July 15, 1974 - CN Tower Construction Nears Completion in Toronto

On July 15, 1974, you're witnessing the moment the CN Tower's concrete shaft reached its full height of 335 metres — the turning point that shifted construction from raw concrete to the seven-storey sphere that would crown it. Over 1,500 workers had spent more than a year pouring and shaping the tapered hexagonal core around the clock. Pod assembly would begin just weeks later in August. There's far more to this engineering story than one milestone date.

Key Takeaways

• On July 15, 1974, the CN Tower's concrete shaft reached its full height of 335 metres, marking a major construction milestone.
• Over 1,500 workers operated 24 hours a day, five days a week to achieve this critical phase of construction.
• The shaft's completion signaled a shift in focus toward building the seven-storey sphere housing observation decks and the revolving restaurant.
• The slipform construction method enabled the shaft to advance roughly six metres per day, ensuring efficient and precise vertical progress.
• Pod assembly began in August 1974, just weeks after the shaft milestone, accelerating the tower toward its April 2, 1975 completion.

Where CN Tower Construction Stood on July 15, 1974

By July 15, 1974, the CN Tower's concrete shaft had reached its full height of 335 metres, marking the end of the most labor-intensive phase of construction.

Built on former railway lands, the site represented a bold step in downtown redevelopment, transforming old railway landmarks into the foundation of something unprecedented.

Over 1,500 workers had poured 40,524 cubic metres of concrete continuously, operating 24 hours a day, five days a week to reach this milestone.

You can appreciate the scale when you consider the slipform mold climbed hydraulically, shaping the tapering shaft from the ground up.

With the concrete shaft complete, crews immediately shifted focus to the next major phase — constructing the seven-storey sphere that would house the observation decks and revolving restaurant above. Workers laboring at extreme heights received danger pay, earning an additional $1 per hour for every 300 feet above 1,000 feet.

The entire project was initiated by Canadian National railway to resolve radio and TV reception issues that had been caused by Toronto's rapid skyscraper boom.

Why the CN Tower's Hexagonal Core Made It So Strong

With the concrete shaft complete, attention turns to what made it so structurally formidable — its hollow hexagonal core. You can think of this shape as the backbone of the tower's aerodynamic stability, since its geometry lowers the center of gravity and reduces the surface area exposed to wind forces. Three inverted angles inside the core house six glass-lined elevators, giving riders unobstructed city views during ascent.

Cable prestressing reinforces the entire structure, with hundreds of tightened steel cable bundles running the tower's full height, limiting sway in both the legs and core. Weighted antenna rings counteract any remaining movement. The three support legs blend into the hexagon below, forming a Y-shape at ground level that distributes overturning moments and shear forces effectively — making the CN Tower remarkably resistant to structural failure. A series of copper strips runs the entire length of the Tower, conducting lightning electrons easily through the structure and feeding into massive grounding rods buried below the ground floor. The tower's external buttresses were specifically engineered to withstand extreme weather and wind loads, ensuring the structure remained stable across decades of harsh Canadian conditions.

How the CN Tower's 335-Metre Concrete Shaft Was Built

Before a single metre of concrete could rise skyward, crews spent roughly three months excavating 56,000 tonnes of earth and shale to a depth of 15 metres, then hand- and machine-smoothing the shale base to create a stable platform for the tapered shaft.

Workers then poured nearly 29,000 cubic metres of concrete from a single batch into a slipform, ensuring consistency throughout concrete curing. Hydraulic jacking lifted the three-storey steel-and-timber form continuously upward, while the form gradually narrowed to produce the shaft's tapered profile. Crews poured 24 hours a day, five days a week for eight months, using plumb bobs and ground telescopes to maintain vertical accuracy. When complete, the 335-metre shaft deviated from plumb by just 28 millimetres. The foundation itself required 450 tonnes of rebar and 36 tonnes of steel cable to reinforce the concrete against the immense structural loads above. For those interested in exploring more historical and scientific milestones like this, online fact tools can help surface concise, categorised information drawn from fields such as physics and engineering.

The Slipform Method That Shaped the CN Tower's Taper

The slipform rig first touched down on June 26, 1973, and over the next eight months, it climbed 446 metres as hydraulic jacks lifted the steel-and-timber platform in discrete 20–25 millimetre strokes while fresh concrete hardened below. You can picture crews advancing at roughly six metres daily, with bursts reaching ten metres in a single 24-hour push.

What made slipform tapering so demanding here was the tower's three-legged hexagonal profile. As the shaft narrowed, workers cut steel and timber from the outer form edges, lowering redundant sections away. The hydraulic jacking system started with 54 R72 jacks and reduced to 24 as the diameter shrank. Throughout, plumb-bobs and ground telescopes kept vertical alignment within just 29 millimetres across the entire height. The slipform itself was designed by Doug Sumner and supplied by Scanada, aka Heede International, whose jack technology underpinned the entire lifting operation.

Construction wrapped up in 1975, and the tower opened to the public in 1976, marking the culmination of years of engineering effort that had begun with the first slipform placement in 1973.

