Summary
Highlights
The CN Tower, standing over 553 meters, has been a dominant feature of Toronto's skyline for nearly 50 years. Opened in 1976, it was the tallest free-standing structure on Earth for over three decades and remains among the top 10 tallest globally. Designed largely by hand before computer-aided design, its construction spanned over nine years and involved innovative solutions to complex engineering problems. In 1995, it was recognized as one of the seven wonders of the modern world by the American Society of Civil Engineers. This video will explore its design and construction from foundation to antenna.
In the 1960s, Toronto's growing skyline with new skyscrapers posed a problem for existing broadcast towers. The solution was the CN Tower, designed to transmit radio and television signals across any foreseeable future high-rises. Specifications called for UHF transmitters at 338m and VHF transmitters between 460m and 540m. The decision was made to exceed the minimum height of 540m to make it the world's tallest free-standing structure and showcase Canadian industry. To help fund the $63 million project (equivalent to $350 million today), tourist attractions like a 360-degree revolving restaurant and observation decks were integrated into the design. The tower consists of a 450m concrete shaft with a Y-shaped cross-section, topped with a 104m steel antenna mast. A 7-story pod houses the restaurant and observation decks, with a smaller 'Skypod' above. Construction began in February 1973 and took over three years, involving 1,537 workers.
The reinforced concrete foundation is 5.5m thick and Y-shaped, extending 33m from the center to support the tower's three legs. Its purpose is to bear the tower's self-weight and resist overturning moments and shear forces from wind, distributing them to the shale bedrock below. Over 56,000 metric tonnes of earth and rock were excavated. To prevent shear stress concentrations and fracturing of the bedrock, the foundation's edges were tapered from 5.5m in the middle to 1.2m around the perimeter, reducing stress in the bedrock. Concrete's weakness in tension was addressed by pre-stressing it with a post-tensioning system. 48 high-strength steel cables (each with 7 steel wires bundled together) were hydraulically tensioned and anchored to counteract tensile forces and prevent cracking, especially crucial as the foundation was built below the water table. A low-cement concrete mix minimized heat accumulation during curing, and the foundation was made hollow with internal caverns for post-tensioning operations. The foundation required over 7,000 cubic meters of concrete and 450 metric tonnes of reinforcing steel, taking approximately 3 months to build.
The 450m tall concrete shaft is the tower's main structural component, supporting equipment and providing access. It features a hollow hexagonal core and three glass-paneled elevator shafts, with three tapered support legs radiating from the core, increasing in size towards the base. The legs extend to 342m, meeting the main pod, while elevator shafts continue to 382m. The central core spans the entire shaft height, housing service lines and emergency stairs. The shaft's walls vary from 0.5m to 1.5m thick, with the outer edges of the support legs being tapered for optimal structural efficiency, resembling an inverted icicle or tree trunk. This efficient design, while sacrificing some efficiency for glass elevators, also addressed aerodynamics, local buckling, and post-tensioning challenges. The shaft was constructed using slip-forming: a continuously moving form on hydraulic jacks. Concrete was poured in layers as the form slowly ascended, allowing lower layers to cure. A tower crane inside the core raised alongside the form. Over 29,000 cubic meters of concrete and 3,600 metric tonnes of steel were used, taking 8 months of continuous work. The finished shaft was only 28mm out of plumb over its 450m height.
The concrete shaft acts as a vertical cantilever, designed to withstand wind loads exceeding 400 km/hr. Extensive wind action studies were conducted at the University of Western Ontario using a 1:450 scale aeroelastic model. Displacements, rotations, and accelerations were used to design two passive tuned mass dampers for the steel antenna mast, enhancing reliability. Under 200 km/h sustained winds with 320 km/h gusts, the top of the shaft sways 0.5m and the antenna 1m. To counteract tensile stresses from bending under lateral loads (wind or earthquake), the shaft was post-tensioned. This system provides uniform compressive stress, keeping the shaft fully pre-stressed for up to a 1 in 50-year wind event. For stronger winds, partial pre-stressing and minor cracking may occur, but without compromising structural integrity. 144 high-strength steel cables, each with 16 to 31 strands, were fed through vertical ducts. Core cables are 450m long, anchored from top to foundation, while leg cables are shorter. Most cables were tensioned from the bottom using hydraulic jacks, applying 1,800 kN to 3,500 kN of force per cable. Because vertical post-tensioning was relatively new, extra ducts and cables were installed to compensate for unpredictable pre-stress losses and construction issues. Despite challenges like dented ducts and snapped strands, 129 km of steel cable (762 metric tonnes) were used, providing a combined compressive force of 365 MN.
Construction of the main pod began in summer 1974. It has seven levels with concrete floor slabs and steel framing. The first level, suspended below, houses UHF transmitters within an air-inflated, Teflon-coated fiberglass radome, protecting equipment from wind and ice. Levels 2 and 3 are observation decks with glass floors and an outdoor terrace. The glass floor, 2.5 inches thick, can support substantial weight. Level 4 features a revolving restaurant that rotates 360 degrees every 72 minutes. Levels 5-7 are for broadcasting equipment and mechanical systems, respectively. The pod's structural steel frame forms a 12-sided polygon, supported by 12 triangular reinforced concrete bracket walls mounted to the shaft's exterior. These walls were cast integrally with the second-level floor slab, incorporating a post-tensioned concrete ring beam. This beam clamps the bracket walls against the shaft, preventing tensile stresses. The post-tensioning system uses 12 high-strength steel cables, each spanning 180 degrees around the beam, staggered to ensure consistent compressive force. The pod's construction involved building concrete formwork for the bracket walls at ground level, which were then hoisted up the tower by hydraulic jacks, rising 68m per day to their final height of 342m. After casting the brackets, the second-level floor slab and ring beam were constructed.
While the main pod was under construction, the SkyPod was built at 447m, supported by a circular concrete corbel and accessible by elevator. Significant smaller than the main pod, it is the highest observation platform in the Western Hemisphere, offering views of up to 160 km. Above the SkyPod, the 104m steel antenna extends to a total height of 553m. It's supported by a massive steel base anchored to the shaft, consisting of 39 hollow, pentagon-shaped steel segments (cans) that decrease in size towards the tip, forming a 95m mast. A smaller 9m square segment tops it, bringing the total steel weight to over 272 metric tonnes. VHF transmitters are installed along the antenna's exterior. The antenna also features warning lights, two passive tuned mass dampers, and lightning rods. To protect broadcasting equipment and prevent ice buildup, the antenna is enclosed by a glass-reinforced plastic skin (radome). Due to a tight schedule, a Sikorsky SkyCrane helicopter, 'Olga,' was leased for $230,000 (over $1 million today) to lift the steel can sections, reducing construction time by an estimated 5 months. The helicopter removed the tower crane and made 56 flights, lifting components up to 7 metric tonnes. Cans were pre-assembled on the ground to ensure fit. Bolting crews installed 30,000 1-inch bolts. On April 2, 1975, the final steel section was placed, recording the CN Tower as the world's tallest free-standing structure at 553.3m. Installation of electronic components and the plastic radome followed, with the tower officially opening on June 26, 1976.
Since its inauguration, the CN Tower has been celebrated as a Canadian icon and an engineering marvel. It attracts over 1.5 million visitors annually and continues to serve its primary purpose as a communications tower, supporting over 16 TV and radio stations, as well as cell phone providers and digital audio broadcasting. Recent renovations include 1,300 programmable LED lights, glass floor panels in elevators, and the EdgeWalk – an open-air walk on top of the main pod at 356m. With an expected design life of 300 years, the CN Tower will continue to inspire future generations as a testament to 20th-century engineering.