One World Trade Center Defines the Lower Manhattan Skyline

WSP USA provided structural engineering for One World Trade Center, the iconic 104-story Manhattan skyscraper that is now the tallest building in the Western Hemisphere.

Long before its rooftop spire touched the Manhattan sky – and years before its gently sloping exterior reshaped the New York City skyline – One World Trade Center (1WTC) had captured the world’s attention, with every design detail scrutinized by public officials, architects, engineers and the general public.

When 1WTC welcomed its first tenant, publisher Condé Nast, on Nov. 3, 2014, it completed the skyscraper’s journey from one of the darkest days in U.S. history to a celebration of the tallest building in the Western Hemisphere.

WSP USA was commissioned by the developer, Silverstein Properties, to serve as the structural engineer of record for the 104-story steel-frame and concrete-core building, as well as for three of the skyscraper’s sister towers on the same site – Two, Three and Seven World Trade Center. In 2006, the Port Authority of New York and New Jersey, owner of 1WTC, took over development and retained WSP as the structural engineer.

“Every aspect of the work was closely monitored by the owners and every stage of advancement was of interest to the public and the media,” said Ahmad Rahimian, director of building structures at WSP and principal-in-charge of analysis and design for 1WTC. “WSP was charged with making early decisions that would affect the entire trajectory of the site structures and evolution of the design criteria.”


One World Trade Center features eight isosceles triangles creating its stunning appearance, and converging to form an equilateral octagon floor plan at mid-level.

Spirit of 1776

The 1,776-foot height of 1WTC is intended to commemorate the year the Declaration of Independence was signed. Its height of 1,368 feet at the roof and its 205-by-205-foot footprint match each of the original twin towers.

The 3.5-million-square-foot mixed-use building features 71 office floors, a two-level observation deck, a “sky” restaurant, retail space, parking and access to public transportation. More than 500,000 square feet of the building is subterranean, and eight levels are reserved to handle the structure’s mechanical, electrical and plumbing needs.

“The four corners of the tower slope gently from the first office level inward until, at the roof, the floor plan again forms a square but with a reduced dimension of 145 feet,” said Yoram Eilon, senior vice president of building structures at WSP and 1WTC project director. The exterior incorporates eight isosceles triangles into its design, and at mid-height, the floor plan forms an equilateral octagon.

One World Trade Center also marked the first time that a building in New York City used concrete with a compressive strength of 14,000psi (pounds per square inch), and it is the first structure with specified dual requirements for both concrete strength and stiffness.

“The concrete design and performance criteria were unique and achieved through collaboration of multiple parties including the client, design team and the contractors,” Eilon said.


One World Trade Center dominates the skyline of Lower Manhattan.

Security a Top Priority

The design followed security requirements established by a team of experts that defined the threats for this site. One formidable challenge was the need to meet or exceed codes and standards not yet written at the design stage.

“One World Trade Center will be unprecedented in terms of life safety and security, and will set a new safety standard for the design of tall buildings,” Rahimian said. The structure consists of a high-strength concrete core paired with a perimeter steel frame that provides tenants with a flexible, spacious, column-free interior. “The core acts as the main spine of the tower, providing support for gravitational loads and resistance to wind and seismic forces.”

Every design solution was meticulously reviewed and supported by a multitude of rigorous analyses and design procedures, added Eilon.

“The design and construction of One World Trade Center is the result of a relentless collaborative effort between numerous design and construction teams over a period of several years, with a resolute focus on the goal of creating an iconic tower affirming the preeminence of New York City,” Eilon said.

A series of wind tunnel tests were performed to ascertain an accurate measurement of the tower’s response to hurricane-strength winds. “The wind acceleration results at the highest occupied level of the building meets the criteria of human comfort for office buildings,” Rahimian said.

Although 1WTC is the most recognized tower, it is just one of the buildings that have been built or are currently under construction at the World Trade Center site. WSP is involved with the engineering of four of those buildings:
- Two World Trade Center, with a diamond-shaped top reaching 1,350 feet in height, will be the second-tallest building in New York when completed.
- Three World Trade Center is 1,175 feet tall. The 80-story building is scheduled to be completed in 2017.
- Seven World Trade Center, the first building completed in the WTC complex in 2006, included numerous life-safety and environmental features never before incorporated into a commercial skyscraper.
- The National September 11 Memorial and Museum, which includes a landscaped plaza containing the memorial, two waterfalls and reflecting pools that occupy the footprints of each of the original twin towers, and the 110,000-square-foot museum.

WSP continues to serve as the engineer of record for fit-out and tenant work at 1WTC.


One World Trade Center, now a fixture of the Manhattan skyline, soars to a symbolic height of 541 meters (1,776 feet).

Award-Winning Design

Recognition of 1WTC has come in the way of numerous awards and honors for engineering and design, including recognition as one of five Projects of the Year in the WSP 2015 U.S. Awards and Recognition Program, presented at a ceremony in New York City on April 25, 2015.

Other recent 1WTC accolades included :
- American Institute of Steel Construction, IDEAS2 Award, 2014 Presidential Award of Excellence in Engineering;
- New York State Society of Professional Engineers, 2014 PEC Project of the Year;
- American Council of Engineering Companies (ACEC), 2014 NY Diamond Award in Structural Systems; and
- National Council of Structural Engineers Association, 2013 Excellence in Structural Engineering award.

The building has also been certified Leadership in Energy and Environmental Design (LEED) Gold for its sustainability design and construction.

Throughout the design and construction process, public scrutiny was unflagging, but the project team met expectations, according to Frank Mannino, project manager for WSP.

“The interplay of several simultaneous projects and numerous project teams, as well as the pervasive spirit of cooperation evidenced in this endeavor, has not gone unnoticed by the media or the public,” Mannino said. “Few projects in modern history have stirred public emotions as the World Trade Center site has, specifically One World Trade Center. The intensity of emotions connected with the World Trade Center and its destiny has resulted in an inspirational and extraordinary triumph for New York City and the country as a whole.”

Engineering News-Record featured an article on its Best of the Best award winners in its March 14, 2016 issue. One World Trade Center was selected for the honor in the office/retail/mixed use category.

Ahmad Rahimian

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