24 April 2011

Footbridge awards 2011 - aesthetics 30m to 75m span

Frame and Form has published images of the next set of shortlisted entries to the 2011 Footbridge Awards - the "aesthetics" category for bridges spanning between 30m and 75m. Again, please see their blog post for the images.

I covered the Knokke Heist Footbridge here near the end of 2009. Designed by Ney & Partners, it's a tour-de-force of structural design, synthesising a number of individually great ideas and producing something which is spectacular but still graceful.

Taking the bending diagram for a three-span continuous beam as their starting point, they stripped away unnecessary material stage by stage to evolve a structure which is a unique hybrid of suspension bridge, stayed bridge, and steel girder. It borrows from the form-finding techniques of structural fabric design, with a thin steel skin draped between points of support, cradling the bridge deck and providing a seemingly effortless response to the three dimensional geometry and load requirements.

In many ways, it's an inverted version of Robert Maillart's Tavanasa Bridge. I admire it for its undeniable panache but also for the ability to take a challenging concept and follow it through what must have been a very demanding fabrication and construction process without losing anything to pragmatist compromise.

Barcelona's Sant Fruitós Footbridge, designed by Pedelta, pairs the classic inclined arch typology with the use of stainless steel and GFRP elements. I always find this kind of design slightly forced when, as here, the deck is straight in plan, as the arch inclination has no real structural justification, it's just an affectation intended to add interest to what would otherwise be a fairly straightforward footbridge over a highway.

Despite the awkwardness of a lift tower and staircase at one end, the design doesn't seem unbalanced, and is a good example of its kind. It's interesting to compare Calatrava's La Devesa Bridge at Ripoli, which also has an inclined arch and the issue of very different approach levels to resolve. The Calatrava Bridge has the elegant combination of a cantilevering end support with a staircase, killing two birds with one stone, but the use of cables rather than "bracket arms" at Sant Fruitós gives a lighter and hence more pleasant appearance overall.

The Paloma Footbridge is quite a different proposition. Spanning a highway in Madrid, this bridge is curved in plan, but eschews the counterbalancing cable-stayed or inclined arch support system that may suggest in favour of a warren truss along only the outer edge of the deck. It isn't entirely unique, and reminds me in concept of the Mancunian Way Footbridge, which also had a truss along its curved outer edge.

On the Paloma structure, the main truss appears to be stabilised by a secondary truss connected to its top chord, and sitting flat to provide stiffness against out-of-plane movement of the main truss. It's a neat conception, mainly because it also carries a canopy and hence provides some level of shade and shelter.

What I like about this bridge is not just the interesting structural form, but the quality of the detailing, which looks to be quite impeccable from the photos available.

The Sant Fruitós inclined arch has a cousin on the shortlist, the Riverside Bridge in Cambridge. It was designed by Ramboll, and adopts the white colour that so many of this kind of bridge seem to share. Its main span over the River Cam comprises an inclined tubular steel arch supporting on one side the footway deck, and on the other a cycleway deck. The two decks are carried by transverse "wishbone" members, and supplementary cables, which balance the loads such that out-of-plane bending of the main arch is minimised.

I'm keen to pay it a visit, but based on photographs, it seems only partially successful. The split deck, which seems clever at first, also complicates the overall geometry, multiplying the balustrades and eliminating the simple elegance which inclined arches can normally boast.

I've covered New Zealand's Te Rewa Rewa Bridge here before, and have little to add to my previous comments. The asymmetry of its arch ribs has no structural logic, but seems visually compelling. The detailing lifts it beyond dozens of minimalist Calatrava clones, such as the way the ribs extend above the arch spine. It's definitely one of my favourite bridges on the shortlist.

RFR's Lyon Confluence Bridge is a fine riposte to the structural showmanship of so many post-Millennium footbridges. Timber decking sits on a relatively straightforward tubular steel arch structure, made very slender in line with a number of other recent RFR designs. Indeed, it looks so slender that I wonder whether a minor impact from a boat wouldn't be enough to bring it down, but presumably they will have considered that.

I can find no information on the Butarque Footbridge, in Spain, online, other than the photograph provided at Frame and Form. It shows a bridge which appears to have a deck with a triangular cross-section, supporting a rail-less balustrade. If anyone can point me to any other images, please leave details using the comments link below.

1 comment:

Anonymous said...

You can find some images in the link below. The footbridge is designed by Cesma Ingenieros (www.cesmaing.com) with the collaboration of s+j arquitectos.