12 December 2010

Scottish Bridges: 16. Roxburgh Viaduct Footbridge

The last bridge from this recent visit is probably the least well known, not being such a historic site as at Gattonside or Dryburgh, but it's the most interesting structurally.

Roxburgh Viaduct (also called Teviot Viaduct) was built across the River Teviot by the North British Railway in 1850. It was designed by John Miller, who also designed the spectacular Ballochmyle Viaduct, which features the second longest masonry arch span in the UK. Roxburgh Viaduct, which no longer carries trains, is much less notable than that.

The Viaduct's four river piers are extended to one side to support a low-level footbridge, of a type which while supposedly once common, has very rarely survived.

The footbridge is 54m long, with three simply-supported spans of approximately 15m each. It is thought to date from the same time as the viaduct. Each span consists of a wrought-iron lenticular truss, 1.2m deep at midspan, with bars curving downwards below a very slender deck which curves upwards. Unlike many trusses, it is not fully triangulated, and relies on the V-shaped 'legs' to hold its shape.

Another way of thinking of it is that each span is a self-anchored suspension bridge, with the lower member being the suspension 'cable' (which would be above the deck in a conventional suspension bridge), restrained by the upper member at its ends.

The footbridge seems to have had very little alteration since its construction. The original parapet railings have been extended upwards in timber, presumably to give a greater sense of security, and infilled with a chain-link mesh (which strikes me as completely unnecessary at a rural site such as this). All the structural metalwork appears to be original.

The bridge has certainly suffered a little from age. This structural form has very limited torsional stiffness, and over time, the spans have twisted permanently, perhaps due to slip in the joints. It makes crossing the bridge a slightly unsteady experience, but not unsafe.

It's a type of bridge which is much under-used, although there are a number of very modern examples. Strasky's book on stress ribbon bridges features several, mostly in Japan, and a recent paper by Ruiz-Teran and Aparicio surveys a number of others. There's also the relatively modern "bridge to nowhere" over Biel Water at Belhaven, although I don’t know exactly when that was built.

Older examples include Robert Stevenson's proposal for an under-deck suspension bridge at Cramond from 1821 (although that was planned to be ground-anchored, rather than self-anchored), and the Micklewood Bridge of 1831, described in Charles Drewry's book on suspension bridges. There were also several examples in Europe, much discussed in Tom Peter's excellent book on Guillaume Henri Dufour.

It's hard to believe that a modern designer, constrained by modern design codes, could produce something as lightweight and charming as the Roxburgh footbridge, but if so, that's a shame. I hadn't even been aware of its existence until quite recently, but it's already one of my favourite historic bridges in the UK.

Further information:

1 comment:

Anonymous said...

your blog is most inspiring to lovers and students of bridge design. I realy enjoyed this article. congratulations and sorry for a crude english.