03 November 2009

Kraków footbridge undergoes metamorphosis

I've touched in the past on a pretty bizarre competition-winning bridge design at Kraków in Poland. This design was featured in New Civil Engineer magazine a few years ago, as an example of the sort of result that can happen when a contest has no judge with suitable structural engineering expertise.

The competition was held in 2006, and the winning entry was by Biuro Projektów Lewicki Łatak. It was seen as a partner to a new replacement for the Podgórski Bridge, a 150m span steel arch footbridge which is currently under construction. Łatak's design, however, has yet to secure definite funding, and no firm decision has even been made on its precise location.

The competition-winning image (shown above) is for an exceptionally slender structure. At first sight, it looks like an underslung suspension span on the left, and an arch-suspended span on the right, until you look at the junction between the two and realise that this point is unsupported. In technical terms, it's a "hinge" - a point of negligible bending stiffness at a position on the span where the bending moment would be considerable. In lay terms, it simply could never stand up.

The fact that this design ever won a bridge competition highlighted the often incomprehensible way in which such contests are organised. I believe this one was part of an architectural biennale, and the image represented the apex of a tendency where it seemed architects could design something that superficially resembled a bridge structure, without it having to have any structural rationale at all. Without an informed public, or a competent jury, the image was everything, and patent nonsense such as this could be declared a winning idea.

Although I believe the project has been officially shelved, the winning design has been developed further since 2006, and it's interesting to see the latest image, shown below. What do you do, if you win a competition with an unbuildable bridge, but people show interest in actually building it?

There are answers to the same question to be found elsewhere (Neptune's Way in Glasgow, where props were added to try and hold up an inefficiently-angled arch, or River Wear in Sunderland, where a decidely sub-optimum structural form was made to work simply by throwing more and more concrete and steel at it). In both cases, the result was a bridge costing considerably more than the promoters had budgeted for when they declared a competition winner.

It's unclear if the Kraków footbridge ever had a serious budget (29 million złoty has been mentioned recently, which is £6m or US$10m, while £5m was mentioned when the design was first publicly criticised), but as at River Wear, the most obvious change to the design has been to thicken everything up, turning the main deck into a stiff shallow arch (or possibly cantilever girders, it's hard to tell from the rendering). It's still somewhat unconvincing, but again, throw enough material at it, add beefy foundations, and it can be made to work. The upper and lower arches are essentially non-structural in this revised design, just glorified staircases to provide interesting viewpoints (so long as you're not mobility-impaired). I hope the lower one has been set above maximum flood level ...

I've located a couple more images of the revised design, which perhaps help clarify how it works (also see the image here). Whatever, the moral of the story is (as ever) that bridge design competitions need an informed and competent jury; that designs likely to require substantial structural and hence financial amendment need to be weeded out; and that architect-led bridge design only works with an architect who has a sympathy for structural behaviour.


Stuart said...

Hey Hp I think you getting hung up on the buildability of a bridge, what are you some kind of engineer? (just kidding don't hit me!)

I looked at the first rendering and I could not figure out how it stood up but it sure was pretty.

Doesn't this story relate to most bridge competitions? Even Calgary which has done a great job has a couple of structures in the mix that cannot be build in their current form.

But I doubt they will be researched enough to be sure they can be built before the final bridge is chosen.

Who says the Calatrava design can work with the member sizes they have shown?

The moral to me is still, win the competition, get the glory, get the money, worry about building it later......It is a sad moral for engineering but it seems to be the norm and not the exception.


Island1 said...

Impossible to build? Damn, I was looking forward to that bridge.

The Happy Pontist said...

Oh, the original design, as visualised, could be built. It just wouldn't stand up once you took the scaffold away ... That's a very black-and-white opinion, I know, but in my view entirely justified by what was portrayed.

The revised visualisation can be built and could be engineered to stand up thereafter as well.

julian hakes said...

Interesting, at first I thought it was a photoshop of this design.


Whilst the original model in our studio does hold its own weight. For real world use the inclusion of the prop between the tied arch - compression and bow slung - tension elements of the bridge is rather important.

J Hakes

J Hakes said...

"bridge design only works with an architect who has a sympathy for structural behaviour"

great comment, seconded...

J Hakes