One World Trade Center: Restoring Confidence

One World Trade Center stands quietly at 285 Fulton Street as a proud but solemn memorial to our fellow citizens who perished tragically on September 11, 2001. But the teams that designed, engineered and built the towering symbol of America’s resilience in lower Manhattan were faced with issues from day one. Never mind all the wrangling by different groups about what the rebuild should be – or the disputes between the Port Authority and the developer. And forget about the nearly 4 billion that had to be raised and spent by the time its spire was set in place. This project was a monumental construction challenge from the ground up.

After rescue and recovery operations were halted, the first order of business was clearing the site. Shortly after the attack occurred, truck after truck began rumbling off with debris from the site. Many pieces would eventually find their way to every state in the union for use in local memorials, and some of the recovered steel was even used in building the Amphibious Warfare Ship, USS New York. But even though removing the mountain of debris was a Herculean task, the real work of constructing the tower was only just beginning.

The site itself posed an extraordinary engineering problem to solve. For countless years, it had been a busy, working landfill that had eventually expanded the land area of lower Manhattan into the Hudson River. Without creating some type of barrier, the river’s waters would ooze the excavation as quickly as the compacted material was pulled from the ground.

The answer was nearly Medieval in nature. In effect, a deep moat (or trench) surrounding the site was dug and filled with a mixture of water and a high-swelling pond clay called bentonite. The result worked because dirt was not as dense as the slurry, and that kept the threat of a cave-in at bay.

Once the slurry moat was filled, a huge steel cage more than 100 feet tall was lowered into a section of it. And when concrete was poured into the trench, it displaced the lighter-weight slurry and created the first section of a barrier that was affectionately known as the bathtub. After the bathtub was completed and the Hudson’s waters were held at bay, workers could start removing the 70 feet or so of former landfill and dirt to get down to the solid bedrock they would need to support the future tower.

For obvious reasons, designers wanted to build the world’s safest skyscraper. But they didn’t want it to look like a cold, forbidding fortress. The answer was to create a core “building within a building.” The reinforced-concrete core’s 2-foot thick (at a minimum) walls shield elevators, stairways, mechanical systems, standpipes, antennas and restrooms within its 110-foot deep square space while ductile steel moment frames guard the perimeter. On each floor a 10-foot wide hallway cuts through the middle of the core. For building visitors and occupants, the result is wide open, column-free space with a generous amount of natural daylight.

During construction, as the tower began to rise above its neighbors in the Manhattan skyline, contractors had to consider ways of minimizing the risk of objects falling from height. With all the pedestrian traffic that New York City is known for, this was no small concern.

 

As an example, self-climbing cranes were used to place the permanent perimeter structural steel. Since they were self-climbing, they didn’t have to disassemble and reassemble the tower crane mast sections to raise them up to the next level. Eliminating that process reduced the risk of falling pieces.

Another example was the use of fully enclosed skid boxes for debris removal so the most waste could be safely loaded and unloaded. Additional measures used were tool tethering, pail covers and other ways to secure tools and supplies as they were used or transferred from place to place.

In an effort to further strengthen the building, an especially strong concrete mixture was developed. With compressive strength of up to 14,000 psi, it is stronger than any rock you could find, and was stronger than any mix that had ever been used in the Big Apple. The Port Authority’s Materials Engineering lab, working with several consultants, thoroughly tested the material, keeping hydration heat down by replacing more than half the concrete with industrial by-products like slag, flyash and silica fume. Normally, high-strength concrete sets up faster, but this concrete needed to be workable for a relatively long time of two hours to accommodate transit time on city roads and up to the tower’s top floors. They used sensors embedded in the concrete to monitor curing as ironworkers and concrete subcontractors constructed the core. During this project, concrete was pumped higher (103 stories) than ever attempted before in the Western Hemisphere.

Escape stairways leading down to street level were completely re-conceived for One World Trade Center. The two descending stairways are each 72 inches wide compared to the previous standard of 44 inches, and are enclosed within the core.

Glow-in-the-dark markings and battery-powered lights will keep stairwells lighted in an emergency. Independent air systems will keep them smoke-free. An additional dedicated first responders’ pressurized stairway and water-resistant elevator will make sure these heroes won’t have to wade through waves of descending occupants in the event of an emergency. Many of the improvements made in this building’s systems have now become part of the New York City Building Code for high-rise buildings.

To create the dazzling appearance of the tower as the sun crosses the sky and light conditions change, architects chose to glaze it floor-to-floor. One million square feet of glass was used, and it was carefully developed specifically for this project.

The extremely high U-value of the glass keeps interior spaces comfortable (contributing to its gold LEED rating) and its high transparency makes gazing across the landscape below a special treat. On the outside, the highly-reflective glass, with its corners covered in laser-finished stainless steel creates a breathtaking daily display. And when the light is just right, you can even see straight through it.

After the original Trade Towers fell on that terrible day, there was no shortage of voices calling for the goal of rebuilding on the same site to show terrorists our collective resolve and restore the confidence of the American people. But an unstated goal was standing by quietly in the shadows.

Architects, engineers and contractors wanted to restore Americans’ confidence in the viability and safety of skyscrapers. After painstakingly analyzing safety issues and what had happened structurally during the tragic event, the builders of One World Trade Center constructed a tower that has rewritten the playbook for supertall buildings to come.