14 January 2013

River Dee Footbridge, Braemar

Many thanks to Moxon Architects for sending me through some images of a pedestrian suspension bridge they have designed with Flint and Neill at Braemar, in Scotland.

The bridge is, as yet, unfunded, but I guess it would make a welcome addition to the locality as there's no bridge across the River Dee in the immediate vicinity, and the area is popular with walkers and other tourists. I stopped off there during my tour of Scottish bridges last year. What's immediately noticeable about the Moxon / Flint proposal is how it consciously echoes a number of other white-painted suspension footbridges in the vicinity, particularly those at Garbh Allt Shiel, Crathie, Polhollick and Cambus O' May. Moxon kindly provide a cut-out-and-keep guide to these spans to emphasise the point (click any image to see it full size).

The longest of those bridges spans 55m, and if built, the bridge at Braemar would dwarf them, being 85m long (the span may be one reason there is no such bridge here already).

The structural design is a trussed suspension bridge, with triangulated hangers offering considerably greater stiffness than the more conventional vertical hangers provide (the cost is the need to prestress all the hangers to prevent any becoming loose). The preview images also show tie-down cables at either end, which will also provide considerable stiffness. The bridge deck consists of white precast concrete units, which will help damp vibrations.

The main cables are inclined outwards and connected to V-shaped masts, so they are curved vertically in plan as well as elevation - this helps stiffen the deck against lateral sway (again, there is a cost, in the need for a more exacting erection methodology). I can't see a structural reason for why the masts are inclined backwards away from the river, but would be interested to hear if there is one.

I've included a few more images of the design below. If I were a betting Pontist, I'd bet this design will prove too expensive for the site, although the history of bridging the River Dee is one of reliance on wealthy benefactors to the community, so I may be proven wrong.

A simpler bridge design would perhaps use main cables in the conventional vertical plane, with inclined counter-cables below the deck to provide combined vertical and lateral stiffening. A timber or galvanised steel mesh deck would also keep costs down, although such a bridge has very little damping and hence be much easier to excite into motion.


Imre (ilaufer(at)mail-bme-hu said...

My MSc graduation project dealt with a cable-stayed bridge with an inclined pylon. I could not find a decisive benfit provided by leaning the pylon backward.
If the pylon is connected to the foundation in a moment-resisting manner (i.e. not hinged), then the bending moment at the base can be reduced by the moment due to the plyon's self-weight. However, this comes at the cost of complicated erection procedure, and requires a considerably heavy mast.

For this bridge, I think it was more a choice of aesthetics: the presumably steel masts are easy to install, and the inclination symbolises that the masts have to "pull back" the structure of the main span.

Anonymous said...

Lovely elegant design, fantastic site.

Having proposed something very similar recently, I would be pleasantly surprised if it could come in around the £1M budget quoted in the architectural press.

Bridge designer