Calatrava springs a surprise

It wasn't due to be made public until early August, but it has come early: the City of Calgary has published Santiago Calatrava's design for a CAN$25m new footbridge spanning 130m across the Bow River.

I've discussed this bridge before: first on the controversy over Calatrava's appointment without competition, then the attempts to have that appointment reversed, the designer's incredible fee, and most recently the foot-dragging over publishing the design.

Since then, events have moved on rapidly. First, Calgary announced out of nowhere that the bridge was to be a tribute to Canada's soldiers, something that had never been mentioned previously, and which seems to some like a cynical attempt to defuse the many critics of the bridge by placing a taboo on further dispute.

Then, the bridge's name was declared: the Peace Bridge. Again, who could possibly oppose that?

But at last the bridge itself has hoved into view, and Calgarians can finally see what they're getting for their money.

The Peace Bridge design is a radical departure from Calatrava's signature style, being a flat tubular structure constrained by height restrictions imposed by a nearby heliport (see left; click on any image for a larger version; all images copyright Santiago Calatrava). The helical truss form is obviously reminiscent of Buro Happold's bridges at Harthill and Edinburgh (precedents spotted by at least one Calgary reporter), as well as the Double Helix Bridge in Singapore's Marina Bay. It might also be seen as a distant relative of Pennsylvania's Weave Bridge, perhaps, although unlike that structure, the Peace Bridge is at heart a conventional truss, with proper top and bottom chords.

It has already been described variously as a candy cane, a "Chinese finger trap", or as resembling a stent. While the design has attracted many positive comments, it is also the focus of local discontent. Even now, with contracts awarded, some would like to halt the scheme, others fear parallels with a delayed Calatrava design in Texas. If nothing else, the intense debate over the design is already focussing attention on Calgary.

It reminds me of skeletal coral, or the cheap paper Christmas decorations I had as a child, which expanded like intricately entwined slinkies.

The colours are intended both to match the Canadian flag and also to make the bridge stand out in winter. For me, it's great to see Calatrava moving away from his more normal spartan white, and there's little doubt that if built, it will be a very striking landmark structure.

Visually, I definitely like the bridge, although the white interior has a little too much of the hospital or airport for me. Opening out the helix (compared to the other helical truss bridges) makes the bridge look far more attractive, and the use of curved glazing and the opportunities for lighting make the design visually more successful. How well it fits into its context is a different question, and one not easy to judge from the visualisations.

It's also not entirely clear why it needs to be a covered bridge. In an interesting interview with the Calgary Herald, Calatrava suggests this is to make the bridge more welcoming on a windy winter's day. As with most covered bridges, you have to brave the cold windswept approaches to get under cover, so it's unclear how worthwhile that really is.

Structurally, the bridge's distant ancestors are the Town Truss and the Howe Truss, although it can also be seen as a set of superimposed Warren Trusses. Like these 19th century designs, it uses triangulated elements in a structure which is both stiff and lightweight. These qualities are achieved because in conventional truss design every structural member only carries axial forces (compression or tension).

However, rolling the truss into tubular form introduces enormous secondary forces - bending moments because the curved truss members are eccentric to their nodal connections, and also torsional effects because the connections aren't co-planar. Because Calatrava's helix is so pleasingly open, these secondary forces will be enormous, much more than is the case on the Happold designs, which also have the advantage of much shorter spans (70m at Harthill). Having prepared a preliminary design for a tube-shaped but non-helical truss bridge on a much smaller scale than this, I can attest that the difficulties are considerable.

Is it value for money? That $25m is about £14m in Brit money, which isn't entirely beyond the pale for a major landmark footbridge. At 130m long and 6.2m wide, that works out at £17.4k per square metre of deck, a yardstick I've used here before on several occasions. The Buro Happold bridge at Greenside Place came in at £14k/sq.m , so allowing for inflation, Calatrava's design might seem to be priced about right. However, it's nearly double the span of the Harthill bridge, so you'd expect it to require up to four times as much steel in parts of the bridge (bending moment being proportional to the square of the span, with shear proportional to the span). The cost of construction over a wide river is also likely to be higher, especially if disturbance to the riverbed is unacceptable and if the heliport restricts crane heights.

The Peace Bridge's current cost estimate is right up there with London's Millennium Bridge (£12k/sq.m) or Brisbane's Kurilpa Bridge (£19k/sq.m) - bridges which are structurally incredibly inefficient but where the opportunity to create something truly unusual may well justify it. Will the Peace Bridge be as big a tourist attraction? I think it may well be.

Nonetheless, given how astoundingly inefficient a helical truss will be at 130m span (pity Calatrava's poor local partner Stantec, whose job it will be to make it stand up), Calgarians shouldn't take budget assurances at face value just yet.

Interestingly, the City of Calgary has published its own set of price comparisons [PDF], using the same yardstick as mine (their figures are a bit different, mainly because they adjust for inflation, which is of course reasonable). This is a very welcome move as there's little other way of justifying the budget at this early stage.

The City has also responded to criticism of Calatrava's fee, suggesting that "Calatrava's design fee is in line with the industry standard for architectural fees for similar projects of about 12 per cent of the total construction cost". In the real world, there are very few architects (if any) charging anything like that on a bridge with this capital budget: architects generally only earn large percentage fees on buildings work, where their input is far more significant than on a bridge, or on schemes with a much smaller budget (where design can be a larger proportion of the total cost).

However, a 12% fee offers encouraging news for prospective designers of Calgary's next landmark footbridge, which will be a larger structure promoted by Calgary's East Village development corporation. Originally, this was slated to be designed by Calatrava again, but it's now going to go out to a design competition, starting on August 17th, with proper public consultation prior to anointing a winner.

This is a complete u-turn for Calgary, who insisted that to get the quality they wanted for the Peace Bridge, directly appointing the best-known bridge designer in the world was the only sensible approach. Does this mean that quality is less important on the second bridge? Will they shoot themselves in the foot by demonstrating that in fact, bridge design competitions can get just as exciting a design? And most importantly of all, will they also be paying the competition winner a fee equal to Calatrava's?

Meanwhile, construction tenders for the Peace Bridge are reportedly to be sought this Autumn (good luck pricing that one without the detailed design complete!), with the bridge to open in 2010 (to coincide with an influx of porcine aviators, no doubt).

I'm off on holiday for a week, so feel free to comment but I won't be moderating comments until I'm back.