25 February 2009

Metro West Bridge Competition Winner

In April last year, the Irish Rail Procurement Agency (RPA) launched a competition with the Royal Institute of Architects of Ireland (RIAI) to find a design for a new bridge to carry the proposed Metro West light rail line over the Liffey Valley. Like other bridge contests run by an architects' trade body, it was not without its flaws (engineers couldn't enter without an architect in tow, for example, however experienced in bridge design), but generally it was one of the better planned contests of recent years, with half-decent rewards for the entrants and a prequalification hurdle to keep out the students and chancers.

The location was sensitive environmentally, with a strong desire to avoid major construction impact on the river valley floor. Certain parts of the floor were specifically marked "out of bounds" on the project plan. The client also aspired to a signature bridge which would give pedestrians new views of the valley, while making as minimal visual impact as possible.

Around twenty-five firms made entries to the competition's first stage (if I find any online, I'll put a post together, but haven't spotted any yet). Five were shortlisted to the second stage, with their identities kept secret until a winner was declared.

The shortlisted entrants were:

  • Alan Baxter / Aedas
  • Arup / Heneghan Peng Architects
  • Buro Happold / Explorations Architecture
  • Flint and Neill / Foster and Partners
  • Scott Wilson Scotland Ltd. / B+M Architecture
The winner has just been announced as Happold / Explorations, with the semi-underslung suspension bridge design pictured below:

It's easy to see why this entry won, especially when you read the jury report [PDF].

None of the other shortlisted entries have yet been made public (if any of my readers were involved, feel free to email details to happypontist at gmail dot com!) However, the jury report implies that the other four entries all had supports within the Liffey Valley itself, whereas the Happold design spans right across (I seem to recall it's about 300m or so). You might think the other four took a reasonable approach, given that the competition brief explicitly permitted supports in the valley, and given that this was an obvious way to keep the spans within sensible economic boundaries and hence within the stated project budget.

However, it's no surprise that a design which avoids piers entirely would be looked on favourably.

One problem is that a suspension bridge does not necessarily keep construction activity out of the valley floor - on the contrary, in the way one is normally built, sections of deck are lifted vertically to be supported on the main cables i.e. in this case, from throughout the valley.

In contrast, bridges with valley supports can be built by the balanced cantilever method, limiting impact to only the base of the piers themselves. So as the winning bridge design progresses it will be interesting to see exactly how it gets built.

One entry appears to have been disadvantaged by being too tall and dominant (Flint and Neill). Without seeing the design it's hard to comment but the entry I was involved with in this competition ruled out any structure that projected significantly above the valley crest, so I sympathise with the jurors here.

Another was seen as too complex geometrically (Arup) which might lead to cost concerns, or aesthetic problems if it had to be simplified later.

But a suspension bridge, particularly one which is part underslung and hence requires substantial additional stiffening to prevent instability, is hardly the least cost solution - for this span it may in fact be the exact opposite.

Suspension bridges require substantial ground anchorages to restrain the main cables - the cost of which can be enormous if the ground is unsuitable. In its infancy, this type of bridge was shunned for use on railways following notorious failures and problems making them sufficiently stiff under high locomotive loads. This is not a problem that has gone away today, particularly for light rail bridges where the design is often governed by stiffness rather than strength.

So while it's a very nice design visually, if I were a juror I'd have been looking for the designer to convince me it had a real chance of being built within budget, without having to alter it visually (e.g. having to make the deck stiffer and hence thicker).

With that in mind, it may be worth noting that nobody on the jury (largely made up of RPA staff) appears to have been a specialist landmark bridge engineer, let alone a specialist in cable-supported bridges. The only outside engineering advisor was John Powers of Engineers Ireland, an electrical engineer. A failure to draw on that kind of specialist advice has been a glaring error in several previous bridge design competitions.

I would certainly hope that the design proves successful - it's striking and highly ambitious, and would become one of Ireland's greatest bridges if built. I'd also be the first to admit that it's somewhat ridiculous to try and dissect the design on the basis of a few website photos without access to the full design submission. But either way, it will definitely be interesting to see how it progresses in future.

Update 26 Feb: I gather the RIAI intends to publish all the shortlisted entries, and possibly some of the stage one entries as well, on their website. I'll post again when they become available.

Further update 26 Feb: I've replaced the images with higher-resolution versions and added one or two more. All images provided by the RIAI, with credit to Buro Happold / Explorations Architecture.


Anonymous said...

The winning design is an elegant and attractive structure - the three quarters view is particularly seductive - although I share your misgivings about the suitability of the structural form for the rail loading (running at up to 100kph, I recall) and the buildability of the design. I am also curious about the pedestrian deck which appears to follow the catenary cable as it dips below the deck, and would be much steeper than normally permitted?

The Happy Pontist said...

Pretty much any footway supported by a catenary cable is likely to exceed the 1:20 gradient which is normally preferred - it's one of the classic difficulties of the otherwise elegant stressed-ribbon design. But for this unusual suspension bridge, it may be in fact be okay: the gradient of the catenary over its central section (below the bridge deck) may well be low enough to be acceptable, and the 1:20 figure isn't something everyone agrees on anyway (I don't). I think it's a neat solution to one of the competition's design challenges, actually.