Weave Bridge

The true geometry of the bridge is comprised of two steel spirals.

Arup’s dramatic new bridge spanning railway lines and linking The University of Pennsylvania with its sports recreation grounds is creating a buzz throughout the industry.

Employing a structural theory never before used in bridge design, a close weave of steel girders is actually a massive double helix that coils over the railway lines.

“The project is not about engineering a new type of bridge structure per se. It is about treating the bridge as architectural space: a space of passage in a master plan that is not unlike a major corridor or link in a large building complex,” explains Cecil Balmond, founder of Arup’s Advanced Geometry Unit (AGU).

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Called The Weave Bridge, the project is part of ‘Penn Connects’, a strategy to connect the main city artery to The University.

The project is key to the university’s vision for urban design which revolves around enlivening a strong network of pedestrian pathways, in particular those located near the Eastern half of the campus, which is currently being extended.

Daniel Bosia, Director of AGU led the realisation of the design concept for the bridge which was part of a master plan originally devised by Cecil Balmond.

In elevation this 42m coil looks like a conventional bridge – a warren truss with diagonally crossed bracing lines. However, its true geometry is comprised of two interweaving steel spirals.

The design has no need for longitudinal beams, as conventionally used. The winding narrows at mid-span over the tracks to intensify and reinforce the structural action. It then flares out at the supports. Psychologically, it’s been suggested that the feeling of enclosure at this point is also reassuring for the pedestrian.

The unusual design is the result of a larger narrative, created by Arup AGU, which develops a combined approach to landscape and tectonic form in one unfolding master plan. The bridge is the first phase of the plan completed which was designed to provide immediate access to the sporting grounds.

However, the concept as intended by Balmond evolves as four lines, which start at the end of the sports field farthest from the railway and wind their way, gathering energy as they approach the railway lines. Initially, a meandering path, rocks, planting and gravel, denote the lines.

As they gain energy, the lines rise up an embankment towards the bridge and also take shape as elements rising from the ground to begin to form tectonic strands.

With a burst of energy the lines then spin across the railway in the close spiral form of the bridge. On the far side of the railway lines, the close coil quickly loses its structured form again.

The four lines descend quickly and unravel rapidly in response to the immediate and contradicting urban environment of a car park and cycle path.

This concept will be woven into the new master plan for the overall site as it is an essential part of how The Weave works within, and activates, its surroundings.

The bridge itself was built as a single element off-site and lifted into place over the live railway lines. Glazed panels provide good views of the surrounding area from the bridge, while negating the requirement for handrails, which would spoil the architectural form of the bridge.