Bridges news roundup

Architect plans Elliott Bay Bridge to replace viaduct
Here's a story that leaves me slightly confused. Seattle architect Roger Patten (pictured, right) has proposed a big bridge in place of a tunnel-and-viaduct proposal, and has been arguing for some years against the local state Department of Transportation. Criticising his scheme, the Washington DOT has stated [PDF] that "the bridge has a center span more than a half mile in length, longer than all but one completed suspension bridge in the world." Really? Half a mile is a piddling 805m, which is shorter than the top four cable-stay bridges and beaten by thirty suspension bridges.

While the DOT is therefore talking nonsense, it's unclear whether Patten's bridge makes much sense. Defending his concept, he noted: "They told Eiffel to take down his tower after the fair because it was an eyesore. They said the same thing about the Golden Gate Bridge. Maybe we should take that down, too." Hubris aside, his design relies on "buoyancy stabilised piers" (i.e. caisson foundations filled with air to offset the weight of the bridge), a daft idea that entirely ignores the fact that heavy foundations are normally required to stabilise the bridge pylons against overturning, which governs design more than the purely vertical load.

What will we name the new Forth bridge?
"Forth Replacement Crossing" is too dull; "Fourth Forth Bridge" likely to cause confusion because it's actually the Fifth Forth Bridge; so just what will they call it? Incidentally, according to the type of people who like to generate ridiculous newspaper headlines, the bridge is being designed by the same people who design Ferrari F1 cars.

Bridge petitioners declare victory
Reprieve expected for Victoria's Blue Bridge (see previous posts): campaigners against the Johnson Street Bridge replacement raise enough votes to put at least a temporary brake on their council's attempt to borrow CAN$42m towards the cost.

Bridges to prosperity
The always excellent Bridge Photo of the Day blog has several interesting posts on low-cost suspension bridges in Ecuador, which make a nice follow-on to my own posts on the same theme.

Low cost bridges: 4. Miscellaneous

Okay, I got a bit distracted for a few posts there, but it's time to round off my series of posts on low-cost bridges with a few more pointers to organisations involved in building low-cost bridges around the world, or who offer standards and advice on how to design such bridges.

Alpin Technik
This German contractor sponsored and helped build a timber suspension bridge at Tres Unidos in Peru (pictured below, under construction). Extensive project details are available at the Alpin Technik website, and their presentation at Bridges to Prosperity [PDF] makes interesting reading.

ILO/ASIST
The International Labour Organisation runs a programme called ASIST (Advisory Support, Information Services and Training) which (amongst many other things) has published in Zimbabwe a set of Guidelines for the Design and Construction of Suspension Footbridges [PDF]. While this has lots of sound advice, it stops short of including their standard designs.

IT Transport Ltd
This UK-based aid consultancy put together Footbridges: A Manual for Construction at Community and District Level on behalf of DfID (UK Department for International Development). This is an excellent and wide-ranging publication, covering bridges made from bamboo and timber as well as simple concrete and steel truss structures. Plenty of references to other sources are made where necessary, and it's based on extensive consultation and real-life case studies.

Overseas Road Note 9
This is the UK bible for designing small highway bridges in places that, to us, are far away. Published by the Transport Research Laborator, and titled A design manual for small bridges, it can be found online at DfID's Transport Links website. It's aimed squarely at civil engineers looking for simple bridge designs and details which can be built with partially-skilled labour, and has particularly useful advice on river hydraulics and reinforced concrete bridges.

Global Transport Knowledge Partnership
The gTKP is funded by DfID, and have developed a set of guidelines for Low Cost Structures for Rural Roads. These overlap to some extent with Overseas Road Note 9, but are generally less technical, and with less focus on bridge structure design. A draft version is available online.

Countryside Commission for Scotland
For a long time, the CCS's publication Footbridges in the Countryside has been the UK bible for small rural bridges, but it is long out-of-print and not available online, as far as I know. It has been essentially superseded by Paths for All's Path Bridges: Planning, Design, Maintenance and Construction, which is not online but is in print.

I'm conscious this is only a small cross-section of the information available online on low-cost bridges; but hopefully it's a useful starting point for anyone investigating the topic.

Low cost bridges: 3. Helvetas Nepal

Both the charities profiled in the last two posts have made good use of technical support and advice from Swiss NGO Helvetas. Founded in 1955, Helvetas operates a number of aid programs in 18 of the world's poorest countries, principally aimed at rural development, drawing on a mixture of government and private funding (they have an annual budget of about 59m Swiss francs).

In Nepal, they support a number of activities promoting modern agricultural technology, expanding irrigation systems and employment training. Over two-thirds of the 27 million Nepalese live on less than US$2 per day.

