St Patrick's Island Bridge Competition entries: Part 4

Ok, here is the final batch of my mini-reviews of bridges entered into the Calgary bridge design competition. Parts 1, 2 and 3 are also available, and part 1 has the proper introduction. Looking for more opinion? Accelerated Bridge Construction is posting a series of video reviews on Youtube, which I think are particularly clear and helpful for the general public without specific bridge engineering knowledge. And the public comments on the designs can be found online at the competition website, along with more images and details of each submission.

Infinity Engineering

This is without doubt one of the most ambitious, striking and unusual designs in the competition (or indeed in any of several recent competitions). Three curved bridges are supported via cable nets from a central mast, giving options to access the island or bypass it entirely. As rendered, it looks like the two island bridges would fall foul of the freeboard constraint and be vulnerable to ice damage, but that's something that could be changed easily enough. It's hard to tell quite what the pedestrian experience would be, as the emphasis is very much on icon rather than intimacy.

Is it buildable? Well, it would be unique for a bridge, but certainly within current technological capability. Cable nets are difficult to design, to assemble, and to stress properly, something that would be made more difficult on this design because the lower edge is fixed to the bridge deck, which would be a more rigid element. Again, that could be changed by introducing an intermediate edge cable along the bottom. The only existing cable net bridge which comes to mind is Schlaich Bergermann's Löwentor footbridge in Stuttgart, although that's a very different design. For the Infinity Bridge concept, I think they're hoping that the cable mesh can stiffen the bridge deck in a way that conventional suspension bridge hangers can't (for this is basically just three very fancy suspension bridges strung together). I'm unconvinced about that, but I'm no cable net expert.

The two shorter spans will be inherently stable, but the main span looks like a huge challenge to design - at this length, torsion in the deck will be very difficult to carry, without the advantage of a counterbalancing inclined element (although again, there are ways of addressing that). It's also far from clear that the cable-net on that span is in the right form, looking convex rather than concave in some of the images. A few days with some form-finding software would sort that out.

My main dislike of the design comes from considering it in operation. It will be a very difficult structure to maintain - replacement of a suspension hanger is relatively straightforward in a conventional suspension bridge design, but replacing cable net elements is far harder. There's also the entirely predictable likelihood of unwanted climbers making it their playground.

I'm also less than keen on the gargantuan scale, which seems completely out of place given the rest of the surrounding landscape.

RFR / Halsall

RFR's design is one of the simpler, more elegant solutions, with a series of slender arches seeking to minimise impact on views on or off the bridge. While it looks far too skinny to survive the crushing impact of an iced-up river, it's straightforward to build and maintain, and should leave plenty of spare change in the CAN$25m budget. That will come in handy for the actual walkway connection down on to St Patrick's Island, which seems to have been largely ignored in the competition images.

It reminds me somewhat of the Pont des Arts in Paris (where RFR are based) in its delicate elegance.

Saucier Perrotte

Three glazed tubes enclosing a steel space truss. All the emphasis here is on what can be done with the glass - fritting to provide a varied visual texture (darker at the ends, lighter in the middle), and lighting inside, with the aim being to provide something that is continually varying.

What else can be done with glass? Scratching it, painting it, breaking it all come to mind. I'm not sure suitably laminated and toughened glass can be formed to these tight curves, although again that's not my area of expertise.

I'm not a fan of this one. The structure will need to be more obtrusive than is suggested, it's too much like a tunnel, and the tubular form is just too straight, particularly viewed in elevation - there's nothing to express the leap that a bridge makes from ground to ground, this would work better as an elevated walkway in an airport, a more highly policed environment.

SPF:a / Arup (O-Bar)

The first of two entries by this team. It's easy to see how this is a love-it-or-hate-it-design, with its monolithic, rust-covered monumentality. I actually quite like it, although I can't think of many settings I'd like to see it in, including this one.

