24/6/2026 · 9:42
CN Tower: How Canadian Engineers Poured Concrete Non-Stop for 40 Months to Build the World's Tallest Free-Standing Structure
In 1973, Canadian National Railway sent 1,537 workers to pour concrete non-stop up a hexagonal shaft — day and night, for 40 months — using hydraulic jacks and a helicopter named Olga to stack 44 antenna sections above the clouds, raising Toronto's 553-metre CN Tower with a vertical deviation of just 28 millimetres.
A weekly 1-minute video series uncovering how the world's most iconic structures were built — from ancient wonders to modern skyscrapers.
In the 1960s, Toronto's TV broadcasters had a problem: the city's booming skyline was blocking their signals. The solution became one of the most audacious construction feats of the 20th century.
The CN Tower was designed by John Andrews and built by Canadian National Railway between 1973 and 1976. At 553.3 metres, it held the title of the world's tallest free-standing structure for over 30 years.
The engineering centrepiece was slip-form concrete — a continuously moving platform that climbed the hexagonal shaft at roughly 6 metres per day. Workers poured concrete non-stop, while hydraulic jacks inched the form skyward. Despite rising nearly half a kilometre, the shaft deviated just 28 millimetres from perfect vertical.
The observation pod — called the Space Deck — was assembled on the ground, then hauled up 447 metres by hydraulic jacks, one bolt-width at a time. Above it, a Sikorsky S-64 SkyCrane helicopter nicknamed 「Olga」 made 56 precise lifts to stack the 44 antenna sections of the broadcast mast — working in winds above the cloud line.
On April 2, 1975, workers bolted the final antenna piece into place. Construction had taken just 40 months and employed 1,537 workers at peak. Today, more than 2 million visitors a year step onto the glass floor of the observation deck — 342 metres above the ground.
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