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Steel Shots: Towering Steel
Posted by Tasha Weiss on November 18, 2013 at 11:04 AM.


CTBUH affirmed One World Trade Center’s height as 1,776 ft, which will make the steel-framed tower the tallest building in North America when completed early next year. This ends the 40-year reign of the The Willis (formerly Sears) Tower in Chicago, also steel-framed, as North America’s tallest. Click on the above recent photo of One WTC to see a diagram showing the CTBUH Height Measurement of the tower to “Architectural Top” and “Height to Tip.” Photo: John W. Cahill/CTBUH. Diagram image: CTBUH (base drawing copyright SOM)


It’s official: One World Trade Center’s height to its architectural top is 1,776 ft.


The Council on Tall Buildings and Urban Habitat (CTBUH) convened its Height Committee earlier this month in Chicago to evaluate its criteria for determining the official height of buildings, and to rule on the official height of One World Trade Center in New York.


Because One WTC is still unfinished and has not received its certificate of occupancy, it cannot yet enter CTBUH rankings as a “completed building,” but its height is no longer in dispute.


There were two central issues for discussion in the ratification of this building’s height against CTBUH criteria.


  • The nature of the mast structure on top of the tower.
  • The datum line (bottom point) from which the height to architectural top was determined.


This determination was made through examination of design and construction drawings and continued through dialogue among the Height Committee’s 25 members. The committee received a 15-minute presentation by the One WTC owner/design team, including the project’s chief architect, David Childs of Skidmore, Owings & Merrill LLC (SOM) and Ken Lewis, project director at SOM.


“We were very satisfied with the detailed information presented by the team, in particular, that which affirmed that the structure on top of the building is meant as a permanent architectural feature, not a piece of functional-technical equipment,” said Timothy Johnson, chairman of CTBUH and design partner at NBBJ.


“The design of One World Trade Center, as explained to us, reinforces its role as a symbol of resurgence on this important site,” said CTBUH executive director Antony Wood. “In particular, the spire which holds the beacon light, shining out at the symbolic height of 1,776 ft, is especially poignant – echoing the similarly symbolic beacon atop the Statue of Liberty across the water.”


The Council measures the height of buildings in three categories: Height to Architectural Top, Highest Occupied Floor, and Height to Tip.


Due to design changes that resulted in the removal of the architectural cladding around the mast at the top of the structure, it became unclear whether the structure (made up of 18 barrel-shaped sections of steel) was in fact a “spire” – a vertical element that completes the architectural expression of the building and is intended as permanent, or whether it was an antenna – a piece of functional-technical equipment that was subject to change.


The spire-antenna distinction makes a difference in the measurement of “height to architectural top,” which includes spires but does not include antennae, signage, flag poles or other functional-technical equipment. Determining the topmost structure to be “functional-technical” potentially could have significantly impacted One WTC’s ranking against other buildings, as the next obvious point to take a measurement of “architectural top” would have been the building’s roof slab, at 1,334 ft, 8 in. – 441 ft, 4 in. lower than claimed.


building-height-comparison-chart.jpgThe diagram at left, courtesy of CTBUH, shows where One WTC will rank among the tallest buildings in the world when it is completed next year (click on the image to enlarge). 


Approximately 45,000 tons of structural steel were used in the One WTC tower in all. Once open and occupied, it will surpass The Willis (formerly Sears) Tower in Chicago as the tallest building in North America and will likely be the third tallest-building in the world.


The Willis Tower used about 76,000 tons of structural steel in its construction. Thanks to advancements in the domestic steelmaking process and an increase in the strength of structural steel, if it were built today versus 1973, the steel package would be much more efficient. It could be done with approximately 16,000 fewer tons of steel, 876,000 fewer labor hours, a 58% lower carbon footprint and 74% less embodied energy.


You can read the full CTBUH announcement and watch news coverage of the Chicago and New York City press conferences on the CTBUH website.

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