Low-cost bridges: 1. Bridges to Prosperity

I've been meaning for some time to post about a side of bridge-building that rarely makes it into the non-technical let alone the technical press. These are community bridge projects, often self-built, often bridges built in developing countries without access to modern design tools or materials. They might offer some interesting lessons for developed-world bridge designers, and a wealth of interesting design resources are available.

What prompted me to finally put a post together was an excellent pair of articles in the latest Bridge Design and Engineering magazine. One covers a project in Kenya by charity Bridging the Gap - I'll cover that in a separate post. The other article features the charity Bridges to Prosperity (B2P).

Founded in 2001, this American charity has built 50 new bridges in about 15 countries worldwide (a typical example is shown on the right). It targets sites where a bridge is not a luxury but a necessity - places where the lack of a river crossing can cut off access to education and health services when rivers become impassable. The bridges are also key in expanding access to employment and business markets, and B2P's aim is not just to build the bridges, but to leave local people with the knowledge and capability to build more.

B2P's preferred design is a lightweight steel and timber suspension footbridge, illustrated below. This is a simple, unstiffened catenary bridge design, developed by Swiss charity Helvetas, who I'll feature in a future post. It requires a minimum of imported material and is hence generally relatively inexpensive to build in locations where access is difficult (they estimate the total cost at about US$350 per metre length). It is also less vulnerable to failure during construction (e.g. due to flash flooding) than a bridge with supports in the water channel.


Manuals for design and construction, including technical drawings and specifications, are freely available online. These make great reading, especially for engineers mired in the complex bureaucratic fog that engulfs much developed-world bridge design. The focus is on simplicity, and when reading them I found it hard not to wonder whether any of the design methods, structural concepts, or construction technologies could be used outside their intended context.

The image on the left shows the simplicity of the main cable tied around a pile-head. It reminded me of the Traversinersteg in Switzerland, where one anchorage is wrapped around a large rock. I'd think that with this kind of non-technical detailing, one thing that will be lost is long-term durability, but I'd guess it would be cheaper to rebuild one of these bridges than to carry out complex maintenance on protective coating systems.

The picture on the right shows the very simple hangers, which are connected to the suspension cable simply by being hooked around it, and also the mesh infill "parapet" system. This also will lack durability but is easy to repair or replace.

That's one area where the context does matter: the economics of mend-and-make-do are more attractive where labour costs are relatively low, and where the costs of maintenance-induced disruption are low. In the developed world, closing a bridge (or the obstacle below, more likely to be a railway or highway than a river) is hugely disruptive, and designers must therefore balance initial capital and ongoing maintenance costs very differently.

There's also the question of cultural attitude to safety risk (image, left), with these bridges being built in ways that clearly wouldn't be acceptable in many countries. It would be interesting to know how many fatalities or injuries have occurred building B2P's bridges, as the design minimises material cost at the expense of maximising the requirement to work at height. I'm not sure whether developed countries value life more highly, or are simply over-protective and risk averse. Perhaps it's the same thing.

The charity continues to push itself into new countries, with work underway or planned in Guatemala, Liberia and Timor-Leste. Much of the work is done by volunteers, and B2P raises money both from sponsors and via donations online. I think they're doing something really quite marvellous, and wish there was a UK equivalent (although I'm certainly not offering to set one up!)

Bridges news roundup

Protesters in action to save historic bridge
Direct action planned to protect Bowstring Bridge in Leicester

New Norwich footbridge opens
Unconventional £2.5m Lady Julian swing bridge formally opened

London's bridges in war of words on Twitter
No, seriously, this is news, honest!

Transporter Bridge gets £800,000 of Assembly cash
Newport bridge to have long-awaited refurbishment

Brunel, Locke and Stephenson: the engineering giants who shaped our world
Telegraph looks back wistfully on the 150th anniversary of Brunel's death

Horny bridge keeps on going

A proposed "iconic" bridge in Sunderland, designed by Spence Associates and Techniker, is moving forwards, following a meeting of Sunderland's city council cabinet yesterday. Claiming the structure would be a "people's bridge", the council leader revealed that the design is likely to be submitted for planning consent in November. The bridge features twin cable-stayed masts shaped a little like antelope horns.


A report available on the council's website makes for interesting reading, and is perhaps worth dissecting in some detail (just a dissection - not quite a full scale fisking).

It notes that "the design aligns with the City's brand values of 'smart', 'life-enhancing' and 'balanced'". I nearly snorted my cup of tea when reading that - what's so smart about a cable-stay bridge with no back-stays? It's balanced only in the sense that a man standing on one leg is balanced - he can stand up, but the effort to remain so is harder than it needs to be.

The report reveals that Techniker have now done considerable design development, backed up by peer review by their own subconsultants, consultation with possible contractors, and validation by the Council's normal cost and engineering consultants. It would be nice to know who the peer reviewers are, in particular, as they would be expected to include specialist long-span bridge engineers, but I can't imagine many of those wanting their names too closely associated with this design.

This is all very good - given the extravagantly unusual structural behaviour of this design, a detailed consideration of technical issues and risks is vital to confirming its feasibility and ability to be built within cost. The report certainly suggests substantial risk management is being undertaken. Hydraulic and wind tunnel testing have both been carried out.

The report still doesn't shed much light on the bridge's actual cost, reporting only the overall scheme cost of £133m, which include 3km of highway, land acquisitions, and utility diversions as well as the bridge itself. It makes clear that funding is not yet secured: the Department for Transport (DfT) have agreed £98m of funds, but those are dependent on construction tenders falling within the budget estimate. £6m comes from development agency One North East. The Council calculates it has to provide the remaining £29m of the total.

That's made up by sticking a £10m dent into their five-year Local Transport Plan funding (presumably that means cutting cycle and bus schemes), with the rest drawing down on the city's strategic reserves. Council tax payers therefore don't see any rise in their tax to pay for the bridge, but at the cost of exhausting considerable sums of money which could have been spent locally on other projects. It also means (as with many regeneration projects) that the Council financing is heavily geared, such that a 10% increase in overall scheme costs could lead to a near doubling of what the Council actually has to pay (although in reality some of that increase would be bailed out by central government). The risks to local finances are therefore not insignificant.

The report does give some simple dimensions for the bridge (336m long by 30m wide), which confirm that square metreage costs estimates I've previously made are based on the right information.

The document also suggests that there is evidence that landmark bridges accelerate and lead to higher quality regeneration and development in their vicinity. This is the assumption made by most commissioning authorities, and it may well be true, but I've never seen any proper quantitative evidence to support it. In any event, greater development would need to be measured against greater cost to make a serious financial case for a landmark structure, a calculation yet to be made.

The cabinet report repeats the statement that "public opinion is overall in favour of having a landmark design ... and to do so while minimising the impact on council tax". This has been said several times in Sunderland, but I've noted previously that the council's own surveys tell a very different story, with half the populace preferring a "tried and tested" solution instead.

The report also offers insight into Techniker's appointment. The council have clearly considered the possibility that their appointment could be ended, with the remaining design carried out by another firm, most probably via design-and-build. The report states that "as this is a landmark design, Techniker have insisted that they must remain as the exclusive designers ... for the full duration of the design services".

It's good to see the designer taking such a strong position, as there have certainly been design competitions in the past where the original designer has been discarded in favour of others, perhaps most controversially on Stonecutter's Bridge. At River Wear in Sunderland, it would seem that Techniker retained design copyright throughout, giving them a strong negotiating position, even in the face of their very limited track record in designing major structures of this type.

So, the choice of bridge is now finally confirmed, four years after the Techniker design won in competition, and the next step is to secure all the necessary consents from statutory bodies and seek final confirmation of funding. I have to admit I thought this was a bridge that would never get built, and it's remarkable that such a bizarre design has survived the design development phase intact. The next real obstacle in its path is likely to come when construction tenders are submitted in 2011 or 2012, when we'll find out whether the budget estimates are right or not (for a classic example of another unconvential structure which failed at that hurdle, see the Neptune's Way scheme).

Bridges news roundup

The 68 most bizarre and perilous bridges
Seen lots of them before, but still plenty of memorable bridges at Weburbanist.com

Safety of new Venice bridge in doubt
Unclear what's new in fresh report on Calatrava footbridge labelled "not fully functional" - a quick go with Google Translate and the Italian press suggests only that bridge serviceability testing has revealed "many dark sides" ...

