Much of what the World Trade Center rebuild worked out from scratch is now simply how tall buildings get designed. That’s the quiet argument three AECOM Tishman veterans make, 25 years on.
Start with the stairs. During the evacuation on Sept. 11, 2001, people reached the ground floor and still couldn’t get out. Narrow stairwells jammed when someone winded or injured had to stop.
What changed in high-rise egress design
“One of the design things that was incorporated into all these buildings was the tower stairs go down to the podium, say, to the fourth or fifth floor, in which case, in that area, it spreads to four separate stairwells that go one out each elevation,” said John Kovacs, AECOM Tishman’s co-chief operating officer for New York, speaking to Construction Dive. A directional monitoring system flags which stairs are blocked and which are clear.
“A lot of those things now have morphed their way into either the building code or normal construction practice,” Kovacs said.
The climbing cocoon and the BIM command center
On One World Trade Center, at 1,776 feet, the team ran a safety “cocoon” that enveloped 16 floors and rose with the building. It was the first on a hybrid steel-and-concrete structure, protecting the trades at the perimeter and stopping material falls, said Eric Reid, New York co-COO.
“That’s somewhat common practice now, looking back 25 years, but at the time it was very innovative, especially for commercial buildings at that height,” Reid said.
The other legacy is coordination. Every drawing was 2D when the work started. MEP across four towers and a transportation hub, under different ownership entities and on different design timelines, forced the conversion to 3D. Tishman set up a BIM command center in the field office at 7 WTC where each subcontractor added its components to shared models alongside the others.
The Bathtub, and the No. 1 train
The hardest engineering sat below grade. The original 1960s construction excavated only the west side of the slurry-walled perimeter that builders call “The Bathtub.” The rebuild needed full-depth excavation of the east side, which meant extending the slurry wall, removing sections of it, and structurally supporting the No. 1 subway tunnel that had been bearing on grade. Crews excavated roughly 100 feet below the tunnel and installed new shotcrete walls and footings.
It was done without an unplanned service outage, which Reid called a testament to the engineering and construction teams on that package.
Turner broke ground this summer on 2 World Trade Center, which becomes American Express’s headquarters when it finishes in 2031. The first rebuild on the site, 7 WTC, started in 2002. Thirty years, one site, and a set of practices the rest of the industry now inherits without thinking about where they came from.