07 October 2009

Johnson Street Bridge, Victoria, Canada

Here's a very interesting (and somewhat controversial) bridge project in Canada, one I spotted recently and felt it deserved more than a one or two-line throwaway in response.

Johnson Street Bridge is a historic heel-trunnion bascule bridge (pictured right, and below left) in the city of Victoria, in British Columbia. Opened in 1924, its two separate parallel decks span 45m and carry both highway and railway (I think the rail span is disused?). It presently carries over 30,000 vehicles every day.

The design, which I guess will be largely unfamiliar to British readers, is to a patent by Joseph Strauss (best known as the designer of the Golden Gate Bridge). As with most bascule bridges, the opening span is balanced by a counterweight suspended behind the main span. It's called a "heel-trunnion" design because the counterweight rotates about a separate trunnion (or axle) to the trunnion used for the bridge deck - this allows the counterweight to be positioned high above the deck, and eliminates the need for a large pit below deck (as is present, for example, on Tower Bridge). In the competing "simple-trunnion" and Scherzer rolling lift designs, the counterweight is rigidly connected to the opening span, and they rotate about a single pivot point.

While the Strauss design is common in North America, the British and European equivalent is a much simpler design, exemplified by the Magere Brug in Amsterdam but also used for larger spans. In both cases, there's a structural mechanism in the form of a parallelogram, which deforms in a diamond shape, maintaining the parallel sides.

In April of this year, the Victoria city authority received a report on the condition of Johnson Street Bridge, which concluded that it was extensively corroded, the mechanical and electrical systems were obsolete and unmaintainable, and the bridge was vulnerable to failure in a large seismic event. The cost of replacement was estimated as competitive against the cost of refurbishment, and Victoria committed to build a new bridge. The consultants MMM and architects Wilkinson Eyre were brought in to draw up options, with a capital budget of CAN$63m in mind.

Conscious of the heritage value of the existing structure, a public consulation committee was set up, and the three design proposals published during September to obtain comments. The options are all unusual and hence interesting, although therefore suffering the disadvantage of a limited proven track record. It's a challenging design: spanning 40m, providing unlimited clearance when open, and it needs to be about 26m wide to carry footways, cycle lanes, highway lanes and the rail corridor. Wide bridges are always difficult to make attractive, and they cause difficulties with opening bridge machinery as well.

All three design options are bascule bridges with raised counterweights. Lifting bridges haven't been considered, presumably because they'd prefer an unlimited air draft, although I reckon a lifting design could be built with less traffic disruption. There's no swing bridge option, presumably because of the site geometry. And there are no conventional bascules, presumably because of the disruption that would be caused building a sunken counterweight pit.

The first design is a "reverse bascule bridge", purportedly inspired by a Vincent van Gogh painting of the European-style heel-trunnion bridge, although it works quite differently. In the Wilkinson Eyre (WEA) design, the counterweight starts horizontally, swinging down to a vertical position. At the same time, a trailing arm on the counterweight, initially vertical, swings up to a horizontal position, pulling up the main span via cables. Essentially, the bridge folds up on itself, and although it's cunningly mirrored and disguised, the principle is essentially the same as on the existing bridge. Instead of a parallelogram, the mechanism flips from a nearly triangular arrangement into a "crossover" quadrilateral.

Without looking at it in more detail, I wonder how well that would really work - bridges of this type are quite sensitive to the position of their centre of gravity, and normally designed to be stable open or closed (i.e. the counterweight tending to maintain the bridge deck in that position). I'm just not sure if that could possibly hold true for this unusual geometry.

Also against this design is its general ungainliness, and the bulk inherent in a 25m wide counterweight - I don't think the renderings really present the width of the structure well, although it's clear that WEA have done all they can to minimise the structure's bulkiness by splitting the footways off onto underslung cantilevers.

The second design is a Calatrava-style cable-stay bridge (i.e. without backstays). I've seen this proposed for bascule bridges, but can't think of any that have been built (any suggestions?) It has the significant disadvantage over the other two options that its centre-of-gravity is somewhere in mid-air, and hence it takes more power to physically push it around.

This was the preferred design in the public consultation, favoured by 50% of voters.

The best of the three alternatives, and the one chosen by Victoria on 24 September, is a rolling bascule bridge, inspired perhaps by the classic Scherzer rolling lift design, but rotating in place rather than back onto a track, rocking-horse style. A trussed main span sits on two ring girders, each of which presumably sits on bogies or roller supports. It's directly inspired by WEA's similar design at Bellmouth in London's Docklands, a concept also used in Mott MacDonald's Foryd Harbour competition entry, and to an extent in the Falkirk Wheel.

I'm always happy to see a new take on the truss bridge, here replacing the latticework with sleek modern lines and an attractively shaped counterweight. Trusses are rarely the popular choice today, but it would be nice to see them used more often on landmark bridges - they're often the most logical structural form.

Wilkinson Eyre have also put together a pleasing arrangement for the footways, and overall, it's a fine bridge which will undoubtedly be cheaper to maintain in the long-term than the existing structure, with its intricate steelwork and multiple moving parts.

The decision to replace the bridge and adopt a modern design has not been without its opponents. Indeed, there seems to be plenty of dissent, mainly from locals upset at losing a heritage structure and questioning why refurbishment was not a fourth option. They even have their own dedicated website to rival the official project website. It's a great campaigning site - lots of information on the bridge, a regular blog, and opinion which is surprisingly well-informed (as it is elsewhere online, including a helpful Q&A with the lead bridge architect at the Unknown Victoria blog).

Having skimmed the excellent condition report [PDF] on the bridge, I can see both points of view - it's a real shame to lose such an unusual structure, but refurbishment will be very difficult, and will be a cyclic process - the intricate nature of the bridge's truss steelwork is such that it will remain as difficult to maintain in 40 years time as it is now.

It's been alleged by opponents that the decision to replace the bridge and to rush through the design process was largely driven by the opportunity to obtain CAN$42m of federal funding. In an ironic twist, the same day that the council decided on its preferred bridge design, it was announced that the funding would not be forthcoming.

As it stands, Victoria reportedly intends to press ahead, but is taking time out to re-evaluate how best to proceed. This may mean seeking alternative funding, scaling down the replacement bridge plans, or revisiting the refurbishment option. A council meeting tomorrow, 8th October, is scheduled to review the situation and work out where to go next.

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