04 May 2015

Bath Quays Competition Shortlist Announced

The shortlist for the Bath Quays footbridge design competition has been announced. Forty-nine entrants have been whittled down to six, and it's a strong field:
  • AL_A / AFA Consult
  • Flint & Neill / Moxon Architects
  • Grimshaw / Buro Happold
  • Heneghan Peng / Ove Arup
  • Marc Mimram / Webb Yates
  • Price & Myers / McDowell & Benedetti
The teams will develop their designs for submission in June, with the winner expected to be announced in August.

02 May 2015

London Bridges: 41. St James's Park Footbridge

I've not visited many new bridges recently, but I did find a moment for a quick walk in St James's Park, London, to see the "Blue Bridge" which spans St James's Park Lake.

London is a city well-blessed with parkland space, and although St James's Park is the smallest of the central London parks, it is still rather lovely. Much of its current layout, including the lake, dates from John Nash's remodelling of the park in the late 1820s. Prior to Nash's work, the park was bisected by a canal, which was bridged by a timber bridge supporting a Chinese-style pagoda from 1814 until 1825.

A suspension footbridge was built across the lake in 1857, designed by James Meadows Rendel. A visitor's guide of 1865 charmingly commented that "a suspension bridge for foot-passengers was thrown across the lake, to the infinite comfort and delectation of the frequenters of the Park, who had been previously compelled to effect a wearisome circumambulation." The suspension bridge was replaced by the present-day bridge in 1957, designed by the Structural Engineering Branch of the Ministry of Works, and built by Higgs and Hill Ltd. The architect was the Ministry's Eric Bedford, better known as one of the designers of the Post Office Tower.

The new bridge was required as a result of Rendel's span being in need of £10,000 of repairs, and additionally it required temporary support whenever crowds were expected (highly visible in a 1937 newsreel). An anonymous donor offered £23,000 to cover the cost of a replacement bridge. Little thought initially seemed to be given to the historic value of the old bridge, and it's interesting to wonder how the discussion on replacing such a bridge would proceed today. By the time the House of Lords debated the bridge replacement plans, the project was already a fait accompli.

The bridge is a three span structure comprising lightly arched and haunched concrete beams. There are two concrete beams side-by-side, with a central span of 21.34m, and side spans of 10.67m. The bridge is 4.1m wide. At the centre, the total concrete depth is 14" (356mm), giving a span-to-depth ratio of 60, impressively slender.

I'm not entirely sure what I think about the resulting bridge elevation, as there seems to me to be too great a disparity between the slender midspan and the heavy piers, and the very elongated elliptical curve to the soffit is a little awkward.

The slenderness relies on three features of the engineering design: first, that the concrete is post-tensioned, which maximises its bending capacity. Second, that it is a haunched continuous beam, which means it is essentially a cantilever structure, with large hogging moments at the piers, and a very small sagging moment at midspan (compare the Forth Railway Bridge). Third, the midspan sagging was further reduced during construction by setting the two end supports higher than their final position, and then dropping them, inducing a hogging moment throughout the centre span.

The designers recognised that the bridge's slenderness might give rise to issues with pedestrian-induced vibration, which was a well-known but poorly understood phenomenon at the time. What little advice was available suggested that the bridge would have no problems if its natural frequency was above 2.7Hz. However, calculations indicated that the bridge would have a frequency below this threshold.

The problem was addressed by making the bridge's supports non-symmetrical. Originally, it was proposed to have bearings below both the main supports. Fixing both bearings, by casting the pier legs in concrete, would solve the vibration problem, but led to problems under thermal and creep effects. The final solution was to fix one pier, while leaving the other partially free as a rocker bearing. This raised the natural frequency to 3.6 Hz, a figure which was validated by testing the as-built bridge with three hundred marching guardsmen.

The bridge's concrete used white aggregate and white cement to achieve a sufficiently pale colour, while the blue painted railings have given rise to the bridge's present name, the Blue Bridge.

As can be seen from the photographs, this is a very popular bridge, although not because of its architectural elegance or structural engineering achievement. This bridge is much more than just a shortcut across the lake, it is primarily a viewing platform.

The axis of the lake opens up perspectives across the park, towards Whitehall and Horse Guards Parade on one side, and Buckingham Palace on the other. Although I've not seen anything to indicate that a bridge was part of John Nash's original plan, the bridge allows the lake to be more than just a picturesque part of the landscape and source of drinking water for plentiful wildlife. It establishes some of the key views in the Park, and ensures that they are uninterrupted by crowds.

