18 September 2011

Concepts published for St Botolph's Footbridge, Boston

That's Boston, Lincolnshire, not Boston, Massachusetts.

Lincolnshire County Council is proposing to demolish and replace the existing St Botolph's Bridge, which spans the River Witham in Boston. It's a remarkably ugly metal girder bridge (pictured, click on any image for the full-size version), which isn't suitable for use by the mobility-impaired and was found to have structural defects earlier this year. They have a £600k budget and a yearning for something better.

Three design concepts have been released for public consultation, all from local architect Neil Dowlman Architecture (there's no mention of an engineer). All three options are supposed to improve accessibility, reduce maintenance, enhance the environment, and provide improved views of the river and the nearby St Botolph's Church.

The first option is for a so-called bowstring arch, with twin tubular steel arches inclined against each other. I say "so-called", because the deck and arch are only connected in the visualisation by a transverse member which would seem insufficient for a proper connection, and hence the bowstring behaviour will be compromised. This is straightforward to amend, however.

It spans about 35m, with 15m ramps either end, and varies in width from 3m to 4m. The available finance therefore works out at about £2.6k per square metre. The concept has structural glass balustrades with concealed handrail lighting, which together are likely to be a costly component, and don't seem to fit well with the aspiration for reduced maintenance problems.

I don't know the town so don't know how well this design fits within the landscape, but I would have thought a lower-profile solution might be more appropriate, if opening up the views is of primary interest.

The second option is described as the "traditional" design, with a deck-type bridge resting on gently curved lattice beams, with guardrail-style parapets. Lighting is from antique-style lamp-posts. Longer ramps are required because the bridge must sit higher above the river level.

The final option is derivative of the first, with "bowstring trusses" of lower height. These are in a Vierendeel style. The relatively low angle of the upper chord at the ends of the bridge will make structural design difficult, but I would see the main objection to this option as being the fact that the lower-height truss interrupts views off the bridge.

To me, there are two obvious options which seem to have been neglected. The first is the traditional half-through girder design, with twin edge girders cradling the bridge deck, and topped by handrails to provide transparency of views off the bridge. At a 35m span, this should just about be achievable without requiring the girder to extend all the way up to handrail height. It would offer better views than the "bowstring" options, and shorter ramps than the "traditional" alternative.

A second option would be a spine-girder solution, with a single central beam raised above deck level and doubling to provide seating. The views would be the most transparent of any solution, although the depth of girder required at 35m would mean higher ramps than some alternatives (albeit still much less than shown for the "traditional" design).

I always find it irritating when a public consultation presents a false or partial choice, but it remains an early stage for this project, and in my own work I have been amazed how different the final design sometimes turns out from the early concepts presented. In this instance, a quick look on Google Street View and at pictures of the nearby church suggests to me that prominence is best avoided.

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