April 2

This day in sketchtory, the CN Tower was completed. 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 t 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. This portion of the construction was fairly rapid, with only four months needed between the start and the foundation being ready for construction on top.

To build the main support pillar, a hydraulically-raised slipform was built at the base. This was a fairly impressive engineering feat on its own, consisting of a large metal platform that raised itself on jacks at about 6 metres per day as the concrete below set. Concrete was poured continuously by a team of 1,532 people, during which it had already become the tallest structure in Canada, surpassing the recently built Inco Superstack, which was built using similar methods. In total, the tower contains 40,500 cubic metres of concrete, all of which was mixed on-site in order to ensure batch consistency. Through the pour, the vertical accuracy of the tower was maintained by comparing the slip form's location to massive plumb-bobs hanging from it, observed by small telescopes from the ground.

At the time it was built, it was the tallest freestanding structure in the world. These days, theres a new tallest structure almost every year.