Brayford Level Crossing Footbridge, Lincoln

Network Rail has released images of a proposed new footbridge at Brayford Wharf East level crossing in Lincoln. This is the first of two architect-designed level-crossing footbridge schemes in the town intended to combat persistent mis-use of the level crossings. Images have been provided for public consultation in advance of a planning consent submission to be made in December.

The Brayford Wharf scheme is by Stem Architects, and by the standards of what Network Rail normally throw up for pedestrians, is architecturally somewhat extravagant. While I'm all in favour of context-sensitive design rather than simply dropping in identikit bridges, it seems at odds with the organisation's initiatives elsewhere to develop a more adaptable range of standard footbridge types.

Based purely on the images, my first reaction is it's over-wrought, a mass of texture that seems unnecessarily weighty. Although some transparent panels have been inserted into the elevation, there seems little interest in opening up the bridge user's views outwards, which strikes me as a key challenge given Network Rail's normal over-prescriptive approach to bridge design.

Design chosen for St Botolph's footbridge, Boston

As I said last time I mentioned this story, that's Boston, Lincolnshire, not Boston, Massachusetts. Lincolnshire County Council and Boston Borough Council have chosen a "bowstring arch" as their preferred design for a new footbridge in Boston town centre.

Out of a town population of 55,750 or so, a massive 60 people voted for this design in a public consultation, against 77 people preferring an alternative. The new 35m span bridge will have twin tubular arches.

A planning application is expected later this year, with construction expected to start in mid-2013.

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.

Preferred design announced in Sleaford

In March, I discussed proposals to replace a railway level crossing in Sleaford, Lincolnshire, with a new footbridge. At the time, four alternative designs (none of them much good), were being presented to the public to obtain feedback.

I noted at the time that all four options were pretty horrible, but that the cable-stayed design, pictured here was the best of the bad bunch.

The findings of the consultation have now been declared, and even the public prefer this option, giving it a not-especially resounding 34% of their votes. They still haven't been told what any of the options will cost, however, making any kind of consultation at this stage largely futile.

Of perhaps more interest, is that despite a very positive report on the consultation, the overwhelming majority of the feedback received was simply that the public don't want a footbridge at all, indeed they would far prefer to keep the level crossing. Many of the comments are also opposed to the length of ramps proposed, which seem to be partly due to a decision to provide a 5.8m headroom over the railway, a height which may be the current standard but is widely ignored elsewhere.

Sleaford Southgate Level Crossing Footbridge

Here's a project which makes for an interesting comparison with the North Sheen scheme I mentioned last week. Both schemes are for new footbridges either supplementing or replacing an existing railway level crossing, but they are very different in approach.

The conventional solution to this situation was illustrated at North Sheen: an off-the-shelf standard Network Rail design for a footbridge with a steel half-through girder main span, and steel staircases and/or ramps leading up to it. It's relatively inexpensive, largely because it combines the parapet and deck support functions together, but it's not especially attractive, especially in an urban environment.

At North Sheen, Network Rail's proposal has received planning consent, but local residents are running a design competition to see whether a better alternative could be found, without, as yet, any idea of how something better could be funded.

At Sleaford, in Lincolnshire, the local council is promoting a new highway scheme to open up access to several regeneration areas, and to replace a level crossing with a new highway bridge (at a different site). When the level crossing is closed, they want to retain pedestrian access by building a new footbridge. Here, the design is being driven by the local authority, and I would guess it is being developed by their consultancy partner Mouchel.

Lincolnshire have prepared four alternative footbridge options, and are seeking public preferences via online voting open until 5pm on Friday 18th March. As yet, they apparently don't know what any of their options will cost. This seems a little imprudent, selling the public a sports car without recognising that they can probably only afford an ordinary family car, especially in the midst of cuts to council budgets.

In contrast to North Sheen, all of the options are expensive at best, and in some cases positively unwise, along the lines of sports cars designed by Lada. I've included images of the four designs below, but more images and supporting text are available at their website.

Option 1 - helical multiple spiral footbridge

Are helical truss bridges still flavour of the month? This design has clearly been inspired by a raft of similar ideas, and shares the flaw of many of them that it's very difficult to detail the ends of the tube attractively.

Much of this is simply because a flat tube doesn't respond visually to the dynamics of the span, but the plethora of ramps and staircases required here makes the task harder still. Glazed balustrades are proposed within the tube, which is fine, but are also shown everywhere else, which would be very expensive.

