20 July 2011

Footbridge 2011: papers roundup pt 3

Time to cover two more papers from Footbridge 2011.

There were a number of papers describing bridges which were, at best, odd, and at worst, downright awful. I won't embarrass the authors here, but the most striking examples were all cases where the architect had been let loose on their own and the engineer left to pick up the pieces later*. It was quite a shame to see such highly talented engineers being employed in this way. (*There were also a number of engineer-designed bridges which could have been improved no end by the presence of a sensitive architect!)

One case which left me with more ambiguous feelings was Tim Black's presentation Optimisation in footbridge design. This is a subject I wrote about recently, so I was keen to see the talk. Black is a director of BKK Architects, who had collaborated with RMIT University's Innovative Structures Group to attempt a new approach to tubular footbridges. They took the basic cylindrical form and applied "BESO" (bi-directional evolutionary structural optimisation), a form of topological optimisation, to it, allowing the process to eliminate and rebuild areas of material in response to analytical criteria such as stress and stiffness.

The resulting design is illustrated above. It isn't what the architects expected (they anticipated a more regular perforated tube), and nor is it what an engineer would expect, as it lacks the symmetry you would expect on a simple symmetrical design problem (a simply supported beam). This is because the architects have steered the design process to suit their preconceptions: they have extracted a segment of geometry from the solution which can be repeatedly tiled both around and long the tube, imagining that the correct approach for ease of real-world fabrication is to maximise repetition. Indeed, they have moved on to digital fabrication and precast prototyping.

No engineer would expect an optimised geometry to be tileable, instead, it would be reasonable to expect material to "collect" in the upper and lower walls of the tube according to the bending moment diagram, and to form diagonal elements in the side walls according to the shear force diagram. I like the idea that new structural forms can emerge by "growing" rather than designing a structure, but the demands of construction (ease of fabrication) and the demands of material efficiency (curved and complex elements) are opposed, and it is not easy to imagine how they will be reconciled. A more rigorous and engineer-led approach to optimisation may yet lead to interesting designs, however, and it's good to see architects thinking in this way as well.

Another paper which echoed a subject I've covered here was Markus Hennecke's Pre-stressed granite bridges: a new generation of granite bridges. This was a showcase for Kusser Aicha Ganitwerke's bridges, which achieve exceptionally high span to depth ratios, as much as 50:1. Their bridge at Stevenage (pictured above), installed late last year, manages 49:1. The presentation attracted a keen engineering interest, with many questioners clearly looking to be persuaded on subjects such as local bursting stresses and cable protection. Indeed, bursting stresses may represent a key constraint on the range of designs achievable, as they are resisted solely by the tensile strength of the granite. I wonder whether that couldn't be extended by some form of localised strapping system, however.

Reading back through the conference proceedings, there are several other bridges which would merit attention here, and some very interesting design concepts to store away in my "for future use" folder. However, I want to move on. I'll put together a couple more posts on some of the most interesting bridges shown at the conference, and I also want to cover some of the bridges in Wrocław itself.

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