30 June 2010

Scottish Bridges: 9. Glasgow Bridge

Moving eastwards from the South Portland Street Bridge, the next crossing of the River Clyde in Glasgow is Glasgow Bridge.

The present structure was built in 1899 by Blyth and Westland engineers, as a wider replacement of Thomas Telford's 1836 bridge (itself a replacement for a bridge of 1772).

As masonry arch bridges go, it's not a brilliant design. The segmental arch ring seems to me to come too close to the parapet, touching the stringcourse and breaking up the spandrel wall. The result is that the arch looks flattened at its crown. It doesn't look quite right, but historic photos do seem to make clear that it's identical to Telford's original design.

The bridge piers each incorporate three secondary transverse arches, presumably to reduce loads on the foundations.

The night-time lighting is much better than at the South Portland Street Bridge. The blue intrados to the arches contrasts well with the white bridge piers and balustrades. It's a shame that one arch is unlit along with several of the piers, but that's just a lack of maintenance.

Behind this bridge, you can see the 2nd Caledonian Railway Bridge, which wasn't illuminated, so I didn't take any photos of it, nor the George the Fifth Bridge, another arched highway bridge immediately to the east (this time in concrete with faux masonry facades).

Further information:

29 June 2010

Scottish Bridges: 8. South Portland Street Suspension Bridge

It's always interesting to contrast bridges by day and by night, so I was pleased recently to make a return trip in the hours of darkness to the Tradeston Bridge, which I'd only previously seen by day. I also took the chance to have a quick look at a few other bridges along Glasgow's River Clyde.

I'll cover the bridges I looked at in order from east to west, starting with the South Portland Street Suspension Bridge.

This footbridge incorporates the oldest surviving elements of any of Glasgow's bridges across the river Clyde, with its stone towers dating from 1853. It replaced a timber bridge on the same site, which had lasted from 1833-1846. Designed by engineer George Martin with architect Alexander Kirkland, the new bridge required substantial reconstruction in 1871, leaving it essentially in its present form, although the hangers have been replaced twice more since then.

The bridge has been illuminated at night since 2005, with well over 2,000 LED lights involved. I first saw the bridge at night, and was surprised at quite how malevolently crimson it is lit, as if the bridge had not long emerged from Vulcan's furnace. You can see the twin sets of bridge chains, one above the other, and the hangers and parapets are reasonably well delineated, but the towers just look somewhat morose.

In the day time, it became clear how awkward the lighting is, totally obliterating the contrast between the stonework, the red steelwork, and the white steelwork lattice panels. It's odd, because the press release issued when the lighting was installed said that the towers would be lit in white, and more use of white light would definitely have looked much better.

Further information:

28 June 2010

Boy Scout Bridge competition winner

Another bridge design contest, another winner, and this time it's the turn of Schlaich Bergermann und Partner, and Hatch Mott MacDonald.

The contest was to design a pedestrian bridge at the Bechtel Family National Scouting Reserve in West Virginia. The two firms' winning design is a 250m suspension bridge, with "walkable" cables running over fanned-out columns, some 60m above the ground.

The bridge is to be built in time for the National Scout Jamboree Event in 2014. Consol Energy are donating US$15m towards its construction.

23 June 2010

"Failed Bridges: Case Studies, Causes and Consequences"

If the news is any guide, bridges around the world seem to be falling down at an alarmingly frequency. In early June alone, there were collapses reported in Idaho, USA, Ohio, USA, Connecticut, USA (1 injury), India and Indonesia (12 dead). The casual onlooker might conclude that the bridge engineering industry was rife with incompetence.

That impression wouldn't be dispelled by reading "Failed Bridges: Case Studies, Causes and Consequences" by Joachim Scheer (Ernst and Sohn / Wiley, 2010, 307pp) [Amazon.co.uk]. This features an impressively long (but still far from comprehensive e.g. no Ynys-y-Gwas) list of bridge collapses guaranteed to strike alarm into the hearts of wary gephyrophobes. Of course, as the author emphasises, these statistically remain a tiny minority of structures. Nonetheless, it's hard to avoid the impression that there is a widespread problem.