The Trinity Trust Announces "Bridging the Trinity" with Santiago Calatrava, Award-Winning Architect and Designer of the Margaret Hunt Hill Bridge
Party (get your tickets here) to celebrate progress on extravagant Dallas design

Dublin bridge wins European award
Spencer Dock Bridge wins LEAF Best Structural Design prize

Spanning the Tyne's Bridges
Exhibition in Gateshead runs until October 11th

Related posts:
Spencer Dock Bridge

Living bridges of Cherrapunji

How have I never seen these bridges before?! Thanks to Dark Roasted Blend for bringing them to my attention (although they've been featured on what looks like dozens of other websites and blogs).

Here's a photo of just one of these incredible bridges in Cherrapunji, India, made by training the aerial roots of the ficus elastica banyan tree. But there are losts of even more amazing photos at DRB and elsewhere.

What I like about them is not just their incredibly bizarre appearance, the ingenuity involved, or their negative carbon footprint, but the fact that they get stronger over time, the opposite of most man-made bridges.

And, you can visit them too!

The Dark Roasted Blend article also has several other truly astonishing things, including the vine bridges of West Iya in Japan. Go take a look!

River Wear Bridge expected to get green light

Sunderland City Council are to meet on 9th September to review a report on Techniker and Spence's River Wear Bridge, and decide whether to go ahead with the project.

The bridge (shown right), which the BBC states would be the tallest in the UK at 180m tall, is a highly unusual cable-stayed bridge which not only pushes at the boundaries but sallies forth far beyond them. The lack of back-stays leads to a design where pretty much the entire deck is suspended from a mammoth tusk-like cantilever (or two).

The press report that the bridge will cost £133m, of which £23m is coming from Sunderland's residents, although I believe that's actually the cost of the entire highway scheme, not just the bridge. The figure mentioned in the original RIBA design competition for the bridge budget was £43m (roughly £4k/sq.m), so assuming the locals are paying the £23m premium solely to get the uneconomic design, its total cost might be estimated at £66m (roughly £6k/sq.m).

Using some different figures provided by the designer, the initial budget had been reduced £43m to £31m some years ago. On that basis, the new cost might be a mere £54m.

This is pure speculation, I should note: the actual cost of the bridge itself has not to my knowledge been made public.

That cost overrun (anywhere from 50% to 75% depending which numbers you believe, and even before the design is complete or construction tenders mushroom the costs further) may cause the unsuccessful competition entrants to shake their heads: how fair is it for a design to win which didn't comply with one of the competition's basic requirements i.e. the budget? This outcome is, however, far from unusual in competition-land.

I last discussed the bridge back in July, when Sunderland released new visualisations of the bridge indicating that the svelte ivory horns which won the design competition had put on some weight as design progressed.

It seems the designers have won their long battle to prove (to Sunderland at least) that their bridge is feasible, affordable, and desirable. That's undoubtedly an achievement when you consider that their design was at first kept secret for three years, and that Sunderland has been repeatedly hesitant throughout the long, painful process of public consultation and further evaluation.

To me, the bridge remains ridiculous, a colossal waste of public money on a structure which is several times more inefficient than even the most extravagantly unbalanced Calatrava creations. To the public, who have little idea of the missed opportunity for an equally iconic, yet structurally more rational solution (Sunderland Council have never made public the alternative entries), this bridge scheme is likely to be judged a success.

As a bridge engineer who most admires bridges which use the challenges of buildability and structural stability as the springboard on which to fashion something remarkable and new, I find the entire project to be a sad indictment of how we now procure landmark structures, and the extent to which engineers have made themselves subservient to architectural or sculptural fantasy.

Related posts:
Who ate all the pies?
Sunderland council decide way forward for River Wear crossing
Bridge competition debris part 12: River Wear
Sunderland gets its iconic bridge. Or does it?
Iconic vs laconic - you decide!
River Wear bridge ready for public battle
Secret bridge unveiled ... and you can see why they kept it secret!
River Wear Bridge design remains top secret

North Coast Harbor Bridge proposals

The Cleveland Plain Dealer has a story up about proposals for a new opening footbridge at North Coast Harbor, at Cleveland, USA. The headline is North Coast Harbor pedestrian bridge proposals by Miguel Rosales are outstanding.

Readers may remember Rosales as the architect behind an ambitious, pricy proposals for a "wave bridge" at Portland, Oregon. That design eventually came to nothing, with Rosales replaced by a different architect, Donald MacDonald. Essentially, Rosales' preferred option was decided to be way too expensive for a large-span light rail bridge.

His* latest designs comprise six options for a much shorter, lighter structure, a 27m span opening footbridge across a harbour breakwater (pictured, right). There are two double-leaf bascule designs, two bobtail swing bridges, a very unusual single-leaf bascule, and a curling bridge.

