Do the Twist

Do the Twist

The University of Pennsylvania has landed a piece of trophy architecture with a definite twist: the new Weave Bridge, designed by structural engineer Cecil Balmond and his legendary Advanced Geometry Unit (AGU) research group at Arup. Now open to the public, the bridge will become part of a second phase of design work this fall with its integration into the surrounding campus masterplan, itself a hefty undertaking to remake the Philadelphia campus.


The unusually ambitious design was commissioned by the university in 2007, in reaction to a city announcement that it would temporarily close an essential campus connection: the century-old South Street Bridge, which had long served as the sole passage over an Amtrak line that runs between Penn’s athletic fields and its Hollenback building, home to athletic and ROTC facilities.

Although Penn officials originally intended to operate a shuttle while the city rebuilt the South Street Bridge, they realized their money could be better invested in something permanent, especially as they are now redeveloping the campus under a 30-year masterplan devised by Sasaki Associates to increase open space and connectivity.

“Because this was such a forward-looking opportunity for Penn,” said Penn’s Principal Planner Mark Kocent, “we wanted to raise the bar a little bit and not do a straightforward Warren truss bridge.” So they turned to Balmond, famous for his innovative designs as deputy chairman of Arup, and currently on the faculty at Penn’s school of architecture.

Balmond’s design is composed of six steel strips woven around a square cross-section that flares from about ten feet wide at its midpoint to 16 feet wide at each end of the 165-foot span. “Structurally, it’s like a rope,” said Daniel Bosia, head of the AGU. “So you wouldn’t be able to take the walls apart from the roof and floor.” He called the design a reciprocal frame, one in which each element is supported by the next, resulting in a rigid, overall interlocked structure.

The AGU group has been experimenting with the concept for a few years, notably in their Serpentine Gallery pavilion in 2005, which was composed of short interlocking pieces of timber (though not welded together as the components of the Weave Bridge are). Applying the reciprocal frame concept to a bridge and in Philadelphia are both firsts.

Arup worked with engineer of record Ammann & Whitney on the structure, which was tempered by concessions to safety and a $2 million budget, said Bosia. For the span, they switched to carbon steel instead of stainless steel, and for the cladding, they substituted a polymer blend instead of timber. “It’s fairly poor material, but the power and the form of the bridge allow you to use simpler, cheaper materials,” Bosia said.

Balmond had also wanted the bridge to provide an unobstructed view of campus, but Amtrak forbade the use of open sides above their electrified rail lines. AGU’s compromise was to fill in the side panels with Plexiglas, but to leave the bridge open to the elements overhead, providing the additional benefit of making the bridge self-cleansing in the rain.

As the next stage of Penn’s masterplanning begins this fall, AGU will begin to connect the Weave Bridge to a future 14-acre park being designed by Michael Van Valkenburgh Associates, a $46 million project scheduled to open in 2011. In lieu of the concrete retaining wall currently installed on its west end, the bridge will rest on a berm and will branch off into pathways winding their way through the site.

“We’re going to have [Balmond] work with Van Valkenburgh and the park team, to merge his vision with the park design so you get a senseof the weave unwinding and becoming part of the park,” Kocent said.

A version of this article appeared in AN 09.09.2009.