22 November 2011

Bridges news roundup

Life is busy, busy, busy, so you will have to make do with a quick roundup of mostly out-of-date news for now ...

Money Secured for 3 Dallas 'Signature' Bridges
Pockets emptied for Calatrava designs.

Race starts for £118m Wear bridge before approval
Sunderland City Council goes to tender on unfunded iconic bridge.

Peace Bridge will be red, inside and out
As delayed Peace Bridge hits one-year milestone, questions surround cost overrun
Costcutting continues but Calgary bridge still not ready.

Invisible Pedestrian Bridge in Netherlands
Yes, it's a delight, but how will they maintain it?

Sneek Bridge / Achterbosch Architectuur with Onix
To hell in a handbasket.

Cidade Nova Metro Station and Footbridge
Mixed feelings about this bridge in Brazil, but nice paint colour.

Kurilpa Bridge wins transport award at World Architecture Festival
No comment.

17 November 2011

"Disaster on the Dee: Robert Stephenson's Nemesis of 1847"

I reviewed Peter Lewis's book "Beautiful Railway Bridge of the Silvery Tay" last year, but initially approached his "Disaster on the Dee" (ISBN 0-7524-4266-X, Tempus Publishing, 2007, 224pp) [amazon.co.uk] with more trepidation.

The book tells the tale of the collapse in May 1847 of Robert Stephenson's bridge over the River Dee, built for the Chester and Holyhead Railway. The bridge had opened to rail traffic in November 1846, and five people were killed when it failed. Stephenson was heavily criticised, but went on to open the more significant Conwy and Britannia Bridges in 1848 and 1850.

The Dee Bridge spanned about 30m, and consisted of two parallel track beds, each supported by paired cast-iron girders. It was one of the longest such bridges then built, and well illustrates the dangers of pushing a proven structural form beyond safe limits. The propensity of cast iron to sudden, brittle failure was already well known, and the collapse was just one of many which dented public confidence in railway developments. It's not at first sight, therefore, such a significant bridge failure in itself as to merit book-length treatment.

Fortunately, Peter Lewis's book ranges far beyond the Dee Bridge disaster, to the extent that it's a very useful history of the structural use of cast iron, the development of metal bridges in the 19th century, and the development of railway bridges. The Dee Bridge may establish the headline, but is only one small feature in a much larger tapestry.

The Dee Bridge was an example of a so-called trussed girder, three separate cast iron segments bolted together and "reinforced" with wrought iron bars in the form of an inverted queen-post truss. The trussed girder concept was not Stephenson's, but had been introduced by Charles Blacker Vignoles some years previously. The truss bars were intended both to provide a back up against cast iron failure, and also somehow to enhance the overall strength of the girder.

Although the Dee Bridge disaster lacks the notoriety of the Tay Bridge, Tacoma Narrows, or Millennium Bridge, it has been a popular subject amongst other writers on engineering failures.

Henry Petroski's Design Paradigms (1994) uses it as a case study, "a paradigm of success masking error". Petroski draws on an unpublished PhD thesis by P. G. Sibly to show precisely the extent to which the Dee Bridge went beyond previously safe spans (click on the graph on the right) - even by the standards of its time, its calculated factor of safety against failure would have been low. Petroski attributes the failure to lateral torsional instability, noting that the almost T-shaped girders had no bracing to prevent lateral deflection; and that the eccentricity of the wrought-iron ties tended to introduce a destabilising force, as does the fact that the deck system loaded only the inner half of the girder bottom flange. The trigger for failure was the addition of 125mm of ballast load to the bridge the day before the collapse.

Lateral torsional buckling, as with any slenderness related effect, is a phenomenon which may not be an issue at all at smaller spans, but which grows in significance rapidly beyond a certain threshold. Petroski's lesson is, however, not about this narrow technical issue, but about the broader dangers of "extrapolatory design".

The bridge also features as a short case study in Failed Bridges (2010) by Joachim Scheer, who essentially follows Petroski's narrative in terms of the reasons for failure, both technical and cultural.

In Understanding Bridge Collapses (2008), Björn Åkesson devotes his first chapter to the Dee Bridge. He notes some wider technical issues. The wrought iron tie bars were unlikely to have been as effective as was imagined: the inclined bars rely on their anchorage in a "bulge" above the girder ends to work, but this bulge is prone to move inwards under girder deflections. The result is that the tie bars may have been almost entirely ineffective. Åkesson also shows that the simple rules for girder section sizing used by Stephenson (originally derived by Eaton Hodgkinson) would have underestimated tensile stresses in the bottom flange, due to the lack of any understanding of the concept of a neutral axis.

