21 December 2011

Bridges news roundup

Moxon completes Taunton bridge
Modesty is a virtue (pictured - more images at Moxon's website).

Polyurethane/steel deck wins Canadian design award
Sandwich plate deck system used to minimise dead load in bridge refurbishment scheme.

Memorial Bridge Design Battle Just Beginning
Busybody seeks to add pointless frippery to New Hampshire bridge design.

New River Wear bridge in Sunderland given the go ahead
Government accepts arguments that "iconic" element of controversial design offers good value-for-money, finally securing the future for the new Wear bridge. This is something of a triumph for designers Stephen Spence and Techniker, who saw their initial design contest entry disappear into a black hole before it was eventually revived. Considerable effort has gone into making this bizarre design stand up, and I hope more details of the engineering become available in due course, through a technical paper perhaps.

Sturgess: An unusual process yields an extraordinary result: Peace Bridge
A quite extraordinary love letter to Calatrava's still incomplete Calgary footbridge.

Wow! That's one nice-looking bridge
The Calgary Herald calls it "iconic but overpriced". Get off the fence - was it worth it? A late January opening is currently anticipated.

Be’er Sheva  Pedestrian Footbridge
Winning proposal for yet another "iconic" project (pictured). Slightly reminiscent of the Gatwick Pier 6 passenger bridge.

Gateshead Millennium Bridge bollards to go

City of Toronto halts new Lovelocks fad on Humber River foot bridge
Joy-killing jobsworths hard at work in Toronto.

20 December 2011

London Bridges: 21. PaddingtonCentral Footbridge

This is the last of four footbridges that I visited in Paddington Basin, London. It is the furthest west of the group.

I will confess that what first attracted me to go and visit this bridge was a photo of it online showing the slightly unfortunate positioning of one of the main support legs - right in the middle of the access ramp! It has clearly been built on a very constrained site, but I'm at a loss as to why they couldn't avoid this.

The bridge design seems to pick up on several common features of a "modern" pedestrian bridge, throwing them together in a haphazard jumble that not only is unnecessary at this site, but would be so at any site. These design features aren't connected by the usual logic.

One feature is the use of cables to support the bridge on one edge. The pylon is connected to the deck with a series of cables, and is anchored by back-stays to the ground. At this sort of span, cable support isn't really necessary, and when you see the bridge from underneath, it appears doubly so.

The bridge is supported on Y-shaped steel frames at each end of the main span, and the main girder looks beefy enough to hold the entire deck up on its own. The cables introduce a horizontal force into the deck which must be resisted by the Y-shaped frames, emphasising the artificiality of the layout (the horizontal force is normally carried as a thrust in the deck directly back to ground). With this arrangement, supporting via vertical hangers from an arch would have made more structural sense.

The circular edge girder is positioned in such a way that there's no clear continuity between the access ramps and the main span's bridge deck. No doubt some of this ungainliness is a result of the constrained site, but I suspect more of it comes from trying to match the bridge's necessary geometry to an arbitrary and inappropriate choice of structural form.

In short, it's an object lesson in how not to design a footbridge, well worth visiting just to see what mistakes to avoid.

Further information:

13 December 2011

London Bridges: 20. Station Footbridge, Paddington

Proceeding west from the Rolling Bridge, I'll skip past a very dreary cable-stayed footbridge and move on to one that is a little more interesting.

Designed by Langlands and Bell with Atelier One, the main section of this footbridge is 44m long, with a 19.6m main span. The deck is 3.5m wide, and is suspended below a glass-clad steel "sail", 8.4m high. Langlands and Bell are artists (I think this may be their only bridge design?), not architects, specialising in an quixotic blend of styles which I would think of as pop-art-meets-minimalism. At Paddington, minimalism wins out - this is a bridge as if designed by Donald Judd.

The effect is quite unlike most footbridges. It's a proper gateway structure, proffering a letter-box entry to Paddington Basin from the west, and helping to hide an unattractive building when approached from the east.

Structurally, it is not especially complex. The walkway is a simple grillage of steel beams, suspended at midspan by a single hanger, with diagonals partially obscured by the glazed enclosure above. The form is an inverted king post truss, which is about as simple as it gets. The walkway cantilevers to either side of the support structure, supporting structural glass balustrades.