Further information:

31 March 2015

Bridge competition debris part 27: Nine Elms / Pimlico runners up

Ah, what a wonderful thing is the internet. The organisers of the Nine Elms / Pimlico bridge design competition helpfully made all 74 entries available to the public online and via public exhibitions. Initially, they made one image of each design available, although this represented only 30% of what each competing team was actually judged on. Initially, all the entries were displayed anonymously in the online gallery, so that the public would view each design in a fair manner, however, the organisers have recently revealed precisely who did what, and for most of the entries have now made available the second design board, allowing us to see 60% of what they were marked upon.

It's an impressive field of entrants. I think all the British specialist bridge architects are represented (although none was chosen as a finalist), and there are also big names (in different ways) such as Zaha Hadid, Ney and Partners, Marc Mimram, Dietmar Feichtinger, RFR, Expedition, HOK, Rafael Vinoly, SOM, Snøhetta, Foster and Partners and more.

Ove Arup and Partners participated in a staggering 17 entries, and Buro Happold in 9. No other entrant even comes close. This is not a new strategy in bridge design competitions, but it paid off, with Arup and Happold together providing three out of the four finalists.

Here are the six "runners-up", entries which the jury felt deserved some kind of recognition. Links take you to their detailed design boards, at least for as long as they remain online. I'm not going to cover any of the other entries here, there are just too many, but I will return to this contest one more time to discuss it further.

Wilkinson Eyre Architects Ltd with Aecom, Atelier One and Schlaich Bergermann
For me, this is an extremely disappointing design, particularly given the firms involved. It's hard to see what the merits are of the structural solution, which is essentially a suspension bridge system turned sideways. The shallow profile for the cable hangers means that they are not efficient at suspending the deck, especially given their connection to a main cable rather than directly to a rigid mast.

What's especially surprising about this design is that there are no cycle ramps, only cycle channels within staircases - these are okay for keen adult cyclists going upwards, but useless in the downwards direction and for younger or less fit cyclists. As this is a key challenge for the contest and the site, how on earth did this get judged as a runner-up?

Farshid Moussavi Architecture with Bollinger and Grohmann Ingenieure
I find it hard to express quite how much I dislike this design. It makes no structural sense whatsoever. It's like someone has admired Anish Kapoor's 110m long outdoor sculpture, Temenos, and mistaken it as an idea for a bridge. It's not. The design boards talk about the arches being inclined backwards to counter-balance the cable forces, but they're not shown with sufficient weight, stiffness or inclination for this to be at all feasible.

The ramps also lack credibility, especially at the south end of the bridge where there simply isn't space in reality for the length of ramp illustrated.

Eric Parry Architects Ltd with Richard Deacon AKTII
I think this is a design where the image doesn't do it justice. Any arch solution would be expensive to build and difficult to do so without disruption to river traffic. In the design's favour, pedestrian and cycle traffic are properly segregated. It's one of very few entries which didn't follow the obvious route across the river, instead disgorging its users half-way along a busy footway at the south bank. I think it's pretty mediocre, certainly compared to some of the rejected entries.

Atkins Ltd with Grimshaw Architects
This twin-mast cable-stayed design is mildly reminiscent of the South Quay Footbridge in London's Docklands, at least in its original configuration, with two inclined masts supporting an S-shaped deck. It's a rational solution, with ramp arrangements and engineering which makes sense, although I think the giant glowsticks have a garishness more appropriate to a children's Halloween party than blown up at this scale. But it's not a big sin.

The deck is very wide, 10m, and this requires cable stays on both sides of the deck, which is a less elegant solution than at South Quays.

Coffey Architects with Buro Happold
It looks like a plank, or perhaps a ruler placed temporarily on a walnut-wood architect's model as a placeholder while someone else in the team worked on the actual bridge design. I like the bold concision of this proposal, which would be London's first stress ribbon bridge, and a remarkable feat if ever built. However ... it is supposed to be a bridge which happily accommodates cyclists, and it's hard to see how forcing cyclists up in the air in giant lift towers satisfies that aspect of the project brief. Because a stress ribbon inevitably sags between its supports, the support towers have to be quite tall, and because it requires a very high tension force in its supporting cables, those towers have to be large and heavy, with correspondingly heavy foundations. It's bold, but the jury would have been very brave if they'd made it a finalist.

Ove Arup and Partners Ltd with Studio Egret West
At first glance, I thought this striking design might involve plastic composites, but it turns out this is a proposal for an all stainless steel bridge. Since the jury generally appeared to be looking for sensible, affordable solutions, it's hard not to wonder quite how such an expensive idea made it to the shortlist.