There's no real logic to the use of the structurally inefficient tube, especially since this isn't a covered bridge, and it's an ugly solution.

Option 2 - bowstring arch suspended footbridge

The second option has absolutely no chance of ever being built. Set aside the name, first of all - it's not a bowstring because it's all bow with no string. Set aside, also, the impossibly low rise of the arch, which is simply not feasible at this span.

Let's also discount the confusing and unnecessary array of hanger cables, the odd choice of "timber-effect" steel arch ribs (whatever that is), and the lack of any structurally substantial cross-bracing between the arches (required because of the transverse angle of the cables). Ignore, if you can, the odd decision to support the arch ribs at the top of tall pillars (how is the thrust resisted?) Perhaps many of these points can be dealt with simply by making the arch members much more substantial and allowing them to act as beams rather than arches.

It's the glazed roof that I don't buy. I've worked on the design of bridges over railways before, and I think there's just no chance that Network Rail will accept the substantial maintenance liability attached to cleaning and repairing that sort of glass structure anywhere other than in their own railway stations. Or can anyone offer a counterexample?

Option 3 - cable-stayed footbridge

For me, the cable-stayed design is the best of a bad bunch, but it is not without its flaws. The towers are pretty, but also a little unconvincing. The glazed parapets, as with all the designs, are still everywhere.

The cable layout is frankly odd - why have three stays connecting into one at each tower anchorage, when a single stay at each position would be more than sufficient to support the bridge deck? The detailing for three stays coming together at a point is unlikely to be elegant.

My main query, however, is simply whether the staircase provides sufficient balance for the main span, particularly with the high vertical curvature and slenderness of the deck beams. For a lightweight steel footbridge, the weight of deck and staircase are probably similar, so any live load on the main span has to be resisted either by bending in the towers, or by the anchorage of the stairs into the ground.

More pertinently, the axial compression in the bridge deck induced by the stays will create a marked tendency to hog upwards if the deck curvature is sufficient. You can cope with this at a small curvature, as the vertical loads counteract the resulting eccentricity of the axial force, but it really doesn't look right here. It strikes me as the danger of preparing visualisations before initial structural analysis has been done, or someone with experience of similar bridges has reviewed the dimensions.

I appreciate that these images are being used to contrast different concepts rather than to give an accurate impression of the details, but as is so often the case, the public may be misled into liking something that could never actually be built in the manner illustrated.

Option 4 - traditional lattice work beam footbridge

The last option is the one I would have hoped to like the most, had I just read the title and never seen the pictures. I think there's tremendous room for creative reinterpretation of such staples of railway bridge architecture as the X-braced truss. Such reinterpretation is what basically invented the "inverted fink truss", used now in a few striking footbridges but originating as a curiosity amongst American railroad bridge designs.

This bridge, which resembles the sort of thing you'd expect to see at a shopping centre in Swindon or Slough, is not what I would have hoped for. The X-bracing is used in a clumsy main truss, and then apparently cosmetic on all the balustrades. Why the obsession with glass, with its propensity to attract breakage, grime and graffiti?

Perhaps the local Conservative Association should draw inspiration from their peers in Richmond and invite open submissions for an alternative design developed by the community, "big society" style (for my non-UK readers, "big society" is government code for getting amateurs to do for free what experienced people used to do for pay).

More productively, Lincolnshire should spend some more time looking at the likely capital and maintenance costs of all four designs before they go too much further. It would be horrible if their ambitions end up dashed against the rocks of yet another standard Network Rail design, but there are ways of interpreting each of the options presented so far which are both more attractive and more economic.

There has been a dearth of proper bridge design competitions in the UK recently, and this certainly seems like a good opportunity for one.

Footbridge proposed over River Trent at Gainsborough

Design proposals have been published for a new pedestrian bridge in Gainsborough, designed by Houchell Studio and TALL Engineers. The 276m long bridge has an estimated construction cost of £6m. That strikes me as a little on the light side, comparing for example the not entirely dissimilar 315m long Derry Peace Bridge which has a price tag of £8.7m.

The bridge is S-shaped in plan, and suspended by cable stays from two symmetrical pylons. The South Quay Footbridge comes to mind as a possible precursor, although that was a swing bridge. What's perhaps most striking about it is the very shallow inclination of the pylons, which seems at least visually to be lower than might normally be expected.

The deck is to be shared by pedestrians and cyclists, and the bridge provides a link in a circular route including riverside paths.

The client, Gainsborough Stakeholder Network, currently has no money to build the bridge, and is now seeking sources of funding.