fabric which can hold the weight of an average adult male yet measures only 1/32 of an inch. Its balloon-like shape results from inflating the skin to five times its normal size then maintaining constant pressure.

BREAKING NEW GROUND

When engineers started to plan the foundation of the CN Tower, they were breaking new ground in more ways than one. Never before had anyone been faced with the task of designing a base so far into the ground and they came up against many construction challenges unique to this project.

After an elaborate series of tests on the soil to assess the condition of the bedrock and determine how it would react to changes in hydrostatic pressure, the work was ready to begin. On February 6, 1973, hundreds of people, engaged in a historic enterprise, moved in and started to carve out the launching pad for the World's Tallest Building.

They removed 56,234 metric tonnes (62,000 tons) of earth and shale before pouring a thick concrete and steel foundation 6.71 m (22 ft) deep on a base of hand-and-machine-smoothed shale. Supporting the World's Tallest Building is a tall order and by the time it was finished the y-shaped foundation contained 7,046 cubic metres (9,200 cubic yards) of concrete, 453.5 metric tonnes (500 tons) of reinforcing steel and 36.28 metric tonnes (40 tons) of thick, tensioning cables. The thoroughness and speed with which the foundation was laid is noteworthy. The complete foundation was in place just four months after the first spade of earth had been turned.

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	The CN Tower under construction

BUILDING A SEVEN-STOREY BUILDING AT 1,100 FEET

In August of 1974, workers began building the Tower's crowning glory, the SkyPod, a seven-storey building that would eventually house two observation decks, a 360° Revolving Restaurant (Horizons), a glass floor and various technical areas. This construction in the sky involved lifting 318 metric tons of steel and wood brackets up the sides of the Tower using 45 hydraulic jacks and miles of steel cable. To build the observation level, workers bolted brackets to tensioned steel bars and placed concrete in the wooden frames, then placed a three-feet-high compression ring around the outside.

The radome (the donut-shaped collar at the base of SkyPod) protects the Tower's sensitive microwave equipment and is essential to its intrinsic purpose as a broadcast transmission facility. All of the important VHF, UHF and television equipment is located here. Incoming signals are monitored and fed to the antenna for transmitting. The radome is designed to protect this equipment from the elements but still enable it to receive transmissions.

For the expanded version of this article, download our PDF version of the CN Tower.

Reprinted courtesy of TrizecHahn Corporation.