World Trade Center
New York, New York
Built: 2005 – 2007
Architect: Minoru Yamasaki, Emery Roth & Sons
Architect of Record: Emory Roth and Sons, P.C.
Structural Engineer: Leslie E. Robertson, John Skilling
Contractor: Tishman Realty & Construction Company
Mechanical: Jaros Baum and Bolles
Electrical: Joseph R. Loring & Associates
Foundation: Port Authority Engineering Department
To allow for an open plan, the structural system consisted of closely spaced columns around the perimeter of the building and a core system. The perimeter column system consisted of 236 welded steel tubes spaced 26 inches apart. There were also 47 columns in the core of the structure. Elevators, stairwells and utility shafts were contained in the core, allowing for maximum flexibility in the rentable space.
At the time, the use of prefabricated building segments was ground breaking and greatly increased the speed of construction. Exterior columns and spandrels were welded into panels that could be erected in pieces.
The floor system embedded the top portion of the bends of the steel bar joists “knuckles” into the concrete slab. This was an innovation in composite floor slabs for tall buildings. To reduce the discomfort of occupants to building sway, the bottom cords of the joists were connected to the main frame by viscoelastic dampers.
The final piece of the structural system was the “hat truss”, a series of steel braces on the top floors. It was designed to support the antenna that would be attached to the top of each tower and provided extra connections between core columns and core and perimeter structural systems. During the September 11 disaster, the “hat truss” was a key player in the redistributing loads.
Vertical Transportation of Occupants was handled through an innovative, three-tiered system of elevators. Express elevators transported people to Skylobbies where they could transfer to local elevators. It is estimated that this system used 25% less space than would be required for a traditional vertical transportation system.
Three fire stairs descended from the roof to the ground level, but they were not continuous. In the vicinity of the mechanical floors, the stairs were broken and connected by corridors ranging from 10ft to 100ft.
Final Report on the Collapse of the World Trade Center Towers. National Institute of Standards and Technology National Construction Safety Team Act Report 1. Federal Building and Fire Safety Investigation of the World Trade Center Disaster. 1298 pages (September 2005) US Government Printing Office, Washington, 2005.