29 May 2009

Tradeston Bridge opens

Earlier this month, on the 14th May, Glasgow's £7m Tradeston Bridge opened to the public. Nicknamed the "Squiggly Bridge" (in sympathy to the nearby "Squinty Bridge"), it was designed by Halcrow with Dissing + Weitling, and built by BAM Nuttall. Following a short preview back in March, the Happy Pontist has sent a special reporter all the way to Glasgow to have a proper look at the finished bridge.

The opening was as much as a year behind schedule, following difficulties with the riverside walls, disputes with the fabricator, and arguments over DDA compliance. The budget has been stated at £7m (roughly £14,000 per square metre, expensive for a landmark footbridge, albeit still only half the budget for the previous Glasgowbridge proposal on the same site).

Previously, I noted that the bridge was somewhat sober and colourless. Its stark, bloodless quality is certainly very apparent in reality, although in spite of its stiff angularity, it remains surprisingly tactile. The bridge was criticised as shortening the pedestrian route across the nearby George the Fifth Bridge by only a little, but was relatively well used while I was there, particularly given the overcast day and lack of riverside attractions. Several people were stopping not just to look at the bridge, but to touch it.

Visually, it is sharp and striking, instantly memorable and in this respect best seen from a distance. Although there’s little of note to be said about its more intimate details (the deck surfacing and parapets), closer up its visual drama and interest is enhanced. In elevation, the arrow-head supports are visually straightforward, but from almost any other angle their curves and tapering faces change constantly as you walk around and across (see picture, left). The scale of the pylons is well-balanced between the human and the inhuman.

Only the ultra-slender deck disappoints in this regard – from certain views, perspective foreshortening makes it appear as if there’s a sharp kink in the edge of what is in reality a sinuously curved deck (see picture, right).

Why is it S-curved at all? Judging from the Glasgow council’s press statements, this is to provide adequate air-draft to river vessels. If that’s right, then curving the bridge in plan gives the bridge enough length to rise up and over the navigation channel without offending disability access gradient limits. The bridge deck’s exceptional slenderness may also have been driven by this demand.

I admire its minimalism, even if it’s in danger of seemingly brutalist: my travelling companion on this trip, not a bridge designer so perhaps more representative of the fabled Joe or Jane Public, thought the bridge ugly, industrial and “a monster”.

I’m less sure how well the uniform pale grey colour will work at different times. Most surfaces offer an attractive flatness (although not all the welds have been ground flush, so there’s still some sign of its making), but only the surfaces underneath had much in the way of reflected light and shadow. Imagine the same bridge in polished stainless steel!

The curved recesses at the foot of each arrow-head are clearly designed to discourage people, particularly on wheels, from mounting the top surface of the inclined “stay” members (for this is in some ways an unusual variant on a cable-stayed bridge). The bridge hasn’t been open long, but there is already considerable damage to the paintwork and several tyre-tread marks, suggesting that Glaswegians are more intrepid than the designers allowed for (as I predicted previously). I was certainly able to walk up the surface, and it wouldn’t be especially hard to get to the top for someone less nervous about the possible appearance of the police.

Despite its slenderness, the bridge deck was generally stable and free from wobble – but not entirely so. Even under relatively light loading, I could feel a very definite vertical vibration at midspan. Whether it’s enough for people other than nosy engineers to notice, I couldn’t tell.

It’s also too early to tell how well the bridge will fulfil its stated aspiration, which is to act as a focus for regeneration investment on the south bank of the Clyde, and establish a high-quality riverside appropriate to the high-value businesses sought on the north bank. It’s not the best stage in the economic cycle to judge how well it will succeed in this aim.

Overall? I do like its simplicity, its flat yet sculpted quality, and the way its curvature generates visual variety out of such limited means. I’m nervous about its susceptibility to trespass and graffiti, and hope it doesn’t have to be disfigured to discourage climbing (a risk that could have been minimised by adopting diamond-shaped rather than rectangular section “stays”). It’s good to see that there are still new tricks to be played with the established repertoire of bridge forms. And after the fiasco of the previous bridge design competition on this site, I’m just glad a landmark bridge did finally get built here. It may lack the showmanship of the original competition designs, but as is always said about minimalism, sometimes less can be more.

London Bridge 800: Design an Inhabited Bridge

Marking the 800th anniversary of the opening of Old London Bridge in 1209, RIBA has announced an "ideas" competition to design a new inhabited bridge on the site. The original bridge (long since replaced with a new concrete box structure) was of course one of the most famous "living bridges" of all time. Entries should be made by 25th June, and the winner will be announced in conjunction with the bridge's Anniversary Fair on 11th July.

