The creators of the new Weave Bridge on the University of Pennsylvania campus in Philadelphia were not content simply to span some railroad tracks. Cecil Balmond and Arup’s Advanced Geometry Unit (AGU) have completed a bridge whose graphic punch ripples into the surrounding landscape. Its structural design eschews the chord, post, and web members of traditional truss systems, ushering pedestrians through a helix of interwoven diagonals.
Opened in June, the $2 million, 145-foot-long pedestrian bridge is among the first physical milestones in the university’s plan to transform a forlorn postal depot into a 24-acre complex of athletic fields, open green space, and recreational facilities to be known as Penn Park. Landscape architects Michael Van Valkenburgh Associates are in charge of site planning for this $40 million project along the Schuylkill River, which separates the campus from central Philadelphia. Both Balmond and Daniel Bosia, director of the London-based AGU, have taught architecture at Penn, so they were well positioned to land the job of designing the signature pedestrian bridge, and to study the site.
What makes the bridge so distinctive is what Bosia calls a “braided rope” structure consisting of six steel strands wrapping around the floor, walls, and ceiling of the walkway. Painted black to stand out from the nonstructural wood and glass infill surfaces (actually polymer boards and Perspex panels), the diagonal steel segments need no horizontal or vertical reinforcement.
“The intent was to dematerialize the box,” says Bosia. “We started with a three-dimensional coiling space. Instead of horizontal continuity, you have winding continuity.”
In order to heighten the sense of seamless spiraling movement, Arup developed a custom connector plate to join roof and wall beams without a supporting frame. This joint detail emerged from Balmond’s 2002 collaboration with Toyo Ito for the Serpentine Pavilion, a lattice of intersecting lines and trapezoids. “The corners of the Weave Bridge yield strong buttress features,” says Balmond, who describes the elevations as “alternating transparency and opacity.” The temporary ramps currently leading to the new bridge are just placeholders. According to Balmond’s plan, the geometric motif of the bridge will continue along the forthcoming approach ramps and onto the adjacent grounds. The bridge’s hovering steel braids will unfurl into strands of meandering walkways, seating, rocks, and plantings.
The 45-ton Weave Bridge is narrowest at the center of the span, flaring gradually toward the openings to increase visibility. Its sleek lightness of figure makes the adjacent rail bridges look cartoonishly massive–and makes the trek to the ballfields a lot more exciting.