Åkesson discards the lateral torsional buckling theory on the grounds that some instability would have appeared prior to the addition of ballast, and also that the very thick girder webs would have provided sufficient U-frame action in any event. He is the only author to prevent what you might think is obvious, actual calculations for the static and fatigue stresses, which strongly suggest that static stress was far more of an issue than fatigue (an example is shown on the right). Åkesson points his finger of blame at the connections between the tie-bars and the girders, suggesting that over time, the harder material of the connecting pin would cause plastic deformation of the cast iron, and ovalisation of the connecting hole. Gradually, this would eliminate any beneficial effect of the wrought iron bars, leading to a simple tension failure once additional dead load was introduced.

I guess the genesis for Disaster on the Dee was the author's 2004 paper Aesthetics versus function: the fall of the Dee bridge, 1847. The paper, and the technical examination of the bridge failure presented in the book, make clear Lewis's background in forensic engineering, particularly metal fatigue, which he posits as the reason for the collapse. This is despite there being no surviving evidence which would support the theory (e.g. pictures of the girder crack surfaces), and a complete unwillingness to offer even the simplest of calculations. This would not have been difficult, as Åkesson's contribution shows.

Lewis discounts the possibility that failure resulted from lateral torsional buckling by stating that "this is not a failure mode, but rather a symptom of an underlying problem involving design". That's the language of a metallurgist. To a structural engineer, it's quite definitely a failure mode. Similarly, Lewis rules out failure under static overload on the grounds that the train successfully crossed several girders before reaching the one that failed, suggesting the failed girder had a prior fatigue crack. This strikes me as spurious: surely the failed girder could simply have had a defect which the others lacked, cast iron of the period being notorious for variable properties and inconsistent quality of manufacture.

The varying explanations of the Dee Bridge failure seem to indicate how strongly perceptions of failure can be influenced by an investigator's background: you find what you look for. Many years after the event, no one explanation can be proven fully correct, and they all offer useful insight into lessons that may be valid elsewhere. To me, however, the value of Disaster on the Dee lies beyond the central case study.

The book's first chapter discusses early iron bridges and gives a useful primer on how iron was cast, with particular reference to Iron Bridge in Shropshire. The next chapter explores the extension of cast iron into canal aqueducts such as those at Longdon-on-Tern and Pontcysyllte, while the third survey early railway history, including the use of iron in steam boilers. The fourth chapter covers the build-up to the design and construction of the Dee Bridge, while bringing in other structural failures in rail bridges and cast iron buildings.

After the exhaustive account of the failure and subsequent enquiries, the book goes on to look at how Stephenson's later bridge designs avoided the pitfalls which befell the Dee Bridge, and recounts many other railway and bridge catastrophes, painting a picture of an engineering industry which was slow to learn lessons from its hubris and folly. I found all these sections of the book to be hugely informative, and would recommend it for that alone.

15 November 2011

Merseyside Bridges: 11. Howley Suspension Bridge, Warrington

This is the last in this current series of posts, another bridge in Warrington spanning the River Mersey.

It was built in February 1912 by David Rowell & Co., who were prolific builders of many similar steel suspension footbridges. Unusually for the genre, it has been considered worthy of a Grade II Listing, although the website linked below has it by the wrong name.

A sign on the bridge states that "swinging or jumping on this bridge is strictly forbidden", suggesting a liveliness that I don't recall noticing.

I'm conscious that I know very little of the bridge's history, and as for its appearance, can only state that I am a huge fan of unpretentious, lightweight suspension footbridges such as this. So enough verbiage, here are some photos!

Further information:

13 November 2011

Merseyside Bridges: 10. Warrington Transporter Bridge

The last two posts were a bit of a diversion from Merseyside, not being very near the River Mersey for a start. This time, it's back to the River, although whether Warrington is part of Merseyside is perhaps a moot point.

There are plenty of bridges in Warrington, but I only had time to visit two.

The Transporter Bridge at Warrington was something of a must-see. It's one of only three surviving transporter bridges in Britain, although sadly it's by far the most neglected of the trio. It has been a presence on English Heritage's "At Risk" register for some years now, I believe.

It was originally built to service the Crosfield chemical and soap works. A railway siding to the east ended at the river bank, but wagons were transferred onto the transporter carriage and brought over the river. The rail tracks, long since disused, still run through an adjacent site. Today the site of the soap works is occupied by chemical firm Ineos Silicas, and the bridge hasn't been used since the 1960s, becoming increasingly derelict has the years have passed. Today, the bridge is maintained by Warrington Borough Council, but as it serves no practical use, it has not been well looked after.

It's also a pretty tough bridge to get to. The west abutment, within the Crosfield site, is only accessible with special permission. The east abutment involves negotiating a convoluted route through the grounds of an industrial works, or, as I did, approaching via an overgrown and somewhat forlorn pathway. The combination of lack of use and difficult access mean that any attempt to preserve the bridge for much longer may be essentially futile. I can't see how it can survive in this situation, and if it were to be relocated, that would sever the link with its historic context.

The photos make clear that this was never a pretty bridge, certainly not by comparison with its fellow transporter bridges at Newport and Middlesbrough. Its portal truss form is squat and inert, and if the latticed metalwork holds any visual attraction, it's surely only for the most committed industrial archaeologists or those besotted with the romance of dereliction.

