Sunday, July 27, 2008
CN (Canadian National) Tower
The CN Tower, next to the city hall on Front Street, Toronto, stands on the shore of Lake Ontario. It transmits television and FM radio for more than twenty broadcasters, as well as serving various other communications purposes. Including the masts, it is the tallest freestanding structure in the world; the top of the transmission antenna is over 1,815 feet (553 meters) high. But at the beginning of the twenty-first century, as technically demanding as it is, height alone does not constitute an architectural feat. The twin Petronas Towers in Kuala Lumpur, Malaysia, currently rank as the world’s tallest buildings, at 1,483 feet (454 meters). Others are proposed that will exceed that, including the 1,660-foot (508-meter) Taipei Financial Center on Taiwan, to be completed in August 2002, and the 2,100-foot (642-meter) Russia Tower in Moscow; at 2,755 feet (843 meters), the Millennium Tower in Tokyo will dwarf them all. The CN Tower is remarkable architecture because of its construction technique. For about a year, concrete, mixed and tested on-site to ensure consistent quality, was poured around the clock into a “slip form” that gradually decreased in diameter, to create the elegantly tapered contour of the post tensioned hollow structure.
Slip forming is a rapid construction technique based on extrusion. It employs a self-raising formwork that continually moves upward as the concrete is being placed, at a rate that gives the concrete time to set before being exposed as the formwork rises on a ring of hydraulic jacks, developing enough strength to support the work above. Continuous slip forming obviously speeds up the construction process while enabling excellent quality control, optimizing labor, and reducing the cost of building plant and scaffolding. It also results in monolithic, seamless structures. Developed in North America in the 1920s—The Granary at Logan Square in Philadelphia (1925) was one of the first examples in the United States—it has been widely used to build grain silos, building service cores, and (normally) any tall structures with a consistent cross section.
Early in the 1970s the number of multistory office blocks in downtown Toronto increased significantly, with a consequent interference with television and radio reception in large parts of the city. Toronto needed an antenna taller than any existing office block, indeed, of any that was anticipated, and the CN Tower was proposed to meet that need. The project was initiated in 1972 by the Canadian National Railway, which commissioned John Andrews Architects, working in collaboration with Webb Zerafa Menkes Housden Architects of Toronto. The structural engineering consultant was Roger R. Nicolet of Montreal; the mechanical and electrical engineers were Ellard-Wilson Associates Ltd. of Toronto; and the manager-contractor was Foundation Building Construction.
The original design proposed three concrete towers linked by structural bridges, but that was developed into a single tower with three hollow “legs.” As well as serving as electrical and mechanical service ducts, the hollow columns provided the necessary degree of flexibility for such a tall structure. Construction started in February 1973, and in four months a Y-shaped, 22-foot-thick (6.7-meter) reinforced concrete base was founded on the bedrock 50 feet (15 meters) beneath the city. The continuous slip-form process then began. When the tower reached 1,100 feet (336 meters), a seven-story “SkyPod,” fabricated on the ground, was raised into position
and anchored by twelve steel-and-timber brackets that were slowly pushed up the tower by forty-five hydraulic jacks. The concrete-walled SkyPod, reached by four high-speed, glass-fronted elevators, houses a 400-seat revolving restaurant, a nightclub, and indoor and outdoor observation decks. Later, a 2.5-inch-thick (6.4-centimeter) glass floor was installed. Beneath the SkyPod, delicate microwave dishes and other broadcasting equipment are protected by an annular radome. The concrete tower continues to the Space Deck at 1,465 feet (447 meters)—an observation gallery that on a clear day provides a view with 100-mile (160-kilometer) visibility. A Sikorsky Skycrane helicopter lifted the tower’s 335-foot (100-meter) communications mast in forty sections, each of about 7 tons (6.4 tonnes), and they were bolted together in place. The mast, erected in three weeks, was covered by fiberglass-reinforced sheathing. The maximum sway experienced at the very top in 120-mph (190-kph) winds with 200-mph (320-kph) gusts is 3.5 feet (1.07 meters).
The CN Tower was completed in June 1975 and officially opened on 1 October. It cost Can$57 million and took about 1,550 workers forty months to construct. It is nearly twice the height of the Eiffel Tower and more than three times as tall as the Washington Monument. Soaring above Toronto, it is struck by lightning about seventy-five times every year.
In 1995 Canada National passed ownership to a public company, the Canada Lands Company. In June 1998, the CN Tower officially opened a 75,000-square-foot (7,100-square-meter) expansion including an entertainment center, shopping facilities, and restaurants.