RIBA asks designers to assume that the existing structure could carry a further six storeys of building, and largely to ignore the impractical absurdity of the entire venture in favour of creativity and inspiration (see briefing paper [PDF]).

I've discussed the Peabody Trust and Royal Academy bridge competitons here previously, both of which sought essentially the same goal, an inhabited bridge over the Thames. The concept has been most recently revived by London mayor Boris Johnson. There's nothing about the present competition to change the view that an inhabited bridge like this is both impractical and visually undesirable, a wall across the river.

Nonetheless, I'd be having a go at the contest myself, were it not restricted only to architects. RIBA, as ever, protects its own, even at the expense to the competition of reducing the chances for entries which are not only creative, but also potentially more practical.

26 May 2009

Peace Bridge in Derry

In April, the winning design was made public for a new footbridge across the River Foyle in Derry, Northern Ireland. I didn't cover it here at the time but I've been looking at some of the design images, and it would be a shame not to feature it.

The bridge (which will cost £8.7m as part of a wider £13.3m project) is part of a regeneration scheme masterminded by ILEX Urban Regeneration Company.

The winning scheme is a design-and-build tender from contractor Graham, with design input from Faber Maunsell and Wilkinson Eyre. Their bridge is a 315m long, 4m wide self-anchored suspension bridge, curved in plan to follow an S-shape. The main span is 101m.

The construction cost works out at about £7,000 per square metre, which is pretty normal for a landmark footbridge and compares favourably with Wilkinson Eyre's new footbridge in Stirling (which costs twice as much). While you can't evaluate a bridge's merits solely on cost, value for money does matter, particularly for a regeneration project (the reality, however, is that most such schemes are not funded by the end-client, and scrutiny of value-for-money is generally absent).

The bridge has obvious echoes of one of Wilkinson Eyre's earliest bridge designs, the South Quay Footbridge in London, which shares the S-shaped plan and inclined masts, but was a cable-stayed rather than suspension bridge.

ILEX state that bridge construction will commence in September 2009, to be completed in October 2010. That's an optimistic programme for a bridge which will be complex to design and difficult to build. Self-anchored suspension bridges are a relatively rare choice, as unlike the more popular conventional suspension bridge (or the cable-stayed option), they are not structurally self-supporting until the deck and all cables are complete. This is because the main cables rely on the deck to anchor them. In contrast, a cable-stayed bridge can be built in segments, and a normal suspension bridge can have the cable completed in advance of the deck.

The consequence is normally a need for substantial temporary support within the river, and for a bridge with unusual geometry such as this one, more careful consideration of the cable connection and stressing requirements, and associated deck precamber.

The bridge form is envisaged as a "handshake", whereby two separate self-anchored structures are overlapped at midspan, and I think it's a very attractive design. The curves of the suspension form are softer and lighter than a cable-stayed solution would have been, and it also allows the overall height of the bridges to be lower and less overbearing.

There were four other tenderers, but I haven't seen details of any of the losing designs. If any become available (or are sent to me at happypontist at googlemail dot com), I'll post them here.

More images

25 May 2009

Bridges news roundup

World's most interesting bridges part 3
Gosh, gee, whizz, sez Dark Roasted Blend blog

Constructing Excellence North East project of the year
Infinity Bridge begins to gather awards

Mersey Gateway Bridge public inquiry opens
Toll protestors out in force. Official documents are also online including bridge drawings, visualisations and design statement

Brouhaha over closure of Sheffield Station footbridge
Rail company closes bridge to non-passengers - just google for dozens more stories on this one!

22 May 2009

Uni. of Bath Bridge Conference 2009

As previously discussed here, every year the University of Bath holds a student conference, where their bridge engineering students are encouraged to each critique an existing structure and present a technical paper. It seems to be a near-unique exercise in the UK, and something I think deserves considerable praise.

Their 2009 event has now been held, and the papers are online. 39 different structures are featured in total, and the papers generally make for an interesting read, especially as in some cases they have had access to information from the designers not otherwise in the public domain

Is your bridge there? What did the students think?

20 May 2009

Bridge competition debris part 14: Thames Water Habitable Bridge

Back in February I featured the various entries to the Peabody Trust Habitable Bridge competition, held in 1995 and involving design of a crossing of the Thames between St Pauls Cathedral and Bankside in London. The idea of "Living Bridges" was very much in the air at the time, with an exhibition at the Centre Pompidou.