"Failed Bridges" is the second edition, in English, of a book which first saw print in German. The decision to publish this expanded version in English recognised that engineers around the world often seem to fail to learn from the mistakes in their predecessors, and it was desirable to expand the book's audience much more widely. The literature of bridge failure is lengthy, but not always accessible to practicing engineers. The aim of this book is to bring as much data into one volume as possible and thus provide a single point of reference documenting the reasons for past failures, and the lessons which might be learned.

It contrasts with Björn Åkesson's book "Understanding Bridge Collapses", which I reviewed previously. Åkesson offered 20 case studies, with detailed technical explanations including calculations where appropriate. As such, it's of interest both to design engineers and to students. Scheer widens the net to capture at least 440 failures, with the inevitable consequence that much less detail is provided for each one. Indeed, for the better known failures, little if any detail on the cause of collapse is given, with the author assuming readers can access the information elsewhere.

Much of "Failed Bridges" comprises data on individual bridge collapses presented in the form of tables, indeed 115 pages are taken up with these tables, which are for reference rather than for reading. Some similar information is available online (e.g. at Wikipedia or BridgeForum), and neither the book nor the online efforts are comprehensive, with numerous bridge failures absent in each case. "Failed Bridges" seems particularly short on failures due to floods (probably the single biggest cause of collapse throughout history) and seismic activity, with the latter category being a new inclusion in this second edition.

Failures are categorised by physical situation or cause: during construction; during service under "normal" load including wind; ship collision; vehicular collision; flooding and ice floes; fire or explosion; seismic activity; and falsework failures.

The broader view of a system like SCOSS's "3 Ps" (people, process, product) only comes out in the book's final chapter, which summarises the lessons learned. Nor is the explanation of failure as thorough as the approach used by the RAIB, which identifies the immediate cause (often physical), causal factors, contributory factors and underlying factors, in an attempt to lay bare the complex web of human error which underlies most bridge failures. None of this is to criticise the author, as for most historic bridge failures, detailed information of this sort is simply not available.

Scheer makes a number of interesting points in his introduction. Recounting his own experiences of bridge failures (he has acted on several occasions as investigator and expert witness), he notes the high proportion which occur during construction: "My own experience reflects what everyone 'on site' knows: building work is often linked with failure; this has always been the case and always will be". However, I suspect very few of the people with the greatest responsibilities for construction of bridges (i.e. contractors) will ever read this book. The challenge for its main audience (academics and designers) is therefore how to bring its lessons to the attention of contractors, and clients (the other group who could benefit but won't read it), without appearing alarmist.

I'd suggest that the risk management processes now common throughout engineering design and construction offer one way forward: do designers completing their obligations under the CDM regulations always make clear the more extreme risks of failure? "Failed Bridges" offers countless examples of what can go wrong, and plenty of advice on mitigating the risks. Perhaps if more risk assessments clearly identified key mitigation actions such as the involvement of the designer in construction supervision, the situation would be improved.

Scheer also notes that: "when analyzing the causes of structural failures today, I find that there is hardly any case which could have been prevented by more detailed calculation". Instead, failures occur because certain possibilities were never even considered in calculation, or they are the result of what with hindsight can be seen as gross rashness. A similar point was made by D.W. Smith in a major survey of bridge failures (ICE Proceedings, 1976), who also warned of the dangers of reliance on complicated and sometimes ambiguous design standards. Reading this, it is tempting to wonder whether the introduction of Eurocodes offers any improvement at all on structural safety, especially where their implicit reliance on probability theory can hide the very real uncertainties that go beyond the boundaries assumed.

There are a number of minor issues I noted while reading the book. Chief amongst these is that while its translation is generally excellent, the opportunity has been missed to introduce references to material available in English. So for example, the failure of the Tay, Dee and Cleddau bridges is discussed with reference to papers in German, rather than the widely available and often informative material in English. For example, for the Tay Bridge (pictured), Peter Lewis's very thorough book "Beautiful Railway Bridge of the Silvery Tay" isn't mentioned, nor the essential technical papers by Lewis, Martin and MacLeod. Given that the author assumes readers will seek more details elsewhere, it's unfortunate that the bibliography remains largely aimed at German-speakers.

