28 January 2013

Manchester Bridges: 20. Leech Street Footbridge

On my same visit to Manchester which took me to the New Islington Bridge, I also spotted another footbridge in the same vicinity which I thought merited inclusion on this blog. It’s not a spectacular, or architecturally exciting bridge, and it doesn’t seem to have been the result of any design contest or had any publicity. Indeed I have searched but failed to find any details of the client, designer or builder.

It's the third interesting modern footbridge I've encountered spanning Manchester's Rochdale Canal. As well as the one at New Islington (which Leech Street is about a quarter of a mile to the south of), there is the minimalist Architects Bridge in Castlefields.

The main reason I thought it was of interest is simply because it’s a representative of a bridge form which is very rare, certainly in the UK, the lenticular bridge.

Terminology is never exact in this area, and the bridge can be thought of variously as an untriangulated lenticular truss (Brunel’s Royal Albert Bridge being the pre-eminent example of a lenticular truss in the UK); as an inverted bowstring arch (where the “arch” is in tension rather than compression, and the reverse is true of the deck); or as an "underspanned" self-anchored suspension bridge, where the suspension cable is below the deck rather than supported above it from towers.

Bridges of this form are rare because it’s normal for the geometry of a span to be determined by the conflicting imperatives of maximising clearance above the obstacle it crosses, and the need to minimise the level of the bridge deck, avoiding costly approach structures. I’ve covered some historical examples here before, at Roxburgh Viaduct and Elgin, and will share a more modern example shortly. Other historical examples include Robert Stevenson’s proposal at the River Almond, and Alexander Nimmo’s proposal in Ireland, while other modern examples in the British Isles include Arup and Wilkinson Eyre’s bridge near Limerick. There are also variants on the form where a level truss has been substituted for the lenticular elevation, as in the rather ugly Mailbox bridge in Birmingham. A list of similar modern bridges is given in Ruiz-Teran and Aparicio's paper (see further information, below).

Where conditions do allow its use, it’s a form of bridge with the potential for remarkable slenderness, because the compression member is combined with the load-supporting bridge deck, both of which require a certain thickness to resist, respectively, buckling and the effects of local concentrated loads. Combining the two therefore minimises the overall depth of material when the bridge is seen in elevation, to an extent only otherwise possible in a suspension bridge, which requires obtrusive towers and often expensive foundations.

Seen in that context, Leech Street Footbridge is not one of the greatest examples of the form. Rather than using a slender bar or thin cable as the tension member below the deck, it is all of tubular steel. This is, on the whole, cheaper to assemble, with the weight of the steel tubes offset by not needing to fix or tension cable elements.

Even so, I think the bridge has considerable simple charm, all of it down to the inherent attractiveness of the structural form. It would be nice to see more bridges of this sort, and looking at this example, it's hard not to imagine they could be built quite cheaply.

Further information:

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