There are some nice features, like a cafe below its southern ramp, and a proposal to build the two halves parallel to the river and then rotate them into their final position. But these have to be set against a bizarre intention to cross-over the cycle and pedestrian routes at midspan, introducing an entirely pointless mid-river pedestrian crossing.

The contest organisers have now released the jury report to competitors, and once I've had time to digest it, I'll return with some final thoughts on this competition.

17 March 2015

Nine Elms / Pimlico Bridge finalists announced

Four finalists have been announced in London's Nine Elms / Pimlico pedestrian and cycle bridge design competition. They were chosen from a field of 74 entries.

The finalists are:

Buro Happold Limited with Marks Barfield Architects, J and L Gibbons Landscape Architects, Gardiner and Theobald

Bystrup Architecture Design and Engineering with Robin Snell and Partners, Sven Ole Hansen ApS, Aarsleff and ÅF Lighting

Ove Arup and Partners Ltd with AL_A, Gross Max, Equals Consulting and Movement Strategies

Ove Arup and Partners Ltd with Hopkins Architects and Grant Associates

It is, of course, grossly unfair to judge the outcome of the contest based solely on these four rendered images. The jury allocated only 30% of their marks to these images, with the remaining 70% being for images as yet unreleased and the accompanying written submissions. However, a few comments may be in order.

What unites the designs is that they are in general structurally feasible (a characteristic not shared by all the entries), relatively modest and undemonstrative, and yet also at the same time tall. Indeed, some of the finalists are far taller than seems reasonable. The Arup / Hopkins design appears to have masts roughly twice the normal height for a suspension bridge, the Bystrup / Snell masts are also a little on the pointy side, while the Happold / Marks Barfield bridge is really striving to dominate its surroundings.

The Happold design is a development of the work they did for the client at pre-contest stage, although with a few unusual features. The image appears to show only single cables emanating from the tower top, then splitting to pick up the deck via a cable net arrangement. This is a pretty diabolical structural arrangement, which would be a nightmare both to build and maintain. The entire load from the bridge's main span must pass through two cable attachments, creating single points of failure which cannot easily be replaced if damaged. The mast is also split into two stems, although whether this two-fingered salute is aimed at the Nimbies on the Pimlico bank, or at the nearby US Embassy, it is difficult to tell.

The Arup / Hopkins bridge minimises physical impact on the riverbanks by locating the necessary serpentine cycle ramps above the river, and hence vulnerable to impact from stray boats in time of flood. Pedestrians appear to share the main span with cyclists, but to pass down via staircases which, like the masts, penetrate vulval openings in the ramps in what seems to me a very awkward arrangement.

Bystrup's entry is shrouded in mist but clearly features a quite improbable cable-stayed arrangement, where the back-stay cable are so steep as to be almost ineffective at restraining the tower. It's possible the weight of the supported ramps provides sufficient balance, but it looks horribly wrong in the image provided.

The Arup / AL_A design is my favourite of the four finalists. The skewed arch arrangement echoes several other similar bridges (the Clyde Arc, Hulme Arch, Gogarburn Bridge, Newport Street Bridge, and River Taff Central Link Bridge) but would become easily the largest of this type in the UK. Given how massive the Clyde Arch is (with a 96m span), this gives some feel of quite how enormous the Nine Elms Pimlico structure would be. This is not a straightforward bridge to build over a busy river, so it will be interesting to see how the designers addressed the "constructability" challenge set as part of the competition's first stage.

Overall, it is pleasing to see that the jury has not been swayed by either celebrity or by irrelevant flash, but I am far from convinced that these four were the best designs available, they instead have a strong scent of mediocrity. However, the promoter was clear that they were not choosing a design at this stage (indeed, the press release very pointedly labels each image as merely an "initial design idea"), but choosing a team. As a result, it will be very interesting to see how the designs evolve through the remainder of the competition and beyond.

The finalists go through to Stage 2 of the competition, described as a "competitive dialogue". Each team will be paid £12,000 to develop and further detail their design proposals, and to participate in a series of workshops and review meetings with both the Technical Panel and Jury Panel. I find this separation into two panels to be quite bizarre: it implies that the jury has insufficient technical expertise to properly evaluate the proposals, while relegating that evaluation to subordinate status. Why not just ensure the jury includes sufficient technical expertise within its ranks?