Indeed, it's an idea that (despite its many problems) still hasn't gone away, with schemes in Hamburg (pictured, right) and Duisburg, for example, not to mention Zaha Hadid's bridge pavilion in Zaragoza.

The problems of "living bridges" are well illustrated in Amsterdam's Entrepotbrug, built in 1993, a monolithic blot on the landscape that would be far better hidden amongst other buildings than allowed to dominate river views.

In 1996, the Royal Academy held its own exhibition on the subject, documented in the excellent book Living Bridges. In conjunction with the exhibition, they held an invited competition to design a habitable bridge at a different point on the Thames, running between Temple Gardens on the north bank and the LWT building on the south bank.

This competition was essentially speculative: there was no funding to actually build such a bridge. It was an attempt to promote the concept itself, in the hope that publicity might get developers to take forward a scheme. The competition was sponsored by Thames Water. Seven designers were invited to submit ideas.

People attending the exhibition were asked to vote for their favourite designs: I recall voting for the Daniel Libeskind proposal, with its interlinking network of walkways likely to make for a more interesting experience than some of the others. The Zaha Hadid design was somewhat unfathomable in its perspex blockiness, but seemed to offer a compromise between keeping the habitation on the banks (to preserve views) and siting it on the bridge (to provide the over-water experience).

Antoine Grumbach's design sought to preserve views by keeping most of the building off the bridge entirely, essentially just riverside apartments joined to a nice pier. Like Ian Ritchie's minimalist slab, it was a sober effort. By contrast, Branson Coates' phallic alien mothership was amusing but impossible to imagine being built at this site.

Much the same was true of Future Systems' dayglo pod, or Krier Kohl's dark tribute to Crystal-Palace era London. What were they thinking?

Now in 2009, London Mayor Boris Johnson has revived the Grumbach proposal, with an estimated cost of £80m attached. This isn't the first time it has been resurrected - it had the dust briefly shaken off in 2000 as well. It remains extremely unlikely ever to be built.

Essentially, the problem of the habitable bridge is still the impossibility of resolving the visual impact on the riverscape against the need for sufficient accommodation to achieve economic viability. In 2000, English Heritage were scathing about the Grumbach design's impact on river views, describing it as "walling off" the river.

With that in mind, the feasible options are reduced. One is to minimise the accommodation and its height, which limits viability to narrow rivers or locations with cheap land (in which case, why build on the river at all?) The other is to find locations where the views simply don't matter, which means leaving the river behind and building above streets and railways - and who would want to live there?

As a concept, the habitable bridge isn't quite dead, but it's difficult to see many being built in future.

(For those interested, there's more on the "living bridge" subject at A Daily Dose of Architecture, covering designs by Steven Holl and Dan Hisel).

The entries

Ove Arup / Zaha Hadid

Marc Mimram / Antoine Grumbach

Buro Happold / Branson Coates

Techniker / Future Systems

Arup / Krier Kohl

Daniel Libeskind

Ove Arup / Ian Ritchie Architects

19 May 2009

Request for bridge competition information

I'm preparing a number of future posts in my ongoing "bridge competition debris" series, but having difficulty finding details of some entrants and their designs.

I'm particularly looking for information on the unsuccessful entries for:

- Gateshead Millennium Bridge
- Stockton-on-Tees North Shore footbridge
- Basildon bridge competition
- St Helier footbridge competition
- London Millennium Bridge

If you can assist, please get in touch via happypontist at googlemail dot com.

14 May 2009

Infinity Bridge opens

Today marks the start of a three day "Infinity Festival" in Stockton-on-Tees, with its centrepiece being the official opening of the Infinity Bridge, a spectacular £15m double arch footbridge (pictured right, image copyright Expedition Engineering). Expect fireworks.

Infinity Bridge is 237m long in total, with the two arches being 120m and 60m. It was designed by Expedition Engineering and Spence Associates (not without controversy), and has been built by Balfour Beatty. It was the winning design in a RIBA competition held in 2003, and more recently it won the ICE Robert Stephenson Award. Quite right too - although I've got a few quibbles with it below, it's a distinctive and impressive structure.

It's not difficult to see why this design was a winner. Working on the assumption that what regeneration bodies are really after is a gigantic logo, not a bridge as such, the twin arches lend themselves to this purpose (see also Spence's River Wear design for the same principle). If you go to the Infinity Festival website, you can see the logo that has resulted.

