10 July 2009

Who ate all the pies?

The latest designs for a proposed iconic bridge over the River Wear in Sunderland are being made available for public consultation next week, in advance of an application for planning consent. Designed by Techniker and Spence Associates, the cable-stayed structure ranks as one of the most audacious proposals ever to come out of a bridge design contest. Plans will be on show at three local venues from 14th to 22nd July.

The projected cost of the bridge has not been made public, but Sunderland City Council acknowledge that it will cost about £29m more than a conventional girder bridge alternative, with the risk of a considerably greater funding gap (and that figure was put together before Techniker were appointed to carry out more detailed design development). Sunderland are currently suggesting that work on the bridge could start in 2012, and be complete by 2014.

I'm not sure what is expected from this latest round of consulation: a previous exercise in 2008 found that while 52% of locals wanted "a striking design", 49% wanted something "tried and tested", and 58% wanted to minimise impact on council tax payers. There's clearly a contradiction here, as a bridge of this outrageously ambitious design will inevitably cost far more than more ordinary options (indeed, either of the unsuccessful competition entries that I've seen would cost significantly less, while still being "striking").

What's most interesting about this latest round of consultation is the set of images that have been released to accompany it. Let's take a look at one of them, and compare it to a similar image used previously.

The first one is the earlier image. The new image is second.

Is it just me, or has the bridge been at the pies? The masts have put on weight, and they're also shorter and more upright. To me, it looks a lot less attractive than the design which won the competition, although this is hardly a surprise given the extraordinarily inefficient structural form which has been chosen - a cable-stay bridge without back stays or even any attempt to counter-incline the mast.

Some of this is just "design development" - contestants in bridge design competitions often tend to slim down their bridge design visually in order to get a positive reception. I've done some of this myself - not slimming it down, but omitting some of the details such as stiffeners which I know will really be there. The competition image is an aspiration, not the finished article, and hopefully the technically astute jury understands this.

But on the River Wear Bridge, the expanding waistline has as much to do with the design's conceptual difficulties as it does that art of deceptive visualisation. I can well imagine the work that has gone into reviewing the calculations in order to eliminate the lack of stiffness and potentially poor dynamic behaviour that would have been inherent in the original geometry. The reason the masts are more upright is simply because in the original concept their own self-weight was only adding to the problems, and straightening them reduces this.

What would be interesting would be to see more of the technical review brought into the public domain. Given the structure's innovative nature, who (if anyone) is peer reviewing the designer's work? Who is giving advice on its construction (e.g. have specialist contractors been brought on board for early input)? Given the scale of the mast foundations required, what are the ground risks? What key risks have the cost planners identified? Sunderland Council's stated game plan is to achieve greater cost certainty before proceeding further with the Techniker design, and these are the pertinent questions.


FF said...

great post! hopefully we will learn something from it.
Is it me, or they also added back stay cables on the new improved solution?

The Happy Pontist said...

I'm fairly sure they haven't added backstays. The single most obvious change they could propose to make the design more efficient would be to introduce cables tying the two masts together.