In this context, it's perhaps worth noting that back in 2003, Tees Valley Regeneration were seeking a bridge costing a mere £4m to £5m. By any yardstick, a 200% cost over-run is a shocking performance, but there's little doubt that Stockton-on-Tees would have got a considerably less special bridge for the original budget.
Why did the Infinity Bridge cost so much more than anticipated? For one thing, the original budget was clearly out-of-step with the client's aspirations for a major river crossing such as this. Nonetheless, I wonder whether the form of bridge chosen plays a big part.
The bridge is designed as a twin tied-arch, where the arch thrusts are resisted not by the foundations, but by tie cables running at deck level. These are visible in the photo on the right (courtesy John Yeadon), just outside the deck edges (there are "bars" running below the deck, but I assume these are service ducts e.g. for lighting cables). As with any tied arch, the structure as a whole is not stable until both the arch and tie member are complete, and any structure which is not stable until complete normally requires extensive temporary support and/or a more complex construction sequence.
From the construction photo on the left, it's apparent that the ties were installed only after the arches were lifted in, implying that the foundations must be designed to resist thrust from the arches' self-weight in the temporary state. This should not be a huge force, and the erection sequence has at least minimised the need for temporary propping. I am also told that it doesn't govern the river pier design (as this has to cope with ship impact, which is larger).
The tie cables are independent of the deck itself (this is shown in the construction photo on the right, again courtesy John Yeadon, where the cables are temporarily lifted out of the way while deck segments are installed). This will have complicated the erection sequence, but substantially reduces temporary propping requirements, as the foundations don't have to resist temporary arch thrusts relating to the weight of the concrete deck.
Essentially, the tied arch is not the most economic option at this site. A cable-stayed design (which can be built segmentally and is normally stable throughout construction without significant temporary works) would have been cheaper. Buildability should come first in any bridge engineer's thought process, but on the Infinity Bridge it has been more of a case of choosing the bridge's structural and sculptural form, and then working through the challenge of construction in an intelligent manner. That's not necessarily the wrong decision, but it certainly reduces economy, and many of the cleverest bridge designs are those that arise naturally from the imperatives of construction, rather than hinder it.
Having said all that, one thing that most impresses me about the design is the extent to which the designer's original vision has been translated into the final construction. Having seen the tender drawings back in 2006, it's clear that the contractor has made very few changes to Expedition's design. That must be a tribute to the thought given to the buildability of the bridge, some of which is illustrated in the assembly images Expedition produced (see example on the left).
Overall, it's easy to nit-pick, but I think this is a very successful design, with an original aesthetic vision which has been followed through with minimal compromise. Unlike several recent bridge competition winners, the design is led by a clear structural concept, rather than distorted for exaggerated effect.
Photo credits: construction photos from North Shore Footbridge website; visualisations copyright Expedition Engineering; all other photos are credited as appropriate.