The level of detail given for individual failures is often uneven, with some, such as the 1940 collapse of the Frankenthal Rhine Bridge accompanied by levels of detail which seem excessive in comparison to others. However, this is as much a strength as a failing, as often the bridges covered in detail are German and little known to the wider world. Scheer also offers more detail where he sees it as necessary to provide a view other than that which has been widely reported. For example, the 1907 collapse of the Lawrence River bridge in Quebec (pictured) was widely stated to have been due to the underestimation of dead loads in calculation. Scheer draws attention instead to the non-compactness of the critical steel section, and hence its inability to redistribute internal constraint stresses in a plastic manner.

For some of the causes of collapse listed in the book, the technical lessons seem to be straightforward, and are largely now a routine feature of design standards. This is the case for failure due to ship and vehicular collision, where events like the Eschede railway disaster (pictured) have led to considerable conservatism in the design codes. The hazards from flooding and earthquakes are also to a certain extent predictable. The chapters devoted to these effects are correspondingly shorter. However, one lesson from history is that new hazards continue to become apparent as technology advances. One example not covered in this book is the phenomenon of ballast instability due to bridge resonance, observed when high-speed trains were introduced to the Paris/Lyon rail line, causing damage to bridges and increased risk of derailment.

Amongst the myriad of other case studies, one that stood out for me was the 1990 collapse of arch falsework for the Lake Street - Marshall Avenue Bridge at St Paul, Minnesota. This was a case where different elements of the falsework were designed by different firms, although the overall strength of the system was highly dependent on the stiffness relationships of each element, including the stiffness of parts of the bridge arch already cast. The result was that scaffolding props carried greater loads than assumed by their designer, and their support beams lacked sufficiently strong stiffeners. When the scaffold collapsed, one person was killed.

This case stood out because it's an example of something I've seen several times in my own work, especially beyond the field of bridge engineering. In building structures, it's entirely normal for responsibility for "details" to be divorced from the main designer (e.g. pile design, or bolted steelwork connections), and there's an accident waiting to happen wherever those details prove more significant to the global design than is commonly assumed (e.g. where the stiffness of a bolted connection is important, or a pile's stiffness against lateral loading).

The final chapter of the bridge seeks to summarise advice on how to avoid bridge failures, both from the author's own perspective and by surveying others who have attempted the same. These are mostly aimed at engineers, although they do touch on lessons for those involved in procurement, such as the need to incentivise the appointment of professionals who are competent (while seemingly obvious, this also seems to be frequently ignored in the delusion that "low cost" is the same thing as "high value").

Scheer also touches on difficulties with regulators, who by promoting particular procurement arrangements often hinder best practice (e.g. the lack of involvement of experienced designers in site supervision which has become increasingly widespread in recent years, resulting from a preference for design-build procurement). He notes that pressure to drive down costs and meet deadlines must have an adverse effect on time available to optimise design and coordinate work correctly. I particularly liked his statement that "designers are forced to commit themselves to a single concept at a much too early stage and to stick with it, at times, against their better judgment", a conclusion which should be read by anyone who is overly enamoured of the bridge design competition as a procurement route.

Scheer's advice extends to every phase of bridge design and construction. On conceptual design, the need for robustness and simplicity of structural form are discussed. In calculation, the key concern relates to unsafe extrapolation - the failure to realise when rules well-understood at one scale can become unsafe at larger scales, principally because effects which were once negligible become dominant. This includes buckling issues as well as the aerodynamic issues that were encountered on the Tacoma Narrows bridge. Scheer notes, quite correctly, that engineers in the modern age are seldom afforded the time, funding, or control required to undertake experiments which might render extrapolation safe, with the example of Stephenson and Fairbairn's experimental work on the Britannia Bridge box girders (pictured) being offered as an example of how extrapolation can be carried out sensibly.

Advice covers how the design process should be coordinated, as well as risks related to modelling and the misuse of computer analysis. Many of these issues have been well rehearsed elsewhere. Scheer records recommendations made by others: Sir Alfred Pugsley, W. Plagemann and D. Kaminetsky. The last of these includes one guideline which might act as a motto for this entire book: "The best way to generate a failure on your job is to disregard the lessons to be learned from someone else's failures".

Amongst Scheer's own advice I particularly like: "Always bear in mind that your model of a load-bearing structure is defective".

He concludes the book with suggestions for how the history of failure might hold lessons for the teaching of structural engineering, concluding that students learn too much about analysis, and too little about design, a view I very much agree with. As Scheer notes, too much maths and science can hinder a strong intuitive feeling for structural behaviour, rather than assist it.