The bridge basically consists of two oversized box girders made of weathering steel, making it one of the very few steel designs to have considered the need to minimise maintenance costs (although in this close proximity to pedestrians thought needs to be given to its vulnerability to graffiti, and the need to ensure hands and clothing can't come into contact with all that rust). The girders are far larger than they need to be, even though they have an undulating shape which reflects the intensity of bending moment at any point (very nicely widening out where the girder is in compression and hence needs to be wider to give stability against lateral buckling). I like that.

Its nicely rendered, although some of the images omit the tie bars from top to bottom girder which would surely be needed across the window slot. There's also an entirely fanciful image of how it could be built out of interlocking modules, which would be a horrendous way to put it together, with the huge number of full-strength butt welds required.

Against it? Well, the way it blots out most views up and down river; the somewhat claustrophobic feel; and the imposing mass amidst the tree-lined Island (although the contrast is interesting). Contrariness is always welcome, but it will never win this competition.

Arup / Frederic Schwartz

Not a bad design, a single masted cable-stay bridge which ticks all the key boxes for buildability, maintainability and minimal ecological impact, but which is generally a bit lacking in inspiration. Bending moments on the mast due to asymmetric loading will be significant, but not unconquerable. The mast could do more to accentuate a needle-like shape, I think, and I think the curved path down to the island is much less attractive than several other entries.

Bosen Lu

I've stared at this design for a long time and still don't really understand how it's intended to work structurally. The plan and some of the other views imply that the cable plane is central to the deck, but some views imply the cables are connected to herring-bone struts protruding laterally from the main mast. It's far from clear whether it's properly back-stayed, with some views suggesting that it's stayed onto the back-span, but not to ground, which would be an unusual choice for an asymmetric cable-stay bridge. I don't feel able to judge it in engineering terms, so can only comment that to me, visually it is over-fussy and out of scale with the context.

Erhard Kargel / ABES Wagner

This is an interesting design, but certainly not a great one. The basic structure is a nice idea - a single masted suspension bridge, with the two spans at an angle from each other so that the mast needs further cables onto the island to restrain it. These cables are then used to support the island walkway. The mast-head cable connection detail will be a challenge, but it's otherwise an elegant, pleasant bridge design.

What lets it down is the lack of attention to detail, particularly at pedestrian level. It's a sketch rather than a finished design (not that this is necessarily a bad thing given the tendency of open competitions to attract ever more preposterously rendered imaginings). It's almost impossible to figure out how well the bridge will relate to its arboreal context, but to me the mast looks far too tall.

SPF:a / Arup (Glide)

That's how many entries from Arup? I make it five, which must be some kind of a record for any bridge design contest, surely. Well done!

As with the other SPF:a / Arup entry, this is easily one of the more radical concepts, visually if not structurally. A precast concrete box-girder deck is supported on steel struts (the edge members are described as tension members although presumably they'd be in compression next to the island - unless it spans the island too, which seems unlikely). The soffit is hidden by zinc cladding. The balustrades are of structural glass, which looks nice when rendered but will be terrible to maintain in this environment.

Picking the flaws first, it interferes enormously with the stated freeboard constraints, and is therefore likely to be a no-go because of its massive impact on river hydraulics during flood conditions. And while the stepped ramps look nice on paper, there would be no usable access to the island when floods occur. The structural form is somewhat dubious in its behaviour - if the outrigger chords are intended to act as the tension member of a deep beam, then how is longitudinal shear transferred? And almost no thought seems to have been given to what the space would be like beneath the structure.

In its favour, it sees the bridge as a public gathering space rather than a point-to-point walkway, and it easily achieves landmark status without the paraphernalia of towers and cables. But I can't see how these can overcome its many problems.

In the next post I'll put together some predictions for the shortlist. I'll also see if there's anything to be said about how the competition has compared both with other design contests, and with Calgary's recent appointment of Calatrava for his Peace Bridge design.