January 2017 January 2017 - Page 21

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Web Exclusive Article from Civil Engineering Magazine at www.ASCE.org

Continued from page 16

The park offers such residential elements as stoops, porches, and swings because features of this type are largely missing from many of the living spaces in the nearby neighborhoods.

"We're putting in an overhead catwalk area, and we're connecting it to the existing girders," Van Schooneveld says. Construction activities there might add more weight than the residents who will eventually use the park, he points out. "We're not too concerned about the strength of the girders, but obviously we had to design the overhang area to handle a small Bobcat loading."

The most unusual load that the engineers had to accommodate was the dynamic one imposed by the 8 ft wide swings on the Callowhill section, which can seat 8 to 10 people at a time. But even those loads were limited because the movement of the swings themselves is limited for safety. "It's not a massive loading compared to a railroad, but it does add a dynamic load," Van Schooneveld says. The swings hang from a steel structure one vertical member and one horizontal member, both tilted back slightly. The structure is bolted into the exterior and first interior girders of the bridge superstructure. "That resists the moment forces," Van Schooneveld says. A frictionless copper pin at the top of the supporting structure connects to a rod that holds the swing. "Welding on the tabs limits the range of motion," he says.

Other areas of the structure were analyzed to determine their ability to hold the plantings. Angelo Waters, P.E., M.ASCE, the vice president and manager of environmental services for Urban Engineers, says, "There will be improvements at street grade as you walk to the park [on the west end]. And then as you enter the park at 13 th Street, you go over a bridge, and there're six through girders there. The middle four will support walking paths, and the outer two girders will be converted into planter boxes." Beyond that point, the rail structure is supported on fill between the retaining walls. "There is a lot of soil to work with there, so there will be larger trees from 12 th to Callowhill," Waters says. "Then as you cross 12 th Street to the viaduct structure, the soil there is a lot shallower, so the plantings will be smaller but maybe more dense, such as shrubs."

The walking paths throughout will be made of chip seal, a low-maintenance pavement surface that combines asphalt with aggregate. The platforms and benches will be made of Ipe, an extremely durable hardwood. "Ipe is hard to scratch, and you can't burn it," Waters explains. Ultimately the maintenance of the park will fall to the Philadelphia Department of Parks and Recreation, so every material had to be approved by it. "This isn't a typical park for a city," Waters says.

The project requires close coordination on the part of the Center City District, Studio Bryan Hanes, Urban Engineers, and the construction company, the local office of AP Construction. "There are no existing or as-built plans, so there will be a lot of verification of dimensions in the field," Van Schooneveld explains. And Noble Street is home to two significant Internet infrastructure companies. "It's a major data center. They tell us that a large portion of the Northeast's Internet traffic goes into those buildings, and there are utilities galore," says Waters. "When you're dealing with landscaping and tree plantings, working around those utilities is no small challenge. And we're still dealing with it. We have good information, but one wrong cut . . . "

The companies also require constant access to their facilities during construction. "So coordination with them is imperative," Van Schooneveld says.

But those companies are just as excited about the prospect of the park as are the residents, and when construction concludes in 2018, the designers hope their efforts will help employees, visitors, and residents of many neighborhoods connect with one another and with the city's industrial heritage.