Snøhetta, Adamson Associates, Buro Happold
Among the towering giants and behemoth cavern currently under construction at the World Trade Center site, it can be easy to overlook the Entry Pavilion of the National September 11 Memorial Museum. After all, it is only three stories high and contains a mere 47,000 square feet, much of which is mechanical equipment. However, the little pavilion serves vital roles in the master plan, both functional as well as aesthetic. For one, it houses the entrance to the museum—a grand stair that descends beneath the recently-opened plaza beside two of the soaring steel “tridents” salvaged from the wreckage of the original twin towers. The building also contains an advanced security apparatus for screening visitors, an auditorium, the aforementioned mechanical equipment, and a special room reserved for World Trade Center attack survivors and the family members of those who lost their lives.
As with every other piece of the massive construction project, the pavilion is also far more complex than a cursory examination of its architectural renderings makes it seem. The design team—which includes Norwegian architectural firm Snøhetta, local architect of record Adamson Associates, and multi-disciplinary engineering firm Buro Happold—faced the very unusual challenge of designing a building that could perch off the edge of two different lower structures: the Path Station and the Memorial Museum. This required developing a series of unique structural solutions that not only meet New York City building code but also stand up to the heightened security concerns of the World Trade Center site.
The majority of the pavilion rests atop the Path Station, specifically atop three massive north-south oriented steel girders, each between 13 and 16 feet deep, which were designed by Port Authority engineers. Only the western tip of the building, which contains the grand stair, sits on the concrete mat of the memorial museum, designed by Aedas and Cantor Seinuk. The challenge for the design team was to create a “foundation” for the pavilion that would both distribute the building’s gravity loads across these two underpinning structures as well as handle the rather intense lateral loads that could occur under the conditions of a blast event. Before anybody starts thinking that was an easy chore, there were additional complicating factors. Two of the Port Authority’s girders—the eastern most and the western most—did not span the entire depth of the pavilion’s footprint, meaning that much of the building would have somehow to be hung off their ends. The northeastern edge of the pavilion also extended beyond the easternmost girder, meaning that as much as 15 feet of the building would have to be cantilevered over the path station. Finally, while the Port Authority engineers allowed the team to transfer north-south lateral forces to the girders, east-west forces were off limits.
The team established “footings” for the building that they termed “drag beams”—3-foot-wide by 7-foot-deep concrete beams, heavily reinforced by structural steel wide-flange sections and two layers of No. 10 rebar. The drag beams follow the perimeter of the pavilion, and one bisects its east-west axis, spanning as much as 100 feet across the underpinning structures. Between the center and southern drag beams, which run east-west, and atop the three Port Authority girders, which run north-south, they established a concrete core that rises the full height of the structure, functioning as both hardened ingress and egress as well as a cavernous ventilation shaft for the underground spaces. The core transfers the building’s north-south lateral loads to the girders. All of the east-west lateral forces are transferred from the drag beams at the western end of the pavilion to the memorial’s concrete mat via structural shear dowels.
Hanging the north edge of the pavilion off of the two short girders called for two different solutions. At the eastern-most girder, which was 16 feet short, the team was able to employ an inclined beam that runs up from the end of the girder at a 45-degree angle to the second floor, where it becomes a column and runs vertically to the top of the structure. The westernmost girder, however, was 20 feet short. There, the team ran a column vertically to the roof and then suspended the remainder of the structure from a 22-foot-deep truss.
The rest of the pavilion’s framing is more conventional in nature—steel post and beam and concrete floors poured on metal decking—though many of the members are encased in concrete and are larger than one would expect for a building of this size. In fact, some of the girders that support the infill beams go up to W40x503—the largest rolled sections available—making Memorial Pavilion a very sturdy enclosure indeed.