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.
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.
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.