So, it sits there and rusts. Some indication of quite how decrepit the bridge is can be gleaned from two websites with photos from unauthorised visits.

The gondola can be seen in the last photo, on the west bank, and it's still just about possible to close your eyes and imagine how it might have looked while still in active use.
Unless someone has the appetite for something radical, such as relocation, I would expect the bridge to be fenced off as dangerous within half-a-dozen years, and gone within the next decade or two.

Further information:

08 November 2011

Merseyside Bridges: 9. Acton Swing Bridge

From the Dutton Horse Bridge, my journey ran south along the River Weaver. The next bridge of significance gives its name to the small town of Acton Bridge.

It was designed by the same John Arthur Saner who designed the Dutton Horse Bridge, and apparently many other structures along the River Weaver. The current bridge opened in 1933, replacing a previous structure which had been able to carry only one line of traffic with an axle weight limit of 8 tons.

It is 83.5m long, with two slightly skew spans of 25m each. It's estimated cost at the time of construction was £52,000, excluding the diverted approach roads. The superstructure comprises twin riveted steel trusses, supported on a mass concrete pontoon chamber. One website describes it as "the first floating swing bridge in Britain", although I think there is some confusion there with the nearby Northwich Town Swing Bridge, which has a plate on it claiming it to be "the first road swing bridge on floating pontoons".

In its structural form it is typical of many truss swing bridges of the period, and not as elegant as some. The overhead bracing seems particularly over-designed. The curved profile of the truss is visually satisfactory but does not optimise the level of stress in the main members. Structurally, this particular profile's main benefit is to keep the overhead bracing clear of vehicles.

The use of contrasting black-and-white paint is common to many of the structures along the River Weaver.

With the truss main members and verticals painted white, and the diagonals black, there seems to be an intention to define a visual hierarchy of members and perhaps de-emphasise some. There is no great structural logic behind this, as the diagonals are as important to the bridge's overall strength as the verticals.

The edge of the bridge deck is painted with black and white bars, presumably to warn of limited headroom. Interestingly, the footway parapets are painted with the same alternating pattern. This makes the parapet appear to dissolve into a series of isolated panels with open gaps between them, which is a peculiar effect, if intentional.

Further information:

04 November 2011

Merseyside Bridges: 8. Dutton Horse Bridge

This is a short detour from Merseyside, however broadly that area might be defined. From Runcorn, the journey heads south-east towards Northwich. I stopped off at two bridges before returning north to the River Mersey. In broad outline, this route follows the course of the River Weaver, which is a tributary to the Mersey.

The first of the two structures is Dutton Horse Bridge. This twin-span timber bridge was completed in 1919 to a design by John Saner, who was the engineer responsible for many works along the River Weaver when its navigability was improved. The Dutton Horse Bridge spans alongside the river's main course, over a secondary channel used to help regulate the water level.

It is historically significant as one of the earliest surviving bridges to feature laminated timber. The laminated arches span about 31m, with two ribs slightly out of line with each other. Triangulated timber struts stiffen the arches and support the deck.

As with many timber structures, it's certainly an attractive bridge, and it's great to see it has survived so well. Two span bridges are reputed to suffer from the problem of the "unresolved duality", where the viewer lacks a primary visual focus and so feels discomfited. This is a phenomenon that has received support from at least one experimental study, where viewers consistently tended to prefer one or three-span bridges to those with two spans. At Dutton, the bridge is rarely seen in full elevation, as the footpath on the opposite side of the river seems not to be the main route. From any other angle, the number of spans is hardly relevant. Even in full elevation, I think it looks fine.

The black-and-white paint scheme is characteristic of several bridges along the Weaver, including the one I'll come to next.

Further information:

01 November 2011

Amsterdam Iconic Pedestrian Bridge Competition

An open international contest has been announced to design an "iconic" [ugh] pedestrian bridge in Amsterdam.

Apparently, the architecture of the bridge should "reflect contemporary design tendencies", whatever they may be. Perhaps that means they want a bridge which looks like a turd shat by a giant penguin across a canal.

It's a single stage contest: to enter, you simply have to pay the registration fee (US$80 until 15th December, rising to US$120 until 15th February) to receive full contest details.

The winner gets a US$3,000 prize, there are smaller prizes for second and third, and there will be seven honorable mentions. Winners will be announced on 15th March 2012.

They emphasise that "there are no plans for the Iconic Pedestrian Bridge to be built." The site for the (imaginary) bridge, which would be about 90m long, is in front of the Amsterdam Heritage Museum, spanning the River Amstel. No engineering calculations are required, instead the structural design is suggested to be "architectonic" in nature. The biggest geometrical challenge is that a 15m navigation headroom is specified.

The competition is organised by AC-CA (Architectural Competition Concours d'Architecture), a private company whose business seems to be running contests for designs which will never get built, the architectural equivalent of vanity publishing.

I really must get back to writing up some more Merseyside bridges to post!