Overall, "Failed Bridges" is an excellent contribution to the bridge engineering literature. It's singlemindedness doesn't make it an easy book to read right through, but the information it contains should be thought-provoking for younger engineers, and likely to cause many grimaces of recognition for their older colleagues.

I do wonder how the material can be more widely disseminated - the reality is that a very small minority of design engineers read books like this, and an even smaller proportion of the clients and contractors who can play an even larger role in preventing failure. So, if you're in any of these groups, please consider what you're missing!

It's a book that's certainly worth reading for those concerned with the education of engineers, and I'd particularly commend it to those involved in the development of standards and codes, procurement strategies, or with overseeing roles in the process of design management.

17 June 2010

Bridges news roundup

River closure planned when new footbridge is hoisted into position
Hull's "iconic" £7.5m swing footbridge to be installed in August.

Iconic Wear bridge plans on ice
Government spending review may kill ambitious bridge design.

Disappointment over Mersey Gateway bridge scheme delay
Government spending review may kill ambitious bridge design.

Is this the craziest bridge ever designed?
More twisty and turny than a twisty-turny thing.

Finalists announced for Prime Minister's Better Public Buildings Awards
Three bridge schemes amongst the finalists (A40 Western Avenue, Forth Bridge cables, Silver Jubilee Bridge) and it's great to see technical achievement recognised alongside architectural schemes.

16 June 2010

"Thomas Bouch: the Builder of the Tay Bridge"

I picked up a copy of John Rapley's biography "Thomas Bouch: the Builder of the Tay Bridge" (ISBN 978-0-7524-3695-1, Tempus Publishing, 2007, 192pp) [Amazon.co.uk] cheaply in a bargain book store earlier this year.

I had previously read Rapley's fascinating "The Britannia & Other Tubular Bridges" [Amazon.co.uk], which is a detailed and evenhanded account of the great joint achievement of Robert Stephenson and William Fairbairn (and some other, lesser-known structures). So his treatment of the life of the Victoria railway engineer Sir Thomas Bouch was bound to be of interest.

Bouch's name is known to posterity pretty much for one thing, and one thing only: the collapse of his Tay railway bridge on 28 December 1879, approximately 18 months after it had been officially opened to traffic. Bouch's design was discredited, as were the construction and maintenance, and he died ten months later, his reputation ruined.

Before that, he had built up a considerable reputation as a railway engineer who could build new lines for far less money than his competitors. Bouch believed that his contemporaries were often far too conservative in their designs, and his quest for economy frequently led him to build railways using secondhand rails, timber bridges which never lasted long, and single-track rather than double-track solutions. While these allowed new railway lines to be built quickly for low capital outlay, they almost invariably led to higher upgrade costs later.

His antipathy to over-design can be seen in one of his most successful bridges, the Hownes Gill Viaduct, built in 1858 (pictured left in a 1906 postcard). In his book "British Railway Bridges", David Walters suggests that "its slender grace recalls Bouch's life-long contention that contemporary engineering work was hopelessly over-designed and uneconomical, through general conservatism and a chronic underestimation of the ultimate strength of materials". Bouch's design was reviewed by Robert Stephenson, who required both the addition of invert arches at foundation level to better spread the loads, and also that the tallest piers be widened to provide greater stability in high winds.

Bouch was unafraid to innovate when required, developing roll-on-roll-off ferries for railway wagons at the Firth of Forth, and his 1871 cable-stayed Redheugh Bridge at Newcastle foreshadowed modern designs such as those of Riccardo Morandi.

Most of the book focusses on Bouch's lengthy career as a railway engineer, working generally on minor regional lines. This offers a good understanding of Bouch's finely matched strengths and weaknesses. His ability to build for a penny what others could only build for a pound seems to have been unmatched. However, as well as frequently requiring expensive rebuilding, his schemes were often blighted by initial cost over-runs, the result of inadequate advance surveys.

For my taste, there's too little offered in the book to shed light on Bouch as a person (perhaps the source material simply isn't there), and I soon tired of the endless episodes of railway woe.

