11 January 2010

I'll huff and I'll puff and I'll blow your bridge up

New Civil Engineer reports that Swiss architects have created an inflatable bridge. Philipp Dohmen and Oscar Zieta have built a 6m long steel-skin balloon that can hold a load of up to 1800kg, according to load test at the Swiss ETH institute.

It's all very exciting, but not really news as such - the ETH's department of Computer Aided Architectural Design (CAAD) has been working on this system for some time, with their so-called FIDU-Brücke having been load-tested in December 2007 (FIDU stands for FreieInnenDruckUmformung, or internal pressure forming). Essentially, it's a prestressed steel shell structure, where the internal pressure prestresses the skin (maintaining it in tension even when subject to applied compressive stress) and hence prevents it from buckling, even in the absence of stiffeners.

Although Zieta's website shows lots of clever applications in product design, including furniture, it's hard to see how any inflatable structure can be robust enough to act as a real bridge, with the danger that any escape of air (e.g. due to failure of a seam or accidental puncture) would immediately eliminate all the structural resistance. The advantages, of course, include the extremely light weight of the structure, which also allows it to be readily deployable in difficult locations.

Nor is ETH-CAAD the first to develop an inflatable bridge.

The US Army has a deployable air-inflated system which can carry military vehicles weighing as much as 80 tons. This however, is really a causeway system rather than a bridge as such: unlike the FIDU-Brücke, it isn't subject to bending.

The University of Maine has also used inflatable plastic arches in bridge construction, although these are filled with concrete after inflation.

More pertinently, the "tensairity" system promoted by Airlight has been proposed for a number of bridges, and applied to some. I wrote about this previously because an Airlight bridge was one of the entries to the Leamouth Bridge Design Competition (pictured left).

The tensairity system uses inflatable plastic beams shaped a little like cigars. These are inflated under relatively low pressure, and the system relies on the use of struts and cables fixed to the inflatable membrane to carry most of the load: the balloon only carries shear and compression between these, and stabilises the whole system against buckling.

An 8m span test bridge was built and shown to be capable of carrying a 3500kg test load [PDF], easily putting the Dohmen / Zieta span in the shade, and with a structure lighter in weight as well. Unlike the metal-skin solution, a further advantage of the Tensairity beams is their translucency, allowing them to be lit internally for architectural effect.

The Airlight website illustrates a number of projects for which tensairity beam bridges have been proposed, but not built. As well as Leamouth, these include footbridges in France and Switzerland, such as the one at Giubiasco shown on the right. They have also been proposed for temporary use to support construction vehicles.

The most significant bridge to be completed is at Val Cenis ski resort in France, a 52m footbridge for skiers (also carrying substantial snow load) - pictured below. This is essentially a timber frame bridge with steel bars providing the suspension cable below - the two elements are separated by the Airlight balloons which hold the main structural pieces in place. It was designed by Charpente Concept, who provide further details of the design and construction on their website.

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acceleratedbridgeconstruction said...

Hp, another great blog post!

In my first class yesterday I told them to visit your site if they wanted to learn about bridges!

Have you had any experience with another cool material, super high concrete (30,000 psi, sorry not metric)in a bridge design?


The Happy Pontist said...

That's a compressive strength of about 200 N/mm^2, for my metric readers - about five times as strong as normal structural concrete. You can achieve a concrete strength in that range with ultra-high performance fibre reinforced concrete (UHPFRC, better known under the Ductal trade name). Ductal has been used successfully in a number of bridges, mainly in slender prestressed beams but most notably in Seoul's 120m span Seonyu footbridge. While the material has a number of advantages, it is inherently expensive, and requires heat curing to achieve a high early strength: another expense. Its relatively high tensile strength compared to normal concrete means that it can be used without reinforcing steel in some applications.