07 March 2009

Farewell to wave bridge

I've previously discussed proposals for a new light rail bridge in Portland, Oregon, carrying the proposed Portland - Milwaukie line across the Williamette River. The point of interest has been architect Miguel Rosales' promotion of an innovative, risky "wave frame" bridge design (pictured above left) as a substitute for the more conventional cable-stayed design that otherwise seemed likely to prevail (pictured below right).

The process for selection of a preferred design seems so byzantinely complex and bureaucratic as to beggar belief. Nonetheless, I gather that it's far from unusual in the USA, where public participation in bridge design seems frequently to be allowed to trump the vision or wisdom of experienced professionals.

The bridge is being promoted by a body called TriMet, and it's worth reading the documents on their website both to marvel at how no firm decision has yet been reached after six months of incredibly detailed debate, and to see how, even when the options should in theory be narrowing, yet more choices are in fact being added to the mix.

The Portland press implies (without stating it explicitly) that Rosales preferred "wave frame" bridge has bitten the dust, with extensive cost studies confirming that it is both far more expensive and more risky than the obvious cable-stayed rival - US$145m for the wave-frame against US$91m for the cable-stay (pictured left). While it's a shame that cost gets in the way of innovative and aesthetically conscious bridge design, the Portland to Milwaukie light rail scheme seems to be struggling to get its budget to add up at all, so it's clearly not the opportunity to place ambition and aspiration ahead of common sense. I can't imagine how an extra US$50m could ever be justified for a bridge of this scale and function.

The cable-stay proponents have recognised that their design visualisations were somewhat unattractive, and have now presented a hybrid solution, combining the attributes of a cable-stayed and a suspension bridge (pictured, right, and below).

This is definitely a softer, more touch-feely alternative to the angular cable-stayed option, and harks back to designs like the Albert Bridge, Brooklyn Bridge, and the Niagara Railway Bridge. So, in search of an efficient, modern, visually appealing bridge, they've stepped back in time by about 150 years.

What's especially unclear is how they reconcile their desire for this visually attractive hybrid bridge with the desire for a least-cost solution: adding a main suspension cable to what was a perfectly functional and efficient cable-stayed bridge can only bump up the construction costs by a significant margin.

There's a reason they don't build them like that any more, after all.

The cable-stay option, which is undoubtedly the most efficient design for a light rail bridge spanning 200-250m, never seems to have been given the attention it deserves - the visualisations, with their classic A-frame design, do nothing to show how flexible this form is aesthetically, how many ways there are to make it attractive without having to go to the expense of Rosales' "wave-frame" solution. I don't get the feeling we're anywhere near seeing the final design yet ...

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