Constructing a single facility adequate for all of Toronto's radio and TV requirements dictated the great height of the CN Communications Tower. An elevation of 1100 feet was needed to clear all Toronto area obstacles for microwave receivers. Above that height a 350-foot separation was required between the receivers and the UHF/VHF and FM antennas, which in turn had to be spread out over a 335-foot antenna mast. These and other design considerations led to a slender, tapering beauty in concrete, and a record height for a freestanding structure 1815 feet 5 inches.
Findings from numerous pictures and test samples taken as deep as 100 feet in four 30-foot-diameter holes led to design of a Y-shaped foundation resting on shale at a depth of about 50 feet and capable of both supporting the tower weight and resisting the great pressures induced by wind. Since the water table was only 19 feet down, the area had to be dewatered before excavation and during construction of the foundation. Post-tensioned cables criss-crossing the Y prevent cracking. Nine caverns within the foundation provided sheltered work space and means of anchoring the vertical tendons of the tower when post-tensioned.
POST-TENSIONING THE TOWER
Selection of a fully post-tensioned construction instead of reinforced or partially post-tensioned concrete was made early in the design process. The fully post-tensioned structure will remain essentially uncracked at all loads. This is important for a tower that will be exposed to great changes in temperature and humidity as well as chemically aggressive air. Post-tensioning also contributed to economy of materials, reducing the foundation requirements and permitting the walls of the hexagonal core to be only about 24 inches thick.