11 May 2011

Footbridge Awards 2011 - technical up to 30m

Okay, I've previously provided some commentary on the 2011 Footbridge Awards shortlists for short, medium and long-span bridges under the "aesthetics" heading. Now it's time for those that fall under the "technical" heading, although there is some overlap.

Frame and Form have posted images of the short-span technical shortlist, and as before, please just visit their blog if you would like to see the pictures!

Three bridges on the technical shortlist are also on the aesthetics shortlist: Castleford Footbridge, the Glass Bridge in Lisbon, and the Buitengracht bridge in Cape Town. I don't have anything to add to what I said previously on the Castleford or Lisbon structures.

In the case of Buitengracht, it appears F&F may have pictured the wrong bridge. I think the right one is the one now posted at Future Cape Town. There are also several images from the contractors, Vusela Construction, and a technical description at Skyscrapercity.

The challenge with this footbridge was to minimise the length of the approach ramps, while allowing access to the bridge from the side. Reducing the ramps means minimising the construction depth, which is the depth between the footway surface and the underside of the bridge. This is normally achieved by raising the bridge girders above the deck (in the "half-through" form), or suspending the deck from above (with a bowstring arch or cable-stayed layout).

At Buitengracht, the designers opted for a "quarter-through" design, raising a box girder above one edge of the deck, and relying on its torsional stiffness for overall stability. That allowed exits from the opposite edge of the bridge deck. It partly explains why the parapets are very different, with a post-and-rail arrangement on one side, and a glazed screen above the edge girder, which also acts as a wind-break.

There are two other bridges on the technical shortlist.

The Marinic bridge in Slovenia, would appear to be a footbridge in the spectacular Škocjan Caves, possibly the one known as the Cerkvenik Bridge. This spans an amazing underground abyss. I can't find much information online, but would imagine the challenge was to design a bridge which could be built in a remarkably inaccessible and difficult location!

The Stalhille Footbridge is another design by Ney & Partners, their fourth on the awards shortlists. This is an opening bridge over a canal in Flanders, with an opening system which may be unique.

Generally, there are six basic systems available for an opening bridge:
  • rotation about bridge's longitudinal axis (tilt, or "blinking eye")
  • rotation about bridge's transverse axis (bascule)
  • rotation about bridge's vertical axis (swing)
  • translation along bridge's longitudinal axis (retracting)
  • translation along bridge's transverse axis (possibly used on some floating bridges?)
  • translation along bridge's vertical axis (lifting or submersible)
There are some feasible combinations of these, particularly for retracting bridges which move at an angle to the transverse and longitudinal axes, and for "driven" bridges such as passenger boarding bridges in airports. There are also a very small number of oddities which involve transformation of the bridge's geometry, as with Schlaich Bergermann's Duisburg and Kiel bridges, or Thomas Heatherwick's Rolling Bridge. And of course, there are transporter bridges which solve a related problem, but aren't really opening bridges as such, perhaps having more in common with the cable ferry.

Ney's Stalhille design is a variation on the second and sixth types, whereby there are two pivot points lying above the bridge deck, allowing the entire deck to remain level while it is swung up and away from the canal. Structurae puts it in a class of its own, a pendulum bridge.

Spanning a mere 26m, it's far from clear why such a system would be chosen in preference to the more conventional options. Ney note that the bridge deck's static system (a simply supported beam) is preserved in all situations, but the same is true of a lifting bridge. I imagine that coordinating the rotating mechanisms so that the bridge remains level requires a carefully designed control system. I also wonder quite how the joints at either end of the deck are detailed, to avoid one being "clipped" as the bridge deck falls into place.

I do particularly like the bridge's filigree parapets, which are a lovely combination of ancient and modern.

Updated 20 June 2011:
The bridge in Slovenia is actually this one, which remains an impressive piece of engineering at a difficult location - I'd recommend following the link to see the photos!

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