Of interest here at the Pontist is their Trail Bridges Sub-sector Project (TBSSP), which has supported the construction of over 2000 rural bridges throughout Nepal since its inception. In the monsoon season, up to 50% of rural Nepalese roads are severed due to the lack of bridges, so these structures are a vital ingredient in supporting rural communities and building a more resilient economy. The image on the right is courtesy of féileacán at flickr, as are the other images below where not credited otherwise.

Nepal of course is famous along with Bhutan for their cantilevered log bridges (like this one), but these can span only a relatively short distance and can be vulnerable to damage when rivers rise. The idea to use more modern bridges did not originate with Helvetas: in the early part of the 20th century, a number of metal suspension bridges were designed by Scottish engineer Louis Harper, and shipped from Scotland to Nepal (see image on the left borrowed from the excellent Harper Bridges website). As is acknowledged in a potted history at the TBSSP website, only a small number of bridges could be built this way.

The TBSSP's dedicated website has plenty of information on their activities including a detailed database of completed structures. At present, between 140 and 180 new bridges are built every year, which puts the activities of Bridges to Prosperity and Bridging the Gap into perspective. Key to the TBSSP's success is that it is less about building bridges, and more about fostering the capability for the local people to do so. Over 30 colleges teach skills related to the programme, and bridges are to a large extent now designed and built by local consultants, fabricators and contractors. This is the culmination of many years of effort: this kind of infrastructure hasn't appeared overnight.

The TBSSP now largely concentrates its efforts on enforcing a national strategy on which types of bridge to build and how, promoting proper maintenance of the existing bridge stock, and developing new bridge types which could support road as well as foot traffic.

I was pleased to find that the British connection to building Nepal's bridges hasn't entirely vanished, with financial support for TBSSP being provided by the UK Department for International Development (see interesting blog piece at DfID from where I've borrowed the photo on the left).

The Helvetas bridge designs rely on standardised lightweight metal components both for their catenary suspended bridges, and their larger suspension bridges. The shorter bridge types, without conventional suspension towers, are documented in manuals (available at the Bridges to Prosperity website), which combine detailed technical information with practical advice on how to build the structures, such as the fence weaving illustration shown on the right.

I find these to be a mine of interesting information, and although the Helvetas bridges are notoriously prone to bouncing, I keep wondering whether better use of this technology could be made in the rural parts of the developed world. There was a time when suspension footbridges were in vogue (as well as Louis Harper, the likes of David Rowell and others installed large numbers of essentially similar designs).

Helvetas are expanding their bridge-building programme into Africa, specifically Ethiopia [PDF](image, left, taken from Helvetas), since 2005. I think that's great news, especially as they are again concentrating on developing the institutional capacity to build bridges.

I'll return to this topic shortly, but will take a break from it for now, as I'd like to take a quick look first at a little bridge design competition currently underway in Calgary ...

Low-cost bridges: 2. Bridging the Gap

Like Bridges to Prosperity, Bridging the Gap (BtG) is a charity whose aim is to build bridges in developing countries. Founded in 1996, BtG focusses its efforts on Africa, principally Kenya, and has completed around 40 spans. As with B2P, the bridges provide access for rural communities to schools, health facilities and markets, and reduce exposure to the risk of drowning or animal injury.

The charity has its own standard bridge design, a simple catenary suspension bridge similar to the Helvetas designs used by B2P, and suitable for spans up to about 45m (see above right). This uses tubular steel supports (see left), inexpensive metal hangers, plank decking, and typically a few horizontal wires to form the "parapets". While the deck and parapets are simpler than the B2P bridges, the supports would seem to require more in the way of a non-local engineering input.

In its most recent bridge, a span of 112m was called for, far beyond what the charity had previously attempted. With the assistance of B2P and Helvetas, they adopted a far more conventional suspension bridge (shown right), albeit one which is still lightweight and adapted for construction in difficult territory.


Costing US$100,000, a very large sum by Bridging the Gap's standards, this bridge is documented in an excellent article at Bridge Design and Engineering magazine. As the image on the left shows, erection of the bridge's steel grille decking requires a head for heights. The images also show that this is a highly engineered structure, with more complex fabricated towers, hangers and decking than is the charity's norm.

While it's good to see them expanding their capabilities, it's hard not to wonder how many of their more normal bridges could have been built for that sum (they estimate US$5,000 for their typical bridge). This is a classic and perhaps insoluble dilemma for any charity.

Bridging the Gap invites donations to support their work.

See also:

• Bombi Bridge at Structurae
• Galana River Project Blog
• Bridging the Gap at Flickr
• Bridging the Gap - The Movie

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!)