The Tay Bridge, understandably, is covered in greatest detail. The difficulties of construction included the collapse of two girders during erection, blown over by the wind, but in line with Bouch's general parsinomy, one was recovered from the estuary and re-used in the finished bridge. Once open, there were problems with scour and with slackening tie bars, with inadequate repairs made on site without reference back to Bouch himself. While the civil engineer was receiving a knighthood for his achievements, the bridge was beginning to wobble alarmingly, and (with hindsight) the collapse of the bridge became inevitable.

The Court of Inquiry which investigated the failure of the bridge led to Bouch being left in disgrace. This was despite the two engineers on the Court refusing to apportion blame, and indeed disassociating themselves from the far more critical conclusions of their colleague Henry Rothery (notably, a lawyer rather than an engineer).

While Bouch was certainly responsible for many of the defects in the bridge's design and construction, his responsibility for the ultimate cause of failure, the bridge's inadequate strength against wind load, is less clear. He had sought advice from the railway inspectorate (who noted that wind load need not normally be included in design for spans only a little shorter than those adopted). He was told by the Astronomer Royal (in connection with his aborted design for the Forth Rail Bridge) that a pressure of 10 pounds per square foot was reasonable. Bouch's assistant used a pressure of 20 psf for the Tay Bridge design, despite there being in general little understanding of wind load amongst civil engineers of the period. Bouch became the fall guy, but it seems many of his peers might have made similar decisions.

Overall, I was a little disappointed by this book, although I suspect much of that is simply because Bouch was more of a designer of railways than of bridges, and hence there were large parts of the tale which were of limited interest to me. It was certainly less immediately appealing than Rapley's book on the Britannia Bridge, but that offered a more straightforward story where extensive source material is available, and with historically significant disagreements between the main protagonists to recount. To its credit, "Thomas Bouch" is well illustrated with archive photographs and diagrams, and I'd think it's likely to remain the definitive work on its unfortunate subject for a long time to come.

15 June 2010

International Wildlife Crossing Infrastructure Design Competition

I blogged about this contest back in December, so won't repeat all the information I gave then. However, it has now been launched in earnest. Expressions of interest are requested by 30th July, from teams of engineers, landscape architects and ecologists able to innovate in reducing wildlife-vehicle collisions in Colorado. Teams who successfully prequalify get a US$15,000 honorarium, and there's a US$40,000 prize for the eventual winner. The competition rules look fairly sensible with an multi-disciplinary jury, the winner retaining copyright on their design, but no commitment to award an actual design contract.

Past examples illustrated by the competition organisers imply a bridge as a solution, but most wildlife crossings I know of are tunnels or culverts, and I suspect they're looking for a more imaginative solution if one exists.

14 June 2010

London Bridges: 4. Sackler Crossing

The last of four bridges I visited in London recently was also at Kew Gardens, the Sackler Crossing.

Designed by John Pawson, engineered by Buro Happold, and built by Balfour Beatty, this 70m long S-curved monument to minimalism connects footpaths across a small lake, encouraging visitors into areas of the Gardens not always well-trodden.

Opened in May 2006. the bridge is intended to give the feeling of "walking on water", and is set quite low against the lake (although not as low as I had expected from comments from other visitors). The bridge deck consists of 564 slender black granite "sleepers", supported on a hidden steel framework which in turn rests on steel piles ever 8m.

There are 990 bronze balusters, each with a curved top but no handrail. I've seen a similar design only on one or two other occasions, as most footbridges provide an upper rail, which serves both as a handrail and also to make the posts more economical by sharing loads between them. A continuous top rail is mandated in the relevant standards (e.g. BS 7818), but it's great to see it dispensed with for such pleasing effect as here.

The shape of the balusters is such that they appear essentially transparent when viewed face on, but solid when viewed at an angle. It's a charming concept, and lends the bridge a visual interest that its minimalist heritage would seem to have precluded.

An unobtrusive bridge was definitely the right response to a very tranquil setting like this, and I was particularly impressed as to how well hidden the supporting engineering was, with the impression given that the granite deck planks just float above the water.

It's a shame the water level wasn't higher when I visited, as it would definitely have looked better that way. You have to clamber down the bank of the lake to see what's really going on underneath, and no sensible visitor would do that.

The bridge certainly merits the several awards it has won, and offers a welcome reminder of the possibilities for bridge design beyond those which emphasise the structure or which feel obliged to at least display it honestly. While I enjoy minimalism in the arts, I'm not normally an admirer of it in architecture, but it makes perfect sense at the Sackler Crossing.

