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OEM Report: Heavy Equipment/Construction

Finishing Touches

By Tom Klemens

Designers, fabricators and erectors are creating the National September 11 Memorial and Museum with extraordinary collaboration and care.

November 2012 - Despite its small footprint and diminutive stature, few new structures have as high a public profile as the Entry Pavilion at the National September 11 Memorial and Museum in New York. The complex is situated on eight acres of the original 

16-acre World Trade Center site. Tucked between two large reflecting pools in the footprints of the Twin Towers, the Entry Pavilion leads to the Museum, which is below the Memorial plaza and now in the final stages of construction.

The Memorial was dedicated last fall on the 10th anniversary of the Sept. 11, 2001 attacks on the World Trade Center and the Pentagon. It honors those who perished there and on Flight 93, as well as in the Feb. 26, 1993 World Trade Center bombing.

The Museum has yet to be completed but already houses the iconic tridents, two seven-story steel columns that stood side by side on the eastern façade of the North Tower. Crews put the 50-ton artifacts into place in September 2010, after which the Pavilion atrium was erected.

A clear view

“The emphasis of the Pavilion design is to mediate between the Memorial and the very tall buildings that surround it,” says Anne Lewison, AIA, senior project manager with Snøhetta, New York, who has worked on the project since 2006. Much of its design has been dictated by other ongoing building projects at the site. The structure straddles the new Port Authority train station, serving the PATH lines, and the Memorial and Museum. Although the Pavilion was completed in time for the Memorial dedication, both the PATH station and the Museum are still under construction. 

The three-story building, Lewison observes, serves as a connection between the commercial buildings looking heavenward toward the future and the Memorial, which looks inward and down.

“The Pavilion really reflects the present,” Lewison says. “Visitors can see themselves reflected in the storefront section as they approach the entry.” To ensure the atrium did not visually conflict with the tridents, its design is purposefully irregular. To accomplish that, Snøhetta skewed and tilted what might otherwise have been a very simple structure.

Angles everywhere

The Pavilion’s flat surfaces belie its complexity, which caught many involved off guard. “It looks far simpler than it proved to be, in terms of building it,” says Bruce Hernsdorf, project manager for W&W Glass, Nanuet, N.Y., which was responsible for the exterior façade excluding the roof. “There are no parallel lines here; nothing’s plumb and square.”

To address the challenge, W&W’s subcontractors used workpoints from the architect’s master computer model as the basis for fabricating the complicated geometries. 

Glass panels in an irregular steel framework surround the Pavilion entryway and provide a clear view of the tridents inside, but roughly 75 percent of the façade consists of stainless steel panels. A. Zahner Co., Kansas City, Mo., fabricated the individual 4-ft.-wide, 9-ft. 11-in.-long panels using its proprietary machinery and processes.

During the panel selection process, Zahner prepared numerous samples with varying reflectivities for the architect’s evaluation. “We bead-blasted No. 4, No. 6, No. 7 and No. 8 finishes,” says Gary Davis, director of marketing at Zahner, “varying the intensities of blast to produce a different specularity in the panels. We also varied the intensity over the perforated panels,” which mask the building’s HVAC louvers. In the end, Snøhetta selected a No. 4 finish for the panels, with a GB-60 bead-blasted finish for those that are not reflective. 

“There are two different base stripe panels,” says Lewison. “All of the No. 4 reflective is the same, but different bead blasts produce a slightly different matte quality.” The walls also have areas with no stripe pattern.

“The two different stripe patterns will be seen differently in different lighting conditions,” Lewison says. “And we continued the stripe pattern from the steel across the first two panels of glass with a graded frit, so there are parts of the site where it becomes very hard to tell where the edges are.”

“It’s a long process, working with the architects to get it right, but it’s very gratifying,” says Davis.

Zahner’s waterjet equipment that cut the panels also marked the attachment location of each stud. These locations also were irregular because of the support structure’s geometry. Zahner craftsmen carefully welded the studs to the panels to accommodate these variations and avoid stud read-through.

The panels were assembled into larger units (dubbed megapanels), 10-ft. wide and as tall as 40 ft., by Island International Exterior Fabricators, Calverton, N.Y. “Working with Zahner, we developed the whole cladding support system and megapanel system,” says Ron Van Steekelenburg, Island’s senior project engineer. “The system allowed us to produce megapanels that basically could be 95 percent finished, trucked to the job site and installed, instead of trying to do that in pieces.” 

Shop prefabrication also facilitated quality control and avoided weather-related issues. 

“Compared to what we typically do, this is not a large project whatsoever,” says Van Steekelenburg. “But the monumental value of such a project in New York, with such sentimental value to everybody in New York, and as we are a New York-based company and all our projects are in and around New York – it’s just very good and satisfying to have worked on such a key project.” FFJ

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