24 February 2010

Prestressed granite bridges

Granite is not a material much associated with modern bridges, other than as a paving or cladding material. It's mainly seen as a historic material, used in masonry arches or retaining walls.

It was therefore a surprise when I recently discovered it being used as the basis of a whole range of prestressed bridges in Germany, particularly so in light of the incredible slenderness achieved. I can only think of one other well-known bridge built from granite for which such wafer-like minimalism is the major feature (the Pùnt da Suransuns).

These bridges are the creation of Kusser Aicha Granitwerke, who have also been employing granite to make sculptures, paving, and water features for about ten years now. They rely on the high compressive strength of granite (at about 200MPa, it's about four or five times as strong as conventional concrete), combined with conventional prestressing to achieve span-to-depth ratios as high as 50 or even 75 for the shortest spans (a range of 20 to 30 being typical in normal bridge construction). Examples quoted are a 300mm slab spanning 15m, or, my favourite, a 40mm slab spanning 3m, for the typical pedestrian live load of 5kPa.

Advantages suggested in favour of the solution are that the structural material also serves directly as the wearing surface, and granite's considerable resistance to weathering and abrasion. The prestressing tendons are placed in ducts packed with grease to provide corrosion protection, although as with any prestressed bridge, these are the weak link in the system, and failure would potentially be sudden and without warning. I would think this is doubly true for the more slender slabs, where any significant deflection will lead to non-linear geometrical effects of a sort which can normally be ignored in conventional beam design.

The bridges can be factory prefabricated, and load tested, and a PDF on Kusser's website shows examples of load testing carried out. As a designer, the short proven history of these bridges naturally fills me with anxiety, and I'd be looking for detailed technical information.

The attraction, for me, is primarily visual, the brutally simple minimalism inherent to the current design. I don't know whether it's a concept that will see wide use, but it's encouraging to see people willing to take traditional materials in innovative directions.


Kusser Aicha Granitwerke said...

Granite is denser, both more elastic and resistant than concrete. It neither shrinks nor creeps, so if we select the very best pieces from our quarry (which we do for prestressed granite) cracks that would let water go through the granite to the tendons become very unlikely. That's the first and major layer of security that protects the tendons. The second and third layer are PE tubes, the inner one being filled with grease, which is the fourth layer of security. The tendons themselves are approved by the DIBt, the German Institute for Technical Approvals, and have been in use for decades.

At the moment we are also in the process of being granted the national technical approval for prestressed granite bridges. All necessary static calculations and tests for that are currently conducted at the Technical University of Munich.

Yes, Kusser Aicha Granitwerke started prestressing granite for major art pieces, and later on for bridges, more than 10 years ago. For almost a century however, we have been quarrying, processing and successfully inventing new ways of employing granite like the Floating Spheres, Rings, Discs, Wheels - a field we have become the market leader in.

Anonymous said...

Interesting and informative blog. The very slender sections must surely make these bridges quite lively. I wonder what they are like for pedestrians?
And the mode of failure is going to be dramatic to say the least - snap through me-thinks!
But I noted a picture on Granitwerke's pdf which set me thinking about its use for covering service trenches on footways when there isn't much available depth...
Food for thought.

The Happy Pontist said...

From discussions with Kusser I believe their view is that the granite offers sufficient damping to avoid unacceptable vibration. I'd like to go any jump up and down on one though!

Anonymous said...

Would this be a useful technique for ultra high strength concrete?

The Happy Pontist said...

I can't see why not, although if ultra high strength fibre reinforced concrete is used, the tensile strength is higher than for granite and therefore there could be advantage in a partial-prestressed approach. As Kusser note in their comment, concrete also has the disadvantage of creep, which will be particularly significant for very slender bridges, as deflection may bring the bridge into the non-linear region of behaviour (including potential snap-through as per another comment above).