Further information:

12 June 2010

London Bridges: 3. Xstrata Treetop Walkway

The two footbridges over the Thames weren't the only bridges I looked at in London recently. I also spent a day at Kew Gardens, and this post covers one of the two very interesting pedestrian walkways to have been built there in recent years.

Opened in May 2008, the Xstrata Treetop Walkway is one half of a visitor attraction within the Gardens (the other half being the Rhizotron, a short underground tunnel with information on tree roots). Rising 18m in the air, and 200m long in total, it allows visitors to wander amidst the treetops, reaching out and touching some of the closer trees. The design is by Marks Barfield with Jane Wernick Associates. It was built by W.S. Britland & Co.

It's a considerable engineering achievement, considering the difficulties of bringing massive steelwork into the Gardens through narrow entrances and without damaging any significant vegetation. Each support had to be positioned very carefully to avoid tree roots, with the assistance of a radar survey.

I haven't been able to find out for sure how much it cost, although The Independent reports £4m (figures elsewhere are less). If true, and considering the bridge is about 1.5m wide, that works out at £13k per square metre. It's actually a bit less than that considering that each 12m span is connected by "nodes", areas with space to rest and absorb the view (one node is intended as an aerial classroom), and I suspect the £4m may include the Rhizotron too. Nonetheless, it's in the big league of landmark footbridges, even though there is no river, road or rail to cross, the structure is essentially conventional, and the spans are hardly challenging.

The money has gone into a spectacular but sadly non-functioning lift (it hasn't worked for most of the walkway's life, and is currently the subject of a legal dispute), the tree-like weathering steel towers, and the unorthodox and seemingly random geometry of the walkway trusses.

The use of weathering steel throughout is a great choice, not just because of the reduced need for future maintenance (especially valuable at height) but because it blends in well with the woodland setting. The support columns appear to have been carefully detailed so that water runs off easily, and the staircase is perforated to allow free drainage.

The walkway has a mesh floor, again eliminating drainage issues. The potential for rust to rub off on visitors' clothing has been avoided by the use of protective mesh panels and by covering the top rail in wood.

I think that in general it looks fantastic, especially from below. The tree-like columns are appropriate both in engineering and visual terms (although certainly not especially economic). As you look up through the floor mesh, you really want to get up there.

Once at walkway level, I was less happy, although this was as much due to mild vertigo as to any issues with the structure itself! The 1.3m high parapets were mostly reassuring, but the view down through the walkway grille was very disconcerting, and some of my companions found the whole experience quite difficult. The lack of reassurance meant that I concentrated more on negotiating the structure, and less on the trees that are supposed to be the real attraction.

In my own design work, I've had the question raised as to whether mesh flooring is in the spirit of the Disability Discrimination Act, as it clearly puts off visitors who suffer from more serious vertigo.

Up close, and indeed from most angles, the walkway trusses look like a jumble of metal struts thrown together randomly. If the idea is to give the impression of tree branches crossingly, it works very well.

However, I gather the pattern is in fact quite ordered, and certainly the floor bracing can be seen to be symmetrical about midspan. There are a higher number of diagonal elements close to the supports, as you would expect broadly in line with the variation of shear force.

The actual density of diagonals is said to vary in line with the Fibonacci sequence, which is philosophically appropriate as Fibonacci numbers govern many patterns in nature, including the branching of trees. It doesn't make such direct structural sense, of course, as the maximum shear force in the trusses does not follow a Fibonacci progression.

The walkway has already several awards, and in my opinion, thoroughly deserves the acclaim.

Further information:

10 June 2010

Te Rewa Rewa bridge opens

The Te Rewa Rewa Footbridge opened on 5 June (see also opening day videos). The NZ$2.8m bridge spans 69m across the Waiwhakaiho River near New Plymouth. I've covered it here a couple of times previously, before it took on its final name.

The steel arch structure was designed by Novare Design and built by Whitaker Civil Engineering.

The photos below were mostly taken the day before opening, and are courtesy of Novare Design. Click on any image for a larger version.

I think they've done really well to carry through the original design vision into the completed structure, without any significant change. I'm not keen on the way the structural behaviour is disguised (the arch springings hide a set of internal steelwork which make the arch behave as a bowstring rather than, as it appears, a thrust arch). Nonetheless, it's visually crisp and appealing, and I like it.