What makes it work as a logo is the sag-curve connecting the two arches, giving the bridge an attractive sinuous quality, looking as if it would rather dance across the river than plunge into it. I've had a problem with that sag ever since I first saw it, as structurally it appears to be doing very little, and actively misleads the viewer as to how the bridge really works. However, in some of the more recent construction photos, the legs of the arch are more prominent visually, and I think you can see it either way, it's a more pleasing detail than I had thought. It also connects the arches such that they stiffen each other, reducing the bending moments in both arches.

I'm less convinced by the river pier, which looks like a set of four paper fans folded and propped against each other. The legs are angled to contain the worst-case arch thrusts, and are also designed to resist ship collision forces. The stepped edges in the pier legs (pictured right, courtesy John Yeadon on Wikipedia) seem fussy and inconsistent with the main arches, although the same detail has been used on the bottom section of the arch to visually separate it from the sag element.

The only other criticism I would offer is the tripodal nature of the split arches, whereby their extreme ends sit on a single point of support, with the river-pier legs offering two supports. From most viewpoints I think it's very elegant (see image on the left, copyright Expedition Engineering), which may owe something to the way the arches have been shaped by form-finding procedures to carry little or no bending under dead loads.

However, it leads to an awkward structural detail, as the arch ties and arch support don't coincide in plan, resulting in a hammerhead detail hidden below the deck to carry the thrust forces. I don't imagine anyone other than an engineer would ever notice, let alone find it unsatisfying. It's not clear what the alternative would have been, as bringing the arch tie directly to the arch springing would be geometrically very difficult.

Noting that the bridge deck is 4m wide between handrails, it has cost about £15,000 per square metre, which is very definitely at the upper end for landmark footbridges (compare Forthside Footbridge, which was similar). This figure seems particularly high when noting that the approach spans are relatively straightforward in structural form i.e. if you looked at the river spans alone, their cost per square metre is actually higher.

In this context, it's perhaps worth noting that back in 2003, Tees Valley Regeneration were seeking a bridge costing a mere £4m to £5m. By any yardstick, a 200% cost over-run is a shocking performance, but there's little doubt that Stockton-on-Tees would have got a considerably less special bridge for the original budget.

Why did the Infinity Bridge cost so much more than anticipated? For one thing, the original budget was clearly out-of-step with the client's aspirations for a major river crossing such as this. Nonetheless, I wonder whether the form of bridge chosen plays a big part.

The bridge is designed as a twin tied-arch, where the arch thrusts are resisted not by the foundations, but by tie cables running at deck level. These are visible in the photo on the right (courtesy John Yeadon), just outside the deck edges (there are "bars" running below the deck, but I assume these are service ducts e.g. for lighting cables). As with any tied arch, the structure as a whole is not stable until both the arch and tie member are complete, and any structure which is not stable until complete normally requires extensive temporary support and/or a more complex construction sequence.

From the construction photo on the left, it's apparent that the ties were installed only after the arches were lifted in, implying that the foundations must be designed to resist thrust from the arches' self-weight in the temporary state. This should not be a huge force, and the erection sequence has at least minimised the need for temporary propping. I am also told that it doesn't govern the river pier design (as this has to cope with ship impact, which is larger).

The tie cables are independent of the deck itself (this is shown in the construction photo on the right, again courtesy John Yeadon, where the cables are temporarily lifted out of the way while deck segments are installed). This will have complicated the erection sequence, but substantially reduces temporary propping requirements, as the foundations don't have to resist temporary arch thrusts relating to the weight of the concrete deck.

Essentially, the tied arch is not the most economic option at this site. A cable-stayed design (which can be built segmentally and is normally stable throughout construction without significant temporary works) would have been cheaper. Buildability should come first in any bridge engineer's thought process, but on the Infinity Bridge it has been more of a case of choosing the bridge's structural and sculptural form, and then working through the challenge of construction in an intelligent manner. That's not necessarily the wrong decision, but it certainly reduces economy, and many of the cleverest bridge designs are those that arise naturally from the imperatives of construction, rather than hinder it.

Having said all that, one thing that most impresses me about the design is the extent to which the designer's original vision has been translated into the final construction. Having seen the tender drawings back in 2006, it's clear that the contractor has made very few changes to Expedition's design. That must be a tribute to the thought given to the buildability of the bridge, some of which is illustrated in the assembly images Expedition produced (see example on the left).

Overall, it's easy to nit-pick, but I think this is a very successful design, with an original aesthetic vision which has been followed through with minimal compromise. Unlike several recent bridge competition winners, the design is led by a clear structural concept, rather than distorted for exaggerated effect.

Photo credits: construction photos from North Shore Footbridge website; visualisations copyright Expedition Engineering; all other photos are credited as appropriate.