02 March 2010
Manchester Bridges: 3. Mancunian Way Footbridge
Now, here's the first of several bridges I visited on my Manchester trip which deserves to be better known than it is. I've certainly not seen it in any of the coffee-table books on modern bridge design, and it's not on Wikipedia or Structurae. It's a fairly short walk from the Hulme Arch, and the next bridges I'll feature are also within walking distance, so the possibility of a walking tour of Manchester's bridges presents itself ...
The Mancunian Way Footbridge was conceived as a replacement to an unattractive subway, connecting the Hulme Park area to the southern edge of Manchester city centre, across the very busy A57(M) highway. The £1.6m pedestrian and cycle bridge was promoted by Manchester City Council with the Moss Side and Hulme Partnership, and funded by a variety of partners, again as part of efforts to regenerate the area.
Following a design contest, Arca Architects and Kingfisher Consulting were awarded the project in July 1999. The bridge was opened to the public almost exactly three years later.
The overall layout of the bridge is governed by the constraints of the adjacent highway layout, and by the need to provide disability-accessable approach ramps. It has one long ramp on each side running parallel to the Mancunian Way, and these ramps extend further in order to achieve the length required. The 50m main span over the highway is curved in plan so that it continues to gain in height as much as possible before passing above the required highway headroom.
This layout isn't especially unusual, but the structural means of support certainly is. Classic ways of supporting a highly curved footbridge deck include the use of cable-stays from a counter-inclined pylon (as on the South Quay Footbridge), or a counter-inclined arch (as on Merchant's Bridge, which I'll be returning to later). In both cases, it's usual to cantilever the bridge deck from a circular or box-shaped hollow member providing the necessary torsional stiffness.
At Mancunian Way, a very different approach has been taken. The bridge deck is supported from a Pratt truss on its outer edge, 150m long in total, braced against an inner tubular ring beam. I assume that resistance of the truss to the consequent torsion is provided by its arching behaviour in plan (assisted by the plan bracing to the bottom chord). The only vaguely similar bridge which comes to mind is on Craigieburn Bypass in Australia.
The truss sits on two inclined concrete piers, which also serve to support the staircases. Small steel struts from the piers hold up the inner edge of the walkway, and seem a little out of place, one of the notable flaws in this otherwise bold and interesting bridge.
The bridge deck and parapet screens are both made from imported cumaru timber (except for the staircases where the parapets use stainless steel wires). The latticework screens are an unusual feature in a modern bridge, and the presence of so much timber is one of the things I found most attractive about the bridge. I liked it so much, I didn't really notice the extent to which it obscures the view off the bridge towards the east. It's also not obvious to me why such a tall screen has been adopted.
Nonetheless, it's the contrast between the steel truss and the timber elements that makes this bridge a success, and one of my favourites from my visit to Manchester.