London Bridges: 2. Millennium Bridge

I've held off on so many occasions from commenting on this bridge. I've visited it several times in the past, but on my most recent visit, several points occurred to me which are perhaps worth addressing here.

I will say as little as possible about the bridge's history: that can all be found online elsewhere. I hope at some stage to cover the losing competition entries here, but that's a project for another day.

For now, I'll just give a quick cost comparison with the Golden Jubilee Footbridges. The Millennium Bridge is 333m long, and 4m wide. It cost £23m in 2000, including the refurbishment carried out after it notoriously wobbled. That works out at £18k per square metre, compared to £13k for the Golden Jubilee Footbridges. The Millennium Bridge is one of the most expensive footbridges of all time (beaten only by Gateshead's Millennium Bridge at £22k, with Kurilpa Bridge and Calatrava's Sundial Bridge at a mere £10k each, although Kurilpa's figure is flattered by the inclusion of its less structurally ambitious approach spans). For those spectacular sums of money, a promoter must get something of spectacular value in return.

The designers, Arup and Foster + Partners, described the bridge concept as "a thin ribbon of steel by day ... a blade of light at night". To achieve this, the extremely low span-to-rise ratio of 62 for the cables is roughly six times shallower than in a normal suspension bridge. As a result the cables (and more impressively, the foundations) carry a horizontal force of over 2000 tonnes, under dead load alone (if you picked the structure up on its end, one set of cables could carry three Millennium Bridges with ease).

I know I'm not alone in finding that the bridge in reality fails to live up to the vision of a "thin ribbon of steel". From almost every perspective, it's dominated by the cables and their outriggers. The low profile of the cables, which pass below the deck at midspan, requires the use of very stiff steel outriggers to hold the deck and cables in the correct geometry. The apparent "wave" created by the varying angle of each outrigger adds to their visual mass, such that from several viewpoints you can't see through them at all. A conventional suspension bridge, with its lightweight hangers, would not have had this issue, with views both onto and off the bridge being far less cluttered. Of course, it's arguable whether the towers for such a structure would have had an adverse visual impact.

Another difficulty with the outriggers is the way they pass below the main deck structure. I'm not entirely clear why this has been done, other than to simplify construction. In their current form, they cradle the deck units, with the deck edge beams sitting above the outrigger crossbeams. Again, from many perspectives this makes the bridge seem very bulky, as the crossbeams add to its apparent depth (something that wouldn't be apparent on a nicely drawn elevation of the bridge). The alternative would have been for the crossbeam to be within the same depth as the edge beam, passing "through" it and giving a much lighter appearance overall. Presumably this would have added to the complexities of fabrication.

The bridge's south abutment (pictured right) has always seemed to be an unhappy way to terminate the cross-river journey, with the deck splitting in two and then doubling back below itself, to a landing area which is often heavily congested by pedestrians as a result. The obvious route off the bridge is blocked by a glass balustrade that offers a view of nothing in particular.

Riverboat and riverside path headroom restrictions mean that the bridge must arrive on the south bank at height. Given the presence of a large plaza in front of the nearby Tate Modern gallery, the obvious solution to me would have been to landscape that area and provide ramps to ground following the various pedestrian desire lines. However, I believe that site constraints may not have allowed that when the bridge was built. At least they didn't build what was shown in the original Arup / Foster / Caro competition entry (pictured on the left).

The dampers which have been added to the bridge to prevent it from wobbling undoubtedly detract further from its appearance, but it would be unfair to criticise the designers for this, as they have clearly made the best of an awkward situation. There are diagonal bracing members next to the piers, diagonal bracing below the deck, little splayed damper "legs" on the riverbank, and more. The "legs", visible in the abutment photo above right, are probably the worst addition, visually. I do wonder whether it wouldn't have been better to go the whole hog and triangulate every bay of the bridge in order to give it a consistent look, but what has been done is presumably the least possible compatible with eliminating oscillation.

To a great extent, the appearance of the bridge is a logical consequence of making certain key design decisions at an early stage, and being locked into those choices thereafter. The design concept - the blade of light - was established by Arup engineers Chris Wise and Roger Ridsdill Smith who decided they "should just make the structure from cables stretched as tight as possible between the two banks, and then walk on them". That brings to my mind a stress ribbon bridge, which the Millennium Bridge has sometimes been described as, but certainly isn't (the deck segments aren't prestressed together longitudinally, hence their potential contribution to the stiffness of the bridge isn't exploited).

