Footbridge Awards 2011 - aesthetics above 75m span

Once again, those kinds folks at Frame and Form have posted pictures of the shortlisted entries to the Footbridge 2011 Awards, this time for the "long span / aesthetics" category. As on my two previous commentaries, I won't repeat the images here, just visit Frame and Form to see them.

I haven't covered any of the bridges on this shortlist previously, which is a shame, as several are very impressive.

The Center Street Bridge, in Iowa, is a variant on the now classic typology whereby a curved or straight bridge deck is hung from an inclined arch. In Iowa, the bridge has a vertical arch, and twin curved decks hung either side which to some extent counterbalance each other. The decks are in the form of closed steel box girders to provide the necessary torsional stiffness, given that they are supported by cables on only one edge.

The twin decks allow foot and cycle traffic to be separated, although there is also a short transverse "strut" deck between the two main spans which provides space to stop and admire the view. From the photos I've seen, it looks to be an attractive, well-detailed bridge, which has a touch of the iconic without appearing inappropriately brash.

Ney and Partners have done well to receive a number of shortlistings, and their College Bridge in Kortrijk, Belgium, is a very interesting design. It's an S-curved suspension bridge, 203m long, with a main span of 86m. The main cable follows the funicular line of forces, and supports the deck using a "warren truss" type arrangement of hangers, which provides greater stiffness than the normal arrangement of parallel hangers. The tightness of the S-bends is sufficient to allow the two masts to be inclined towards the deck. The masts are held against movement by tie-down cables connected via the deck to ground, which must contribute enormously to the overall stiffness of the system.

The deck is stiffened further by lateral edge trusses, although I'm not sure I see how these work, as I would have though the deck itself was wide enough to provide sufficient lateral stiffness. Vibration dampers have been avoided, which is a respectable achievement for a lightweight cable-supported bridge like this. In some photos the bridge looks inappropriately long and massive, but in others it looks just right.

The bridge at Esch-sur-Alzette is also by Ney, but is a very different beast. It's remarkable in many ways, including its bold response to a very challenging site, where it must span a railway, avoid overhead service cables, and carry people across a considerable difference in levels (achieved via a lift and stair tower at one end). Unlike a number of structures, the lift tower is completely integrated into the structural form, which lies somewhere between stressed-skin construction and truss design. The truss frame can be seen either as a membrane structure with stiffening ribs, or as a simple truss where the gusset plates have been allowed to grow unhindered. Overall, it takes the form of one half of a three-pinned portal frame.

The frame is painted grey on the outside, red on the inside, and is remarkable in its appearance, particularly the illuminated interior at night. Ney & Partners are easily amongst the most consistently interesting European bridge designers, and it's great to see them getting recognition on this shortlist.

One firm who have also long held such acclaim is Schlaich Bergermann & Partners, who have two bridges on this particular shortlist. The first is the Passerelle La Defense, in Paris, which was designed in collaboration with Feichtinger Architectes. It's an 88m long bridge in the almost-popular "inverted Fink truss" form (which really needs a new name, since it simply doesn't match the topology of an actual Fink truss, inverted). What makes it remarkable is that it curves round the outside of an existing building, seemingly defying gravity. Most of the truss masts appear to hang mysteriously in mid-air, balancing on a set of horizontal cables which are actually there to restrain twisting in the deck, not to hold it up.

SBP have designed some excellent bridges over the years, but I'd say this is probably one of their best efforts.

Their other shortlisted design is the Grimburg Harbour footbridge at Gelsenkirchen. This owes more of a debt to classic SBP designs of previous decades, being another hi-tech reinterpretation of ring-girder and suspension bridge forms. It's not at all a bad bridge, but it seems a lesser achievement when set next to the Paris design.

From a technical standpoint, the cable layout leaves me slightly queasy. The curved deck is supported on its outer edge by a suspension bridge type arrangement, with parallel hangers extending to a suspension cable. The main cable isn't supported directly on towers, as would be normal, but from two subsidiary cable which are held up by a guyed mast. What makes me queasy is simply the thought of how you might maintain the structure should any of its main cables ever require replacement. The arrangements required to maintain temporary stability would be awkward, at the very least.