07 December 2015

Mimram wins Bath footbridge competition

I'm catching up on news from recent weeks, so don't expect anything here you didn't already know.

Bath and North East Somerset Council (BANES) announced a winner back in mid-November in their competition for a new £2.5m footbridge in Bath. I discussed the shortlisted entries here back in September.

The winner is Marc Mimram with Webb Yates (the latter were listed as part of the shortlisted team, but curiously aren't acknowledged on the BANES website). I'd be interested to see a jury report or other details of how the winner was chosen, as BANES had initially said the choice would be on a combination of design quality and fee proposal. There's a complete lack of transparency around the entire competition process.

The winning design has echoes of Mimram's Pont de Solferino in Paris, which shares the vierendeel truss ribbed girders. The Paris bridge is a marvel of complex craftsmanship, but the complexity of its detailing led French bridge expert Michel Virlogeux to comment that "fantasy governed the detailed design, a fantasy which had not been tempered by the rationality of a serious engineer".

Mimram's design for Bath follows an S-shaped curve in plan, and coupled with the sinuous variation in girder height, this means that no two pieces of metalwork will be the same.

The designer suggests that the bridge "adapts to the scheme of stresses", by which he essentially means that the girder depth follows the bending moment diagram for a two-span bridge with one short and one long span. On closer examination of the elevation, this is, however, not actually the case. It's broadly true for the longer span, but it isn't true for the short side-span, which is deepest half-way along rather than over the pier, which is where the forces are greatest. This is a consequence of having the footway deck follow the girder chord - the "correct" girder shape would have a kink in it, which is clearly visually unacceptable here.

The pier supports the bridge not directly from the edge girders, as would make most structural sense, but from a hammerhead cross-beam. That leaves the visual emphasis on the bridge's sensual shape, but in my view also leaves it looking strangely incomplete. The hammerhead as shown doesn't seem deep enough at its connection to the main girders to carry all the loads properly.

It will be interesting to see how the project develops. £2.5m is an ample budget for a roughly 60m long footbridge, but Mimram's approach to detailing his design must be one of the biggest potential sources for a cost over-run.

Setting aside the structural behaviour, I think it's an appropriately modest design, which could offer a pleasant promenade over the river.

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