As well as being evaluated on their designs, the four finalists are also required to submit formal financial tenders for the design commission. As in the recently announced Bath Quays footbridge contest, the final evaluation of the competition will blend both the quality and commercial aspects, although the way in which this will be done remains opaque at present.

Right now, it's difficult to judge whether this project will ever actually make it to site, with Westminster Council, the authority for the north bank of the River Thames, actively opposing it. The project promoter, Wandsworth Council does at least now have four designs all of which should in principle fit within their £40m budget, so one of the normal issues with architectural bridge competitions has been successfully negotiated.

06 March 2015

More on Nine Elms / Pimlico Bridge

The finalists for the Nine Elms to Pimlico Bridge Design Competition are due to be declared on Friday 13th March (an auspicious date!), but while the jury panel and organisers work feverishly behind the scenes, the project is provoking some interesting commentary.

Writing for The Guardian newspaper, critic Oliver Wainwright contrasts the Nine Elms scheme with the more controversial Garden Bridge proposed for central London. Both bridges span a similar distance across the same river, but have very little else in common.

The Garden Bridge is to cost £175m, and has seen a main designer appointed with large sums of money with little or no genuine competition. At Nine Elms, the budget is a much more credible £40m, and the design and designer are being chosen in an open competition which allows different concepts to be compared for feasibility and desirability.

The Nine Elms bridge makes a feature of encouraging cycle access, and is to be a public structure open at all times. In contrast, cyclists are to be prohibited from the Garden Bridge, which as a semi-private space will be shut at night, and also regularly closed to the public to allow private parties to take place.

Troublingly for the competitors, Wainwright also draws attention to growing opposition to the Nine Elms bridge proposal on the north side of the Thames, with cross-party politicians objecting on the grounds of visual and environmental impact at the most obvious landing site in Pimlico. This is nothing new: Pimlico residents have been nervous about the plan since it was first mooted, horrified at the thoughts that hordes of lycra-clad cyclists from South of the Thames might invade their genteel residential neighbourhood.

Wainwright also offers a well-deserved skewering to some of the contest's most absurd designs, dubbing them "The gushing mandolin", "The spaffy tangle" and most memorably "The flaming mouth of Hades". These are the sorts of offerings that make you genuinely wonder for the sanity of those responsible: why would the architects concerned (and clearly, they are architects) spend more than half-a-minute creating images for something that, surely, has absolutely no prospect of being shortlisted?

Further comment on the contest comes from Rory Stott at ArchDaily, not normally a place I look to find coherent criticism of modern bridge design. Stott considers the budget of £40m, which is largely to be funded by a Community Infrastructure Levy paid for by the developers of adjacent sites, and suggests this is a little pricey. In comparison, London's Millennium Bridge, which was definitely no bargain, would cost about £35m at today's prices.

There's little doubt you could build a lovely bridge over the Thames for a lot less than that. Stott suggests the promoter, Wandsworth Council, are setting their sights high, and further suggests that this is responsible for the lavish over-exuberance of many of the design competition entries. I doubt that, and I think the absurdity of many entries is simply down to the involvement of designers who have no real common sense, understanding of contruction, or interest in the practicalities of a scheme.

The main target of Stott's ire is that this is a bridge for the super-wealthy, who are investing heavily in developments in the Nine Elms area. Some apartments in the area are already changing hands for well above the original sales price, well before they have even been built! It's an investment opportunity for the non-domiciled rich, with the potential to create a highly-ornamented ghost town. With this in mind, it's easy to see why the good burghers across the river in Pimlico can see little value in the bridge proposal.

Personally, I don't think the bridge should be judged so hastily. It will not only serve those who live closest to it, but has the potential to be a useful walking and cycling route for a far wider area, if the right connections can be made. Whether it is a project that Wandsworth can be proud of will depend upon what emerges from the competition jury's deliberations next week, whether they have been blinded by bling or whether their chosen finalists exhibit both sense and sensibility.

01 March 2015

Tower Bridge ... some musings

I recently posted about Tower Bridge in London, and observed that it is one of the world’s most unusual suspension bridges.

Most suspension bridges have a suspension cable or chain from which the deck is hung. Tower Bridge, unusually, is suspended from a suspension member made up of riveted steel plates (pictured).

Additionally, most suspension bridges have the deck stiffened with trusses or girders to prevent excessive deflections. On Tower Bridge, the suspension system is itself stiffened, comprising trussed elements in the form of an inverted three-hinged arch.

Both these features are unusual, and I wondered whether the latter one is in fact unique.

