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Engineering One World Trade Center: Structural Design Ahead of Its Time

One World Trade Center, the tallest building in the Western Hemisphere and the third tallest building in the world, is the new icon of the New York skyline and a landmark like no other. From its monumental architecture and groundbreaking structure to the emotional role it has played in the lives of New Yorkers, One WTC is one of the world’s most famous construction project.

Watcht the story (in brief) of the reconstruction of One World Trade Center, featuring the key players involved in this truly amazing project.

Simplicity and Geometry


One WTC begins from a 200ft square footprint – exactly the same dimensions as the original twin towers. From a height of 20ft, it begins to gently taper at the corners. By the time it reaches the uppermost 104th storey, the floor plan is again a square, but slightly smaller and twisted through 45 degrees. At its midpoint, it is a perfect octagon. From base to parapet, the building is also the same height as the twin towers – 1,368ft [417m] – before its crowning mast takes it up to the symbolic height of 1,776ft [541m], reflecting the year of the signing of the Declaration of Independence and making it the tallest tower in the western hemisphere.

“We always thought this building should be about simplicity and geometry,” says architect TJ Gottesdiener, managing partner at Skidmore, Owings & Merrill (SOM), who led the design team. “We tried to make it look as clean, strong, monumental as possible and that meant making it look as simple as possible – although this is far from a simple building.”

 

Revolutionizing Structural Engineering and Restoring Confidence in Tall Buildings


The project has had a revolutionary impact on structural engineering, redefining the way towers are constructed in New York. As the project team worked to make One WTC the safest ever built as well as to restore confidence in tall buildings, they have developed new technologies and techniques that have since been adopted into the city’s building codes and in buildings worldwide.

 

Designing for a Future Code that Had Not Been Written Yet


“After 9/11, construction just stopped,” says Ahmad Rahimian, director of building structures at WSP | Parsons Brinckerhoff in New York, who led the structural design of One WTC. “The entire engineering and construction community was trying to understand what had happened and see what lessons we could learn to make future buildings better. We knew that current building codes would not be sufficient, so we had to design a building for a future code that had not been written yet.”

To do this, the team reviewed best practice from around the world and consulted the emergency services and the designers of emergency systems. But fundamentally, it came down to a judgement call, says Yoram Eilon, senior vice president of building structures at WSP | Parsons Brinckerhoff in New York. “We didn’t want to make new rules that would be too conservative and cause a dramatic increase in costs. It all had to be reasonable but obviously address the issues.” Gratifyingly, when the new building codes were finally produced, many of the solutions that the team came up with had been adopted.

 

Hybrid Concrete-and-Steel Structure


The design of One WTC is very different from that of the twin towers. While they were held up by external steel columns, it has a hybrid concrete-and-steel structure. The key to its strength is a massive, extremely strong concrete core. This rises all the way to the uppermost storeys, acting as the building’s primary support to resist gravitation, wind and seismic loads and impact, as well as housing all means of egress. Steel beams set into the concrete core support the floors, enabling vast column-free expanses. The core contains two interlinked access stairs and a dedicated first-responders’ stair – to allow first responders to climb the building quickly in the event of an emergency, while people escape – a feature that is now standard in New York building codes.

Concrete Stronger Than Any Rock


The core is made of ultra-high-strength concrete, at 14,000psi the strongest ever poured in New York. “It’s much stronger than any rock you could find,” says Rahimian. This strength also helped reduce the thickness of the walls, maximizing the lettable area, while minimizing the weight of materials required. Replacing more than 50% of the cement content with industrial by-products further cut the carbon footprint of the building, and more than 95% of the steel in the structure is recycled, contributing to a LEED Gold environmental rating.

Underground Extension of the Tower


As impressive as the above-ground structure is, the tower also has a significant – and challenging – mass below ground, which was a Herculean task in itself. The building’s five underground storeys go down 70ft. The masterplan demanded that the tallest tower be built in the north-west corner of the site – but this meant threading the columns through the tracks of the fully operational train station underneath. “There is a whole city below that extends way beyond the footprint of the tower,” says Eilon. “One World Trade Center has about half-a-million square feet below grade – that itself is bigger than many tall buildings.”

 

Unique Facade and LEED Gold Rating


While this is largely hidden from public view, back above ground the architects were responsible for perhaps the most highly visible façade on the planet. Aesthetics were therefore of paramount importance. Aside from the sheer size of the building, it’s the way that it catches the light that makes the greatest impression, the luminescence of the structure somehow lightening its monumental presence on the skyline.

This is thanks to the floor-to-floor glazing – the exterior of the building is composed of 1 million ft2 of specially developed glass. It has a very high U-value to maintain comfortable conditions inside the building and support the LEED Gold rating. It’s also highly transparent, with a reflective, mirror-like coating that creates a constantly changing kaleidoscope. The corners are clad in laser-finished stainless steel that glints in the sunlight.

“It reflects the sky and the buildings around it, and when the light hits it a certain way you can see right through it,” says SOM’s Gottesdiener. “Sometimes it glows a bright orange or red. It’s got a very beautiful quality to it.”

 

Office Space Flooded With Natural Light


Of course, notwithstanding its symbolic and monumental role, One WTC is a commercial office building. On entering, workers are immediately faced with the interior’s most impressive feature: a 50ft-high atrium. Above that, there are four levels of mechanical space, before the office floors begin on the 20th storey. After 71 storeys, there are further mechanical floors topped by a three-storey public observation deck on levels 100-102.

“It has to function as a very stable, very efficient environment where people want to come and work,” says SOM’s Gottesdiener. Key to this appeal are generous 9ft 6in ceiling heighs and column-free expanses of up to 45ft from the core to the perimeter, as well as the floor-to-floor glazing.  “We wanted to allow as much natural light to penetrate the building as possible. People don’t have to turn on the lights 90% of the time, because there’s so much natural light flooding in.”

Three World Trade Center, located opposite the WTC Memorial and Cultural centre, will be 357m high. The 80-storey building has unusual design with an exposed steel structure. The building includes office and trading floors above 5 levels of retail. The three-level high lobby offers tenants and visitors a picture-window view onto the WTC Memorial.

Seven World Trade Center was the first building completed in the WTC complex in 2006. Not only was it constructed quickly, but it included a host of life-safety and environmental features never before incorporated into a commercial skyscraper. The first 10 floors house an electrical substation servicing the entire lower Manhattan area.

 

Architects Skidmore, Owings & Merrill (1 and 7 WTC) 
  Rogers Stirk Harbour + Partners (3 WTC)
Clients Port Authority of New York and New Jersey (1 WTC)
  Silverstein Properties (3 and 7 WTC)
Our Services Structural Engineering