Once again, those kind people at Frame and Form have posted pictures of some of the shortlisted bridges in this years Footbridge 2011 Awards, this time in the medium span / technical category.
I've discussed the Knokke, Paloma and Riverside bridges before, so won't repeat that again.
Glasgow's Tradeston footbridge was the more pragmatic afterthought to a failed bridge design competition (see previous post). The contest had produced an extravagant and unaffordable winner, and the promoter switched to a design-and-build approach to procure the bridge which was eventually built.
I admire its stark aesthetics, and it's always great to see that quality rather than corner-cutting crap can emerge from the D&B process, but I'm not so sure of the bridge's technical merits. The deck is exceptionally slender, but it's hard to see what else on the bridge was a huge technical challenge.
The Peace Bridge, in Tbilisi, Georgia, is a very different creature. The architect, Michele de Lucchi, was personally invited by the President of Georgia to propose designs for a 160m long pedestrian bridge over the River Kura. His three designs were all covered structures, with the as-built organic form being chosen on purely aesthetic grounds over folded-shell and "parachute" alternatives (de Lucchi's paper at the 2010 IABSE Venice Symposium provides full details). What was built differs considerably from the architect's original proposal, chiefly due to the need to provide a more rational structural form for the glazed steel grid-shell.
Clearly, construction of a bridge of this sort is a very real technical achievement. The grid-shell supports the bridge deck on hangers, and its geometry has been simplified by extruding a common parabolic section along its length. However, it is not a structurally efficient means of support, and even if it were, its shape has not been optimised for the loads upon it. Does technical merit for a bridge reside in responding to a difficult architectural challenge, or in establishing an efficient and effective solution to the structural requirements? Both are valid, and the engineering designer has done well, but this bridge still leaves me a little uncomfortable.
The 73m span Robert I Schroeder footbridge in California is a more coventional bridge, but still impressive. It is suspended from twin steel "butterfly" arches, with the thrust into the ground taken by inclined concrete struts. The arches themselves are in the form of triangular space trusses, and the logic of the structural form has been translated into good looking details, particularly the treatment of the parapets.
It's an admirable looking bridge, although certainly less technically exciting than the Knokke footbridge competing in the same category.