(*I'll follow the Plain Dealer in attributing the designs essentially to Rosales, as there isn't much evidence of the contributions of his collaboratoring consulting engineers, Schlaich Bergermann and Wilbur Smith Associates.)

The designs are out for public consultation, with the City of Cleveland looking to narrow the six options down to two. They have a handy report [PDF] adding technical detail to the pretty pictures.

The choice of opening bridge types seems a little odd, but may be driven by site constraints which include a height limit due to a nearby airport. There's no lifting bridge option presented (although they are very difficult to make attractive), and for a footbridge span of 27m, I'd expect a single-leaf bascule to be the obvious choice, as it can be designed to have low operating costs while opening quickly, and they give the possibility of combining low maintenance with interesting aesthetic choices (see the recent Foryd Harbour Bridge design competition for two examples).

Twin bascule leafs are more difficult to operate and maintain, requiring mechanical interlocks to stitch the two leafs together in the closed position. These are vulnerable to dirt and jamming, especially where the geometry will exaggerate the problems of thermal expansion, as is the case on both Rosales' twin-leaf options, which are cranked or curved in plan (pictured, left).

Rosales seems to have made little or no effort to counter-balance either the bascule or the swing bridge options, which adds to operating cost and decreases operating speed. The swing bridge options (pictured, right) may alleviate this somewhat, as they are back-stayed cable-stayed bridges, but only at the cost of failure-prone rotating mast-heads (because the forestays rotate in plan while the back stays don't move).

Even Calatrava's moveable bridge designs (Samuel Beckett, Puente de la Mujer) are better balanced, with proper "bobtails". Not one of the six design options has its centre of mass anywhere near its centre of rotation, increasing machinery costs and also the consequences of failure (an unbalanced opening bridge is far more likely to be damaged if critical equipment fails).

Even more odd is the fact that three of the designs (all the bascules) are drawbridges. In a conventional opening bridge, the deck is moved directly by a pinion or hydraulic ram mechanism - only the lifting bridge is normally moved indirectly, by cables. To reduce fatigue stress in the cables, they pass over large-diameter sheaves, which are hard to hide in an aesthetically pleasing way.

However, in a conventional lifting bridge, the cables and machinery are only stressed while the bridge opens - in the closed position, the deck sits on fixed supports. (The lifting bridge also has the advantage that the deck is generally counterweighted). This would not be true of Rosales' drawbridges, as visually you would expect the back stay (which is also the draw cable) to be required to support loads even while the bridge is closed. This would imply that the mechanical elements are permanently subject to fluctuating load, and the consequences of mechanical failure are all the more significant (not to mention the difficulties of maintaining the equipment). I'm sure there are ways of addressing these issues, but I'd think there are very good reasons why virtually no modern opening bridges are drawbridges.

A more rational layout would dispense with the back-stays entirely and have the bridge operated by hydraulic rams (or a rack-and-pinion mechanism, if counterweights were added).

So far as I know, the layouts shown for the bascule and swing bridges are therefore essentially without precedent, which ought to give the City of Cleveland a few concerns over likely maintenance and operational problems.

The sixth option is however not without precedent: it's to some extent a blown-up version of the Paddington Basin Rolling Bridge, although with simple (and as a consequence less attractive) mechanisms (pictured left, closed, and below right, open). It again has the problem that the mechanical elements are always under fluctuating load, and it would be amazing if it could be done on the scale of North Coast Harbor without regular maintenance problems.

It's also difficult to see quite how it works in the closed position, as it seems to consist of a series of four hinges, with no obvious way to carry any significant bending moment.

I'm not convinced that the bridge selection process adopted by Cleveland will lead to the best solution. The lead designer has discarded every conventional design option before the public are consulted, so their choice is between six challenging, high-maintenance designs, none of a type which has been built before. Since there are more economical solutions which can still be designed as landmark structures, this seems a little odd.

One problem with this kind of false design contest, where the public are asked to pick from a single designer's work, is there is little opportunity for technical challenge, and also artificially constrained visual options - here, you can have any bridge you want, so long as it's white. A proper design competition would have thrown up more idiosyncratic ideas, and perhaps motivated those involved to maximise the quality of their work.

Cleveland have a survey online where you can choose your favourite of the six design options. I tried really hard to choose one, but I dislike them all pretty much equally. What do others think?