06 February 2013

Manchester Bridges: 22. Spinningfields Footbridge

This is the second of two new footbridges over the River Irwell, connecting central Manchester to neighbouring Salford. I covered the other one in the previous post.

Neither bridge could be justified on a cost-benefit analysis of journey time saved. Greengate Footbridge is particularly absurd in that respect, being only 5m away from an existing foot-accessible bridge. The £1.5m Spinningfields Footbridge is a little better, shaving maybe 3 or 4 minutes off a journey walking across the nearest existing bridges (Bridge Street to the north New Quay Street to the south). However, both bridges act as gateways, as ways to improve the permeability of the urban fabric, enhancing the feeling of accessibility as much as the reality of it.

Greengate Bridge's minimalist appearance shows the structural engineer in full retreat, a functionary whose role is quite literally to support the architectural vision with as much discretion as can be mustered. Sited between Greengate and Spinningfields, Santiago Calatrava's Trinity Footbridge is the exact opposite, the structural engineer as flamboyant showman, the centre of attention.

Spinningfields Footbridge lies somewhere between the two extremes. The structural engineering takes centre stage, but not at the visual expense of everything else in the immediate vicinity. It's a very sophisticated piece of engineering, but retains a sense of modesty and a great deal of elegance. I reckon it's the finest contemporary footbridge in Manchester, and one of the best in Britain.

Designed by Ramboll (based on appearance, their former Whitbybird team rather than their former Gifford team), this is a design that distils many years' bridge design experience, matching a simple and beautiful overall concept to a super-careful attention to detail. It was built by Eric Wright.

The River Irwell at this point has high banks and so far as I can tell is not regularly navigated, nor prone to flooding. This gives designers the opportunity to "hide" all the bridge structure below deck level, something that is not always an option. Ramboll have used that as a cue for a lenticular form, deepest in the middle where the highest bending forces are found.

I described a generally similar bridge in Manchester two posts back, and discussed how the form can be seen in several different ways, one being that it is an "under-spanned" self-anchored suspension bridge. This is like a normal suspension bridge but with the suspending cable below the bridge deck rather than above it. Instead of being hung below the cable, the deck sits above it on struts. Instead of the main cable's tension force being anchored into the ground, it is anchored to the bridge deck itself, which therefore must resist compression. It's a design which can sit much more lightly on the ground than the normal suspension bridge.

Bridges of this type can be prone to torsional flexibility, a tendency to twist. I've seen several designs where the deck and cable are connected by a triangulated structure to create a skeletal "torsion box" which can resist this twisting – indeed, the Leech Street Footbridge is of that type. The Spinningfields bridge is considerably more adventurous.

The key feature of the bridge is that it is curved in plan, as is the supporting cable, although it follows a different line. The two are connected by individual struts, with no triangulation. The struts themselves are of folded steel plates, giving them necessary stiffness, and this is cleverly integrated with folded-plate deck bracing. There are clear echoes here of the ribbing on Lune Millennium Bridge (by the same designers), which I featured here not long ago. I have to say that I find it startling that so trimmed-down a structure is stable, and would need to see the engineering drawings to really understand how it works.

The Y-shaped parapets and the knife-edge fascia are reminiscent of the Chelsea Bridge Wharf Link Bridge, yet another Whitbybird / Ramboll design, and in my view are very attractive. Lighting is incorporated into the handrail, and there's further lighting below the deck level to illuminate the bridge structure at night. The triangular fascia does a great job in making the edge of the deck look extremely slender. The decking is permeable aluminium sheeting, allowing rainfall to drain freely through to the river.

I found vibration to be quite perceptible, although the amplitude is low enough not to cause any distress. Feedback on the Skyscrapercity website indicates I'm not alone in noticeing the movement.

A seating bench is provided on the deck, which I guess may make it a popular sandwich or smoking spot for workers from nearby offices. The deck is up to 4.5m wide, which is pleasingly generous.

Overall, I think this a real gem of a bridge, structurally daring yet in no way ostentatious, and one which deserves wide recognition.

Further information


Imre (ilaufer(at)mail-bme-hu said...

The bridge is an interesting design indeed. I can't help but going through all pictures again and again to puzzle together how it may work.

It would be helpful to see the abutments, to see how the bearings and the edge beams contribute to the torsional stiffness of the deck.
I find the idea of combining two different curvatures (one for the deck and one for the cable) to restrain the bridge from twisting to be exceptionally witty and elegant.

Ed Hollis said...

The folded plate struts and bracing completely make this. Care taken in the detail has really set it apart.

Another one I had no idea existed!

Big fan.

Imre (ilaufer(at)mail-bme-hu) said...

One more thing:

It would be interesting to see how the free-draining decking collects the water. There are no runoff stains on the girder - unlike on the Glass Bridge in Coventry -, and the deck isn't transparent either.

The Happy Pontist said...

The deck is slightly perforated. No water stains on the structural members yet, but they will develop over time, I'm sure.

eddablin said...

You are right that it was the whitbybird team. Mark Whitby discussed this at last month's IABSE Future of Design Conference. I believe they designed it as far back as 2002 when it got planning permission.

Here's a picture I took recently illustrating the lighting.