24 July 2014

Yorkshire Bridges: 3. Smithy Wood Footbridge

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 ...

Further information:


David said...

Thanks for your Smithy Wood footbridge post with your photos that allow us to see some of the construction details more clearly. I also find this bridge fascinating for a number of reasons. I hope that it is retained, well-maintained, and treated as a heritage structure.

Firstly the way the structural concept originally conceived by Ernest M Wichert in America in the 1930s was adapted in the 1960s to these concrete structures by the design team thinking creatively, and confirmed by laboratory testing. The Ron Bridle interview transcript is fun to read and informative.

Like you I also wondered what this bridge is doing in its rural location. It must be on the route of an ancient footpath? There can't be too many pedestrians crossing it now except for weekend walkers and maybe the occasional herd of cows.

The design & the construction phasing would have required close cooperation between designers and builders. There is a brief description of the “concrete tri-hinge”, the Wichert truss detail as adapted to concrete, plus some aspects of its construction here:

It would be nice to know a little bit more about the construction of this bridge. The original Wichert truss would have been quite tricky to build too. The patent describing the construction method devised for the original steel version, registered by Ernest M. Wichert in 1934, is visible on the Internet as a PDF file. (search term “EM Wichert”) There is an historical description of the first Wichert “self adjusting” trusses in the Homestead Grays steel truss bridge here:

The other two M1 footbridges with a similar concept mentioned in the Motorway Archive link, Birdwell Quarry and Stainborough footbridges, were both replaced by steel truss through-bridges in 2006, then demolished. I wonder why? This is the new Birdwell Quarry Bridge (no.3 in the album): http://www.britonsltd.co.uk/projects/bridges.html


Imre Laufer said...

Regarding the analysis of the Wichert truss, I have to disagree.
I've tried it shorthand in a FEM/beam analysis program, both with simple truss elements (for an arrangement similar to http://www.in.gov/dnr/historic/3697.htm, in steel), and with the concrete slab and beam arrangement of the Smithy Wood bridge.

If there's only one fixed lateral support, then the subsidence of the support below the rhomboidal part will introduce no normal forces. Even the lateral supports are all fixed (i.e. the strucutre is statically indeterminate), then the additional forces due to settlement will be small if the superstructure is "soft" in bending.

The answer why the bays don't "squish" is quite simple: because the normal stiffness of the struts don't let them, as long as their inclination is steep enough.

Anyway, I too find the idea of this bridge very interesting and innovative.

Best regards:

Stephen said...

I have also run a quick analysis of a plane (2D) Wichert truss, using Microstran software. I analysed an arrangement nominated in Spillers's Introduction to Structures as unstable (45 degree strut inclination), and the software correctly identified the structure as unstable.

Changing the strut inclination (either steeper or shallower) provided stability and the analysis was completed successfully, so current software is capable of analyzing the Wichert truss.

Various tests were able to indicate nearness to the unstable strut inclination: eg ratio of support reactions to applied load (very large near instability); and magnitude of unsupported node displacements in response to a forced settlement of the internal support at the quadrilateral bay (also very large near instability).