25 October 2008

Bridges news roundup

Steel? Psshaw. Concrete is best for iconic bridges

The new chairman of the Concrete Bridge Development Group, David Ball, reckons concrete is under-utilised in British landmark structures, pointing to the material's now much improved durability. Maybe so, but steel remains popular for landmark bridges because it's far easier to erect, and in many cases more compatible with lightweight and geometrically unusual structural forms.

Glasgow bridge in trouble. Not quite

It seems there's always a bridge in Glasgow in trouble of some kind. And now the virus is spreading, as the completion of the Forthside Footbridge in nearby Stirling will apparently be delayed until next year. The Sunday Herald speculates idly on possible common factors, noting the presence of cable supplier Macalloy and main contractor Nuttall both on Forthside and on the unlucky Clyde Arc bridge. (Fortunately for Nuttall, the Herald hasn't recalled that they are also building Glasgow's Squiggly Bridge, which like Forthside has also suffered delays.) Nuttall state that stressing the Forthside Footbridge's stays is proving difficult, something which was acknowledged in the designers' paper at the Footbridge 2008 conference this year (see PDF).

Personally, I think it's entirely to Nuttall's credit that they have taken on several ambitious bridge schemes. Difficulties and set backs aren't entirely unexpected on innovative designs, and perhaps engineers should make more of the opportunity to explain this to their clients, and the public in general.

Weave Bridge to open in November

Cecil Balmond's colourful Weave Bridge is nearing completion at the University of Pennsylvania. Balmond seems to have little sympathy for the conventions of bridge design, taking standard concepts like the arch or truss and transforming them into stained-glass geometric puzzles. In his use of colour and advanced computer geometry, he's an engineer with a distinctive style, a refreshingly eccentric contrast to normal engineering methods.

The Weave Bridge looks from the pictures to be a Warren truss with overhead bracing - a design that's essentially about 160 years old. Balmond's design seems to arise, however, mainly from playing with geometry on a computer, rather than from the purely structural imperatives which drove James Warren's solution. Similarly, Balmond's Coimbra footbridge looks in elevation like an arch, but in reality it's a twin cantilever bridge whose main purpose is to offer a platform for Balmond's complex, startling glazed parapets.

2 comments:

Paul Kassabian said...

The main difference between a standard Warren truss and Balmond's Weave bridge, as I understand it, is that the "Weave" does not have top chords. So the bracing is working overtime to carry longitudinal axial loads (picture lazy tongs which must not "scissor" by providing rigid joints).

The Happy Pontist said...

Good point, not sure why I didn't spot that!