The decision to use the low-rise suspension bridge to achieve this was locked in place at competition stage, and had to be made to work thereafter. The relationship between cables and deck lead directly to the very high cable forces, the unfortunate lateral frequency of vibration, and the tangle of steelwork that now defines the bridge's aesthetic, in quite marked contrast to the original concept. Some of this is a flaw in the design competition route, which generally sets the final appearance of a bridge very early, unlike "conventional" design development where different structural options can receive a more thorough evaluation (often at the cost of daring, imagination, and star quality, of course).

Further information:

08 June 2010

London Bridges: 1. Golden Jubilee Footbridges

I was in London at the end of May, and passed by a couple of bridges on the River Thames very briefly, en route to somewhere else. So the next couple of posts aren't intended as comprehensive reports, just as quick commentaries.

The first of the two visits was to the Golden Jubilee Footbridges, two nearly identical structures jutting out to either side of the Hungerford Railway Bridge. These were the result of a bridge design competition, won by WSP and Lifschutz Davidson in 1996 against over 40 other entrants. The aim was to replace one existing footbridge supported from the east side of the railway bridge, which was narrow and unattractive. The bridge forms one of London's key pedestrian crossings of the Thames, linking two railway stations, the concert halls of the South Bank Centre, the Thames Embankment, and various routes running north towards the city centre.

The bridges were completed in 2002 as a design-and-build project, with the detailed design by Gifford, and Costain and Norwest Holst as contractors. Some changes were made to the concept design, but its basic appearance is largely unaltered and hence Gifford can be absolved of its various visual deficiencies.

Costing £39.5m in total, each footbridge is 315m long and 4.7m wide (that works out at about £13,000 per square metre of deck). The seven spans range from 50m to 65m. The steel pylons support a reinforced concrete deck, which was erected by launching from the river bank.

Many of the bridge's peculiarities relate to the site's challenging geometry and ground conditions. One pier, the A-frame now visible on the north river bank, had to be moved to reduce the risk of setting off unexploded bombs above an underground rail tunnel. Others are positioned according to the need to align the new piers with the existing rail bridge supports, one of which still sits where Brunel's original Hungerford suspension bridge was originally supported (the brick towers were reused in the railway bridge).

I dislike the use here of a flotilla of low-height cable-stay pylons anyway, but the two odd supports (an A-frame on the north bank, and a V-shaped pylon inclining the opposite way to every other mast at the main southern pier, pictured right) mar the otherwise relatively regular layout.

The overly complex arrangement of cables and bars at this particular pier strikes me as particuarly ungainly. If you click on the picture for the larger version, you can just about make out the pylon caps, which are very clumsy, with subsidiary "angel wing" elements hung below the main pylon head in order to collect the large number of cables used.

Each bridge employs 228 separate stays, and I think all these photos make clear that's simply too many stays. The visual complexity generated seems completely unwarranted by the structural demands, and surely the bridge would have looked simpler, cleaner and more elegant with fewer stays. A key design challenge, to me, would be how the new bridges should relate visually to the existing truss railway bridge, and there seems almost an attempt to hide the existing structure, or at least heavily distract from it.

The spacing and inclination of the stays is also very odd. They cluster together at the span third points, with no stays at all near midspan, and a wider spacing towards the support positions. There is some structural rationale for this (the more vertical the stays are, the stiffer they are and hence the greater load they share - so fewer stays are required in the vertical regions, and more where they incline more steeply). However, I think it looks terrible, both because of the odd spacing, and because there are none of the more shallow stays that you would expect to see towards midspan in a conventional cable-stay bridge.

Overall, the footbridge is visually unbalanced, overly fussy, and structurally over-engineered.

Incidentally, the bridge was named for Queen Elizabeth's Golden Jubilee in 2002. The Runcorn-Widnes Bridge had previously been renamed the Silver Jubilee Bridge 25 years previously, and this makes me wonder which bridge, if any, will get the honour of being the Diamond Jubilee Bridge. I can't help but notice that a certain bridge in Sunderland is proposed to start construction in 2012 ...

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