Since my post, suspension bridge expert Bridgemeister has introduced me to a number of other bridges which share either or both of these features, and I thought I'd cover some of them here.

I already knew of one other suspension bridge which has riveted steel plates as its suspension "cable", the Grunwaldzki bridge in Wrocław, which I visited in 2011. This bridge was built in 1947 and it's really difficult to understand why plates (pictured, left) were used instead of cables.

Bridgemeister also highlights the Trukhanov Island footbridge in Ukraine; the Lahn River Bridge at Nassau in Germany (pictured right); and a hybrid bridge over the Salzach between Laufen in Germany and Oberndorf in Austria.

Of those the 75m span Lahn River Bridge can be counted twice, both for its 2005 incarnation and its 1926 predecessor (this is the one in the photo). It is a thoroughly delightful structure, especially the detailing of its towers. It deserves to be much better known.

The plated suspension elements on these bridges are in some cases very different from the riveted plates of the Tower Bridge. The welded steel flats on the Lahn bridge are simple and elegant by comparison, yet the bridge still retains a great deal of character, mainly thanks to the marvellous towers.

Bridges with a trussed suspension "cable" are also hard to find, and often not well known. The 1895 Mill Creek Park, pictured left is a good example, but others are more questionable.

An 1889 suspension bridge in Rome has only minimal stiffening, while several examples were either never built (Gustav Lindenthal's spectacular Hudson River design in New York, pictured right) or have been dismantled (St Louis's Grand Avenue bridge, and Lindenthal's Seventh Street Bridge and Point Bridge in Pittsburgh).

The Kindee bridge at Ellenborough in Australia adopts broadly the same form as Tower Bridge, a three-hinged arch formed from two crescent trusses. It's a glorious monument to engineering idiosyncrasy.

The 1947 Lumberville-Raven Rock bridge (pictured left) has two-hinged stiffening trusses, and is another triumph of engineering. In this arrangement, there are actually two suspension cables one each side of the bridge, separated by an arrangement of lightweight trussed steel ties and struts. These are held together with a fascinating assemblage of custom-made clamps and connecting nodes. It's like a modern-day Schlaich Bergermann design transported back in time.

Another example of the genre can be found at the former John Roebling offices in Trenton, New Jersey, providing a high-level walkway connecting two buildings (pictured right). This shares the Lumberville bridge's two-hinged truss arrangement, although it has had to be further stiffened with tie-down stays below the deck.

It's not hard to see why both variants on the normal suspension bridge have found little favour. The plated "cable" solution is a step up from historic chain bridges, being less vulnerable to single-point failure than chains. However, it simply can't compete with any kind of wire cable, as wires are inherently stronger than plain structural steel, and don't need to be peppered with rivet or bolt holes, which further reduce the capacity. Wires are lighter and much easier to install. However, I do like the Lahn River Bridge and wonder whether there are not some nice bridges still to be designed along similar lines.

As for trussing, again, this is not an efficient means of construction – the trussed "cable" requires expensive temporary support while it is assembled. It's much easier to install a relatively lightweight cable first and use that to provide all the necessary support while the stiff bridge deck is assembled.

Picture sources: Bridgemeister (Lumberville-Raven Rock, Trenton); out of copyright (Hudson River, Lahn River); rg998 at Wikipedia (Mill Creek Park); the author (Grunwaldzki, Tower Bridge).

23 February 2015

Nine Elms Pimlico Bridge Design Competition entries published

O frabjous day! Callooh! Callay! A gallery of the entries for the Nine Elms Pimlico bridge design competition has been published today, and my, what an entertaining box of delights there is to behold.

As with so many bridge design competitions (see past blog posts ad nauseam), I'm left wondering exactly what motivated some of the entrants to this one. This is a serious competition for a genuinely useful river crossing which appears already to have much of its funding in place. However, quite a number of the entrants seem to have put forward entries which nobody could even contemplate building, gaudy scribbles which could be held up only by a wing, a prayer and the magic of Photoshop. Why would anyone put so much effort into something which clearly will go straight into the discard pile?

Of the more sensible entries, it's interesting to consider the many different ways in which the bridge designs have been depicted, from razor-sharp photomontage through to whimsical artists impressions, even one that depicts a 21st century bridge being used by 19th century Londoners.

With 74 entries on display, it seems foolhardy to attempt any proper consideration of their merits. I reckon there are at most a couple of dozen which have a serious prospect of further consideration by the competition jury panel. And how could anyone top this ultra-quick skewering?

So, with the aim of avoiding all that hard work on my part, what's the reaction on Twitter?