03 July 2013

Pont Jean-Jacques Bosc bridge design competition

The lengthy design contest for a new 110m euro highway bridge over the River Garonne at Bordeaux is heading for its concluding stage. Launched in June 2011, five design teams were shortlisted in November of that year. The local municipality has recently agreed to enter negotiations with the two top-placed teams, featuring Rem Koolhaas and Dietmar Feichtinger. A final winner is expected to be announced in December this year, with bridge construction taking place from mid-2016 to late 2018. Since the scheme has been consulted on since 2009, this makes even the snail-like pace of project development in the UK seem less bad.

This is a substantial construction project, as the river must be about 550m wide at the location of the bridge, and all the designs exhibit a very generous deck width, particularly given that the actual highway itself is not that large and most of the width is given over to footway and cycleway provision.

Here are the five shortlisted designs:

Setec TPI / Marc Barani

From the images, it's hard to see why the bridge needs triple masts at each of its four support piers - the walkways are exceptionally wide, but it looks like a more conventional twin masts or portal pylon solution could have been chosen. That said, it's an attractive looking design at first sight.

One thing that is unclear is how the spans are stabilised, as none of the usual solutions adopted for multiple span cable-stayed bridges are clearly visible (very stiff towers - Rio Antirrio; intersecting cables - Forth Replacement Crossing; tower ties - Ting Kau; very stiff deck - Mersey Gateway). Without one of these measures in place, the bending in the deck and towers becomes excessive, as the load pulling down one span (and pulling its towers together) is not well resisted by the adjacent spans being pulled upwards.

Marc Mimram Engineering

I like much of Mimram's proposal, especially the first image above with its elegant approach span arches and gently sloping footways. However, the main-span arches feel confusing to me, particularly the way they penetrate the deck and spring from below - the relationship between arch and deck seems to lack any structural clarity.

The choice of slightly fanned quadruple arch ribs also seems to offer little in either architectural or engineering terms. The ribs will do little to provide lateral stability to the arch, which will be governed by lateral bending at the deck level or at the arch supports. As with so many competition designs, there seems to be a pursuit of originality purely to make the bridge unique, not for any other justification.

Dietmar Feichtinger / Schlaich Bergermann und Partner

This design bears more than a passing resemblance to Feichtinger's Passerelle Simone de Beauvoir, which also features undulating walkways separated in level from the road deck, held apart with a series of steel struts. Unlike the lenticular Paris bridge, the Bordeaux proposal does not derive so directly from the structural form, allowing the spatial requirements to take precedence. At the middle of the bridge, the depth between walkway and roadway becomes sufficient that a partly-sheltered pedestrian plaza is created by joining the two walkways together below the roadway.

While the desire to give pedestrians a variety of different experiences is laudable, the effect overall is to make the bridge significantly deeper than it really needs to be, and I wonder whether the benefit makes this worthwhile. Road users are obliged to pass over a mid-river hill, obscuring views of their destination.


Dezeen has more to say on this design. From the images provided, it's difficult to say much about the structure, which seems to be self-consciously minimal, a beam bridge on a series of pier columns. I am told that the deck is a simple concrete slab on steel plate box girders, designed to be easily launched across the river.

As the images make clear, the emphasis is not on the bridge but on its flexibility of use, a huge tabula rasa which can be reconfigured for different traffic modes, events, fairs etc. This seems quite admirable, but it still seems a surprise for  a bridge where the architecture is so quiescent to be considered a potential competition winner.


The final shortlisted entry is the most structurally expressive, a multi-span suspension bridge which seems to me to be poised carefully between restraint and flamboyance. The challenge of providing stiffness in such an arrangement is addressed by using "A-frame" towers to carry the cables (traditional vertical towers work well for a three-span suspension bridge, but less well for more spans).

The towers are inclined laterally which creates interesting cable profiles but seems to me to add an unnecessarily jaunty air to what is otherwise a straight, non-skewed crossing of the river. The deck widens out at the tower positions, creating large balcony-type areas. I'm reminded a little of the unhappy Golden Jubilee Bridge in London, although this is certainly a more elegant design. My main concern is simply that this is quite an odd choice of solution for such a wide bridge. The suspension system will normally permit a very slender bridge deck, but here its width will govern its depth. It is also likely to be many times more expensive than some of the alternatives, as this is just not an economic construction technology for a highway bridge of these spans.

No comments: