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7 World Trade Center uses type 316 stainless steel

By Catherine Houska, TMR Stainless (Consultants to IMOA)

The original 7 World Trade Center building consisted of an office tower, electrical transformer vaults and access ramps to the World Trade Center’s service levels. Like the twin towers across the street , this 47-story building completely collapsed as a result of the events of September 2001. Immediate reconstruction was required, primarily due to the need to rebuild Manhattan’s power grid.

As the first building constructed on the site, 7 World Trade Center was highly visible. The architects, Skidmore, Owings & Merrill (SOM), were challenged by the owner, Silverstein Properties, to improve upon the original building’s function and appearance.

The current building was completed in 2006. The first 26 meters (85 feet) of the office tower contain the Con Edison electrical transformer vaults, which require a significant amount of ventilation. Offices occupy the higher floors of the building. Because it functions as both an industrial facility and an office building, there were unique design challenges.

The stainless steel and glass exterior surfaces were designed in collaboration with artist James Carpenter. Molybdenum-containing Type 316 stainless steel was selected, because the building will be exposed to both coastal and deicing salt and moderate levels of urban pollution. The owner desired a building that would remain attractive over time and provide a long service life. Molybdenum alloying additions increase a stainless steel’s resistance to corrosion and staining by salt and pollution.

7 World Trade Center

SOM’s rendering of 7 World Trade Center, Courtesy of Skidmore, Owings & Merrill.

Street level view of 7 World Trade Center. Photo: N. Kinsman

SOM and James Carpenter used the different purposes of the upper and lower building to create a study in reflected color, light, and depth, which adds a sculptural element to the building. The exterior of the office tower on the upper floors are comprised of floor-to-ceiling clear glass with concave, Type 316 spandrel panels between the floors. The spandrel panels are embossed with a light pattern that simulates abrasive blasting and were then corrugated. The horizontal corrugation pattern enhances light reflection. Below each panel there is horizontal sill made of blue, electrochemically colored, Type 316. The blue color reflects onto the concave spandrel panels giving them bright, bands of reflected color.

A rendering of the spandrel and blue-colored Type 316 panels used between each floor in the office tower. Courtesy of Skidmore, Owings & Merrill.

Placement of the blue panels was determined by varying the panel angle relative to the spandrel to obtain optimal reflection onto the spandrel panels. Courtesy of Skidmore, Owings & Merrill.

The exterior walls surrounding the transformer vaults have to accommodate high airflow requirements and meet the increased structural requirements of the project.  The commonly used structural stainless steels provide much higher impact resistance then carbon steel.  A welded Type 316 grating was selected. Woven mesh was not considered, because there are tiny crevices at each point where wires cross and Type 316 can be susceptible to crevice corrosion when exposed to salt.

Welded gratings had previously been used only in industrial locations, so SOM and Johnson Screens created a more aesthetically pleasing product. By using custom triangular wire profiles, by changing wire orientation and finish, and by varying wire density, a surface with varying reflectivity was used to create depth, surface variation, and visual interest. The two layers of Type 316 stainless steel bars are separated by a 165 mm (6.5 inch) cavity. The inner layer is lit at night with blue and white LED sources. The color is programmed to shift in color tone from day to night and the surrounding surface pulses and moves subtly through the night because of the bar profiles and their orientation

Day  view of the welded mesh encircling the bottom 26 meters (85 feet) of the tower. Photo: N. Kinsman

Light and color variation was obtained by varying the bar shapes and their
orientation within the welded screen.

Courtesy of Skidmore, Owings & Merrill.

The owner and designer wanted to increase the structural performance standards for the perimeter wall at the  tower’s base. The welded mesh around the base of the building inherently created a structural barrier, but there was a desire to create an open, transparent lobby for the tower.  These seemingly different requirements were met by using high-strength 2205 duplex stainless steel and Type 316. The storefront glazing and doors are set below a girder and within a grid of mullion posts. Both the girder and posts are made from built-up 2205 plate. Built-up, type 316 plate beams cantilevered from the girder support the glass canopy, and the type 316 cable net wall above it is supported at the ceiling and sidewalls.

“High-strength duplex grade 2205 alloy stainless steel was necessary to accommodate the tremendous loads imposed on these stainless steel framing elements by the tensioned stainless steel cables, maintain desired minimal visual sightlines, and meet the enhanced structural performance standards”, said SOM’s Christopher Olsen, AIA. The 2205 has a fine directional brushed surface finish. The visibility of welded joints was minimized and some built-up assemblies were mechanically joined with concealed fasteners.

The inherent color of duplex stainless steels like 2205  is slightly different from austenitic stainless steels like 316. The canopy separates the stainless steels and the angle between the elements would hide the minor color variation. Different finishes would naturally create tonal variations and this aspect of design detailing was used to harmoniously on this project.

7 World Trade Center’s entrance combines high strength 2205 duplex and Type 316L stainless steels. Photo: N. Kinsman