28 September 2009

St Patrick's Island Bridge Competition entries: Part 3

Onwards, onwards, before I either wear out my typing fingers or, more likely, just get plain bored.

This is the third of four posts reviewing the entries to Calgary's contest for a new CAN$25m footbridge. Go to their website for a proper set of images and text describing each design (or just to gawp at the public comments). The first of the previous two posts in this set has the proper introduction, so without more delay ...

Delcan / du Toit Allsopp Hillier

This is the only one so far I've included three images for, and that's just because the Christmas card view is so lovely.

At first glance, this looks like a hundred other footbridges with curved decks slung from inclined arches, but on a closer look it is clearly different, and indeed puzzling. The plan layout is simple and very attractive, with plenty of scope for landscaping on the island, but the first puzzle is the presence of lattice cables tying together arches which clearly have no structural need for them. If you're going to create network arches, why bother with the "vertical" hangers, and vice versa? It's really the cables that are odd, because the stiff "hangers" may be required to stablise the deck.

And it's that thought of deck stability that really confused me when I gave it another look. On the majority of this type of bridge, the arch inclines in the opposite direction from the deck curve, so that the two both counterbalance and brace each other (see for example the Sturgess design below). Even then, the deck requires substantial torsional stiffness to work. But here, both spans have the arch on the outside edge of the curve, and inclining still further out. That puts a far, far greater torsion on the deck, which would have to work exceptionally hard not to have vibrational problems, and the arch becomes far less efficient because both arch and deck tend to buckle together. I'm sure you could just throw money at it until it stands up, but it's a fundamentally flawed concept.

IBI Group / ipv Delft/ Williams Engineering Canada Inc. / et al

Without a doubt this would be the biggest "landmark" of any proposal, a 180m span arch that doesn't touch the island at all, except for a little walkway. We're not told if it's a bowstring or not, but it's rendered as a thrust arch, which would require some quite spectacular foundations, even though the depth to bedrock is not great. The deck is offset from the arch, so there's a reasonable lateral bending load on the arch to transmit to ground.

While they make a virtue of the lack of impact on the island, they don't really address the huge impact of construction, all the temporary propping or tie-backs that would be required. And the bridge's biggest problem is simply that not enough attention has been given to the details of the design, particularly the shaping of the arch in cross-section.

Sturgess Architecture / Halcrow Yolles / IBI Group / et al

Compare this one to the Delcan design above. The designers for this one say: "A unique feature of the bridge is that its structural efficiency and delicate ribbon arches are achieved by counterbalancing the forces of the arch and pathways". Of course, it's nothing like unique, designs which balance like this are ten-a-penny, but what makes the Sturgess bridge different is simply the way the pedestrian and cycle routes are structurally separated.

While I'm in favour in principle of separating the two, I think in this case the structure just gets over-complicated and over-heavy, with the raised struts between the two somewhat unsatisfactory. The Gateshead Millennium Bridge did it better.

CH2M Hill / LeBlond Partnership Option #2

Ah, it's a relief to see something that's understated, elegant, and consciously trying to avoid undue flamboyance. Two very shallow arches are proposed here, with the aim of putting the emphasis on the Island and not on the structures. It's a thoroughly admirable sentiment, and it's a shame the single image doesn't really do it justice. But it won't win.

CH2M Hill / LeBlond Partnership Option #1

Meh. It's not bad, it's just dull. There's just so much more that can be done even with such a default cable-stay iption. Sure, it ticks the boxes - reasonably straightforward to design and maintain, and it keeps out of the river freeboard. It's the pylon I particularly dislike. The designers call it the "needle". I call it a grey carrot.

Endres Ware / Ammann & Whitney

While it's nice to see several entries which have a clear structural rationale, it's disappointing that few of them go the extra few yards that would really make them special. Where a bridge design is engineer-led, with the architect subsidiary, it's vital that the architect's attention to detail, to the bridge's intimate experience, is used to really step up the quality, and there's little sign of that here. Sure, the cables are where they should be, and the pylons are nicely arched, but overall it's just far too busy, and the island pathway is poorly layed out. It seems out of place and out of scale to the island.

Arup / Kasian

Ordinarily, I'm a great rollercoaster enthusiast, but I somehow don't think there will be a runaway mine train rattling over the crests of this bridge. As with another design, this one is partly inspired by fly-fishing - could someone tell me what precisely is wrong with a bridge design inspired by "getting across the river"?

If I understand it correctly, it's a relatively conventional bowstring arch bridge, with two arches on the long span and one on the short. Plus a gigantic squiggle of metal pipe that's mainly there to confuse the issue and hence make it "hip" (does anyone still say that?). It's irrational, effervescent, and sadly just a little too confusing to have any logo appeal. I think I'm mostly neutral on it - structurally, it's completely daft, but some city somewhere should have the right to wave a big flag about how postmodern and ironic they are. I'm not sure Calgary is the right place, nor that it shows any consideration for the river-and-island context.


I have to admit I did think for a second about just photocopying Calatrava's Peace Bridge design and submitting that as an entry, just for a laugh.

I do hope this fashion for helical tube bridges doesn't last long, it's a deeply inefficient concept and very limited in how it can be applied well visually (as witnessed on the very poor end treament for this one).

In their favour, they've pushed the helical members closer together than the Calatrava bridge, making it a bit more affordable, and made great play of how it can be built with locally available steel (Calatrava's bridge is likely to be fabricated by Cimolai in Italy). It is at least curved in plan, which makes the helical truss choice slightly more pardonable than Calatrava's. Against it, setting aside the inevitable accusation of plagiarism, there's an entirely superfluous concrete box inside it, and too little thought given to the island landing. Like the Peace Bridge, it will be a bugger to maintain as well, with all that intricate steelwork, some of it in too-close proximity to the concrete box.

Read Jones Christoffersen / Riddell Kurczaba / Simpson Roberts / et al

I thought for a moment I'd get to the end of today's batch without liking a single design, so I'm glad to finish on this one. It may not be entirely clear from the images I've picked, but this bridge has two opposing suspension spans, presumably not self-anchored due to the high deck curvature. There are no back-stays on the masts, which therefore carry the main cable loads in pure bending, assisted at least a little by being inclined backwards (in contrast to at least one of the other submissions). The foundations will therefore be on the large side, but the two masts will to some extent balance each other out under normal load conditions, so it's not quite as bad as it may seem.

It respects the freeboard limitations, the path to the island is complemented with a nicely cantilevered viewing platform, and it's relatively straightforward to maintain, notwithstanding the many corrosion problems for which suspension bridges are notorious (likely to be easier to solve with the cable sizes required here). It will be a real challenge to build, mainly because of the very shallow trajectory for the main cable. At that angle, the support to the deck is very limited, and there will be significant changes in cable shape as the structure is built piece-by-piece. Lots of careful adjustments in cable tension throughout construction may be required.

It's also nice to see a design that in some way echoes the suspension bridge which is already there, which achieves landmark status without getting too ridiculously out of scale, and which is driven primarily by the inter-relationship of its geometrical and structural form. Even better, the initial public comments seem to agree, so it would be no surprise to see this one on the final shortlist.

Ok, that's it again, for now, I'll return with the final set of designs when I get a moment.

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