The CN  Tower

The CN Tower (French: Tour CN) is a 553.33 m-high (1,815.4 ft) concrete communications and observation tower in Downtown Toronto, Ontario, Canada. Built on the former Railway Lands, it was completed in 1976, becoming the world's tallest free-standing structure and world's tallest tower at the time. It held both records for 34 years until the completion of Burj Khalifa and Canton Tower in 2010. It remains the tallest free-standing structure in the Western Hemisphere, a signature icon of Toronto's skyline, and a symbol of Canada, attracting more than two million international visitors annually.

Its name "CN" originally referred to Canadian National, the railway company that built the tower. Following the railway's decision to divest  non-core freight railway assets, prior to the company's 

privatization

 in 1995, it transferred the tower to the Canada Lands Company, a federal Crown corporation responsible for real estate development. Since the name CN Tower became common in daily usage, the abbreviation was eventually expanded to Canadian National Tower or Canada's National Tower. However, neither of these names is commonly used.

In 1995, the CN Tower was declared one of the modern Seven Wonders of the World by the American Society of Civil Engineers. It also belongs to the World Federation of Great Towers, where it holds second-place ranking.

History

The idea of the CN Tower originated in 1968 when the Canadian National Railway  had a desire to build a large TV and radio communication platform to serve the Toronto area, as well as demonstrate the strength of Canadian industry and CN in particular. These plans evolved over the next few years, and the project became official in 1972. The tower would have been part of Metro Centre (see CityPlace), a large development south of Front Street on the Railway Lands, a large railway switching yard that was being made redundant by newer yards outside the city. Key project team members were NCK Engineering as structural engineer; John Andrews Architects; Webb, Zerafa, Menkes, Housden Architects; Foundation Building Construction; and Canron (Eastern Structural Division).

As Toronto grew rapidly during the late 1960s and early 1970s, multiple skyscrapers were constructed in the downtown core, most notably First Canadian Place. The reflective nature of the new buildings compromised the quality of broadcast signals necessitating new, higher antennas that were at least 300 m (980 ft) tall. At the time, most data communications took place over point-to-point microwave links, whose dish antennae covered the roofs of large buildings. As each new skyscraper was added to the downtown, former line-of-sight links were no longer possible. CN intended to rent "hub" space for microwave links, visible from almost any building in the Toronto area. The CN Tower can be seen from at least as far away as Kennedy Street in Aurora, Ontario, approximately 40 km (25 mi) to the north, 60 km (37 mi) east of Toronto, in Oshawa, and from several points on the south shore of Lake Ontario, 48 km (30 mi) to the south in New York state in the United States.

The original plan for the tower envisioned a tripod consisting of three independent cylindrical "pillars"  linked at various heights by structural bridges. Had it been built, this design would have been considerably shorter, with the metal antenna located roughly where the concrete section between the main level and the SkyPod  lies today. As the design effort continued, it evolved into the current design with a single continuous hexagonal core to the SkyPod,  with three support legs blended into the hexagon below the main level, forming a large Y-shape structure at the ground level.

The idea for the main level in its current form evolved around this time, but the Space Deck (now named SkyPod) was not part of the plans until some time later. One engineer in particular felt that visitors would feel the higher observation deck would be worth paying extra for, and the costs in terms of construction were not prohibitive. It was also some time around this point that it was realized that the tower could become the world's tallest structure, and plans were changed to incorporate subtle modifications throughout the structure to this end.

Events :-

·       When the CN Tower opened in 1976, there were three public observation points: the SkyPod (then known as the Space Deck) that stands at 447 m (1,467 ft), the Indoor Observation Level (now named Indoor Lookout Level) at 346 m (1,135 ft), and the Outdoor Observation Terrace (at the same level as the Glass Floor) at 342 m (1,122 ft). One floor above the Indoor Observation Level was the Top of Toronto Restaurant, which completed a revolution once every 90 minutes.

·       Stuntman Dar Robinson jumped off of the CN Tower on two occasions in 1979 and 1980. The first was for a scene from the movie Highpoint, in which Robinson received $250,000 for the stunt. The second was for a personal documentary. Both stunts used a wire decelerator attached to his back. 

·       On June 26, 1986, the ten-year anniversary of the tower's opening, high-rise firefighting and rescue advocate Dan Goodwin, in a sponsored publicity event, used his hands and feet to climb the outside of the tower, a feat he performed twice on the same day. Following both ascents, he used multiple rappels to descend to the ground.

·       The Glass Floor, at an elevation of 342 m (1,122 ft), opened to the public on June 26, 1994.

·       From 1997 to January 2004, TrizecHahn Corporation managed the tower and instituted several expansion projects including a C$26 million entertainment expansion and revitalization that included the 1997 addition of two new elevators (to a total of six) and the relocation of the staircase from the north side leg to inside the core of the building, a conversion that also added nine stairs to the climb. TrizecHahn also owned the Willis Tower (Sears Tower at the time) approximately at the same time.

·       In 2007, light-emitting diode (LED) lights replaced the incandescent lights that lit the CN Tower at night, the reason cited that LED lights are more cost and energy efficient than the incandescent lights. The colour of the LED lights can change, compared to the constant white colour of the incandescent lights.

·       On September 12, 2007, Burj Khalifa, then under construction, surpassed the CN Tower as the world's tallest free-standing structure.

·       In 2008, glass panels were installed in one of the CN Tower elevators, which established a world record (346 m) for highest glass floor paneled elevator in the world.

·       The CN Tower was closed during the G-20 summit on June 26–27, 2010, for security reasons, given its proximity to the Metro Toronto Convention Centre.

·       In 2011, EdgeWalk (where at an elevation of 356 metres (1,168 ft), visitors tethered to an overhead rail system can walk on the roof of the main pod) opened to the public.

Lighting

The CN Tower was originally lit at night with incandescent lights, but they were removed in 1997, because they were inefficient and expensive to repair. In June 2007, the tower was outfitted with 1,330 super-bright LED lights inside the elevator shafts, shooting up over the main pod and upward to the top of the tower's mast to light the tower from dusk until 2 a.m. The official opening ceremony took place on June 28 before the Canada Day holiday weekend.

The tower changes its lighting scheme on holidays and to commemorate major events. After the 95th Grey Cup in Toronto, the tower was lit up in green and white to represent the colours of the Grey Cup champion Saskatchewan Roughriders. From sundown on August 27, 2011, to sunrise the following day, the tower was lit in orange, the official colour of the New Democratic Party, to commemorate the death of federal NDP leader Jack Layton.

Programmed from a desktop computer with a wireless network interface controller in Burlington, Ontario, the LEDs use less energy to light than the previously used incandescent lights (10% less energy than the dimly lit version and 60% less than the brightly lit version). The estimated cost to use the LEDs is $1,000 per month.

During the spring and autumn bird migration seasons, the lights will be turned off to comply with the voluntary Fatal Light Awareness Program, which "encourages buildings to dim unnecessary exterior lighting to mitigate bird mortality during spring and summer migration."