Why the CN Tower Used 1,500 Workers on 24-Hour Shifts?

Driving the CN Tower upward at such relentless pace demanded an enormous human engine: 1,500 workers rotating through 24-hour shifts, five days a week. Tight shift logistics kept concrete pouring continuously, letting crews hand off progress without losing momentum. Night shift workers earned guaranteed overtime, which pushed weekly earnings well past $2,400 in today's dollars — a direct investment in labor morale that kept experienced tradespeople showing up and performing under pressure.

Iron workers earned $8 per hour, with danger pay climbing an extra dollar for every 300 feet above 1,000 feet. You'd understand why men accepted that risk when the compensation reflected it honestly. That combination of structured scheduling, financial incentive, and rotating crews transformed a punishing 40-month build into something both manageable and historically unprecedented. Despite the dangerous conditions and limited fall protection equipment used throughout the project, no climber fatalities were reported during the entire construction of the tower. The hydraulic-powered slipform used during construction enabled a continuous concrete pour, forming the tower's tapered hexagonal core along with three support arms that gave the structure its distinctive shape.

What CN Tower Crews Built After the Concrete Shaft Was Done?

Once those 1,500 workers locked the concrete shaft in place, crews shifted focus to everything the shaft was built to carry. Pod assembly kicked off in August 1974, starting at ground level where teams built concrete formwork around the shaft's base. Six steel and timber assemblies went up, reinforced internally with steel, then twelve giant bracket forms climbed 45 hydraulic jacks over roughly one week to reach their final positions.

Simultaneously, you'd see crews raising a seven-storey sphere to house observation decks and the revolving 360 Restaurant, while the upper SkyPod took shape at 447 meters. Antenna installation followed once concrete pouring wrapped up, with white transmitter and lightning rod components bolted into place at the SkyPod level, completing the tower's broadcasting capability. Much like Michelangelo's David, which was carved from a single block of marble and designed with intentional scale adjustments to appear correct from a low viewing angle, the CN Tower's proportions were carefully engineered to read as visually unified from street level.

A Sikorsky helicopter named Olga was used in March 1975 to lift 39 individual antenna pieces into their final positions atop the tower. The tower's construction had begun in February 1973 on a CN railway siding, originally tied to the ambitious Metro Centre redevelopment plan for surplus railway lands in downtown Toronto.

When Did the CN Tower's Pod and Observation Deck Get Built?

Pod assembly kicked off in August 1974, when crews began building the seven-storey sphere designed to house the observation decks and revolving 360 Restaurant.

Workers raised the brackets for the pod structure that same month, marking the shift from the concrete shaft to the tower's upper components. The antenna itself was later lifted into place by a specially outfitted helicopter as the final step in reaching the tower's full height.

The Key Construction Dates That Defined 1974 for the CN Tower

The year 1974 proved pivotal for the CN Tower, with two major milestones bookending its construction progress. In February, the concrete shaft surpassed every existing structure in Canada, claiming the title of the country's tallest. You'd notice that crews pushed through weather impacts and material sourcing challenges to hit this benchmark without delays derailing the timeline.

Then in August, workers broke ground on the seven-storey sphere, housing observation decks and the 360 Restaurant. This launch accelerated the project's final phases considerably. With 1,537 contractors running 24-hour shifts five days a week, the team maintained relentless momentum. These two dates—February and August 1974—didn't just mark technical achievements; they confirmed that the CN Tower was firmly on track toward its April 1975 completion. For those wanting to explore further details about the tower's history by category, a fact finder tool can surface concise, organized information on demand.

How Long Did It Take to Build the CN Tower?

Building the CN Tower took 40 months in total, running from February 6, 1973, to April 2, 1975. That construction duration covered everything from foundation excavation to the final helicopter-placed antenna.

You'd be surprised how much ground crews covered in that time — literally, since foundation work alone removed 56 metric tonnes of earth and shale within just the first four months.

To manage such an ambitious schedule, organizers relied heavily on workforce turnover across 24-hour shift rotations, keeping progress continuous while maintaining five-day work weeks.

Over 1,500 workers and 1,537 contractors contributed throughout the project. The tower topped off after 26 months of active construction, and the structure officially claimed the title of world's tallest freestanding building on March 31, 1975 — days before full completion.

How the CN Tower Became the World's Tallest Freestanding Structure

Reaching that April 2, 1975 completion date meant more than just finishing a construction project — it meant claiming a title. On March 31, 1975, at 9:52 a.m., the CN Tower surpassed the previous world record, officially becoming the tallest freestanding structure on Earth at 553.3 metres.

These record milestones didn't happen by accident — they resulted from deliberate engineering breakthroughs, including 998 km of post-tensioned steel and a six-sided concrete core that delivered unmatched structural integrity. Canadian National Railway built something that held this record for 32 years until Dubai's Burj Khalifa surpassed it in 2007 at 829.8 metres.

Today, you'll still find the CN Tower ranked as the tallest freestanding structure in the Western Hemisphere and tenth tallest globally.