18 July 2011

Footbridge 2011: papers roundup pt 2

I've had time to jot down a few more comments on the papers presented at Footbridge 2011. Again, this is a highly selective bunch, there were many others which were interesting but which I simply won't have time to cover.

The Arganzuela helicoidal bridge over the Manzanares River, designed by MC2 Engineering Cosultants with Dominique Perrault Architecture, belongs to the increasingly popular genre of helical truss bridges, a genre I have surveyed previously. There are plenty of images online, but I've just chosen one from the conference paper.

It's actually two separate bridges, with a total length of 278m. Each is in the form of a cone, varying in diameter from 6m to 12m, and structurally they are helical trusses, with straight upper and lower chords intersected with spiral diagonals, taking the general form of a through Warren truss. This is pretty much as rational as this helical form ever gets, but as the authors noted, it still imposes significant secondary stresses on the steelwork, requiring substantially more material than would the more conventional solution.

I've included it here because while I have my doubts about the conic geometry (attractive in the widening direction, but visually constricting in the other direction), this is one of the best of its genre. This is largely, I think, because of the cladding treatment, which with its varying densities of mesh, distracts from the structural frame and uses its apparent solidity to create an object with a greater sculptural weight.

I enjoyed Andreas Keil's presentation, Passerelle sur Nanterre - spatial play of forces, on the La Defense footbridge designed jointly by Schlaich Bergermann und Partner and Deitmar Feichtinger Architectes. This is a remarkable 88m long bridge which passes around the perimeter of a curved building, without touching it, and with supports only towards the ends. This is a challenge in itself, but the choice was made to support it on only one edge using the "inverted Fink truss" typology (a system which has only limited resemblance to a genuine Fink truss).

The result is a truly superlative example of high-tech structural engineering. The main vertical support comes from the truss, which is essentially a series of successively cantilevering cable-stays. The number of cables in each bay of the truss are varied according to their order in the cantilever i.e. the number of cables used is proportional to the force to be carried. This system is assisted by a set of ring-cables offset below the deck level, which are cranked back up to deck level at the ends and hence carry a share of the vertical load in a manner similar to an external prestressing tendon. This also reduces the possibility of uplift forces on the bridge's end bearings.

The deck is a hollow steel box, providing some of the much needed torsional stiffness. However, this is far from sufficient on its own, and the lower ring-cables provide the remaining torsional restraint. The vector sum of their forces at each mast position imparts a net inwards force, which, because of its level, acts to counterbalance the torsion resulting from the deck dead and live loads only being supported on one edge. For some reason, the bridge as a whole reminds me of a set of gimbals, as in a gyroscope, possibly because of the way the whole ensemble appears to teeter on only a few pin-point supports.

It's a remarkable bridge.

Another bridge which impressed me was Link 27 - a new footbridge in the west of Vienna, Austria, presented by Rudolf Branstötter. This 38m span bridge was completed on site in September 2010, and was the end result of a student design contest which Branstötter had won. I thought this was an exceptional structure, where the structural form, although unusual, was a carefully thought-through response to the site's very specific constraints.

It spans over both a railway line and the River Wien. The different permissible clearances led to a structural form which has a low level springing on the river bank, and which is supported over the railway from above. One end resembles a concrete arch, while the other is a concrete cantilever, with fixity provided by steel tie-down bars. However, both aspects appear fully integrated. The method of construction was also unusual, with both edge girders precast on site, lying on their side, before being rotated and lifted into place.

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