I first became interested in the Smithy Wood Footbridge when Tallbridgeguy posted about it on his blog.
It's another great example of how the West Riding County Council engineers in the 1960s applied real innovation to deal with a specific technical challenge. Their concern when designing new bridges over the Sheffield to Leeds motorway was with possible mining subsidence, which could lead to foundation settlement and severe damage to their new structures.
Three new footbridges were designed as concrete Wichert trusses, possibly the first such designs in concrete, and almost certainly the first such designs in Britain. Only one now survives, the footbridge at Smithy Wood. The concept, originally developed by E.M. Wichert in Pittsburgh in 1930, is for a bridge which is statically determinate but also continuous, such that pier settlement can occur without causing damage. It achieves this by incorporating a quadrilateral bay within the truss immediately above the support pier or piers.
Don't ask me quite how that works: any first-year structural engineering student will look at it and ask why the quadrilateral truss bay doesn't simply "squish". I look at it and I don't really know why either, although I think it has something to do with the feasible displacements of the two adjacent spans. To make it work, the designers had to undertake basic research and testing for the tri-hinge, the point directly above the pier. The upper hinge also has to be prestressed.
It's interesting to think that engineers of the time were designing structures with hinges partly to deal with settlement but also to make them determinate and hence amenable to analysis. Today's engineer would struggle to analyse this bridge: modelling it in any conventional structural analysis software would be fruitless, as the quadrilateral bay will be treated as a mechanism and the software will just output an error and come to a halt.
The bridge looks to be in much worse condition than it really is. It looks like it had a concrete coating which is now peeling away.
Perhaps the thing I found the strangest about this bridge is not its supremely odd structural system, but its incongruity. One end of the bridge is terminated with a series of switchback ramps. This gives the bridge a very urban feel, it's the sort of arrangement you often see in an urban environment. But at Smithy Wood, it's in the middle of a field, and it seems completely out of place. This is probably what I liked most about the bridge ...
- Google maps / Bing maps
- An Oral History of British Science: Ron Bridle (The British Library, 2012, especially pages 81-82)
- Bridges on the Ashton - Sheffield - Leeds Motorway (Concrete Quarterly, No. 80, 1969)
- The Motorway Archive