I think the glazed wall is visually very effective, particulary in images showing it illuminated at night. It's non-functional, in the sense meaningful to an engineer - it makes the structure harder to design, by adding enormously to the wind load, but the central cut-out prevents it from acting as a wind-screen. It allows for multiple alternate crossing experiences, and is boldly assertive but visually permeable.

Even visiting it on a dully grey day didn't harm it much. It helps distract from the dreary nature of the nearby building, and even from an adjacent construction site.

I like it.

Further information:

11 December 2011

London Bridges: 19. Rolling Bridge, Paddington

Heading west from the Helix Bridge, the next footbridge at Paddington Basin is the Rolling Bridge, easily the best known of the set, and justifiably so.

I would guess few of my readers will be unfamiliar with this bridge, as it is one of the most remarkable and audacious footbridge designs from the last decade. As with the Helix Bridge, it's something of a white elephant, spanning a very small side-arm of the canal, which goes nowhere and has no real reason for continued use by boats. The bridge is useful in providing a direct route along the edge of the main canal basin, but it doesn't really need to open.

Nonetheless, it does open, every Friday at noon. Sadly, I was there on a different day of the week. The opening mechanism is unique: the bridge slowly curls up from its level position, ending up in the form of an octagon when fully open. Its initial form, which broadly resembles a modified Warren truss, is distorted by extending eight hydraulic cylinders in each of the trusses.

Most moving bridges operate on the principles of translation (lifting and retractable bridges) or rotation (bascule and swing bridges). Very few adopt the principle of transformation, indeed about the only other examples which come to mind are Schlaich Bergermann's folding bridge at Kiel and arching bridge at Duisburg.

There is a sense that the Rolling Bridge's design is just a gimmick, but on balance I think not. It's delightfully inventive, playfully so, with the resemblance to a curling caterpillar responsible for a significant cuteness factor.

It was designed by Thomas Heatherwick, with structural engineering by Packman Lucas and SKM Anthony Hunt. The steelwork was fabricated by Littlehampton Welding. It cost £330,000 in 2004, which seems a lot for a 12m long bridge, but was surely worth it.

Up close, the detailing is nicely done, although the bumpy profile of the top rail in the at-rest position looks  a little odd. It's often a problem with "iconic" opening bridges that their sense of spectacle is lost when they are closed. With the Rolling Bridge, it still retains a sense of potential, like a muscle tensed and ready to flex.

Further information:

06 December 2011

London Bridges: 18. Helix Bridge, Paddington

The next few posts follow something of a guerilla tour around the footbridges of Paddington Basin, London. This canal terminus has undergone considerable redevelopment over the last decade, and a number of bridges have been installed both to improve access and to gain attention through their novelty value.

Before visiting, I'd heard of a fellow bridge photographer who had been unable to take pictures on the site thanks to over-zealous security guards. I'm a pontist, not a terrorist, but I thought it best to take photos quickly and not hang around. You'll therefore have to bear with me if some of the photos aren't of the best quality.

Somewhat prosaically also known as the "East Bridge", the tiny Helix Bridge provides access across the east end of the canal basin at Paddington. Completed in 2003, it spans a mere 7.2m. It was designed by Buro Happold with artist Marcus Taylor, and built by Davy Markham.

It is designed to retract, giving the impression of a corkscrew as it does so, albeit very, very slowly, moving at a snail-like 75mm per second. The "corkscrew" effect is simulated: the enclosure assembly rotates on bearings within the two circular end hoops. These in turn are supported from a retractable 2m wide walkway structure, which cantilevers from one end. This sits on a powered trolley, which runs on rails. As the walkway retracts, the enclosure rotates.

The 3.5m diameter enclosure comprises 15mm thick curved glass panels, glued to the tubular framing. The main spiral element is a 140mm diameter stainless steel tube. The bridge has clearly been altered since construction, with what were previously transparent panels now replaced or covered in blue material. I presume this is a response to vandal-related glass breakages.

I gather that the bridge has not been able to operate for some time, and have seen a comment from British Waterways on an online canal forum that it would "cost a considerable sum of money to repair", clearly disproportionate to the value that opening provides. BW also report that the bridge is due to be replaced by a new lifting bridge, presumably one with a less complex mechanism, and hopefully not destined for a similarly early graveyard.

Further information:

04 December 2011

London Bridges: 17. Challenge of Materials Footbridge

I spotted these photos from earlier this year lurking in my files, and realised it was a bridge I hadn't yet featured here. These aren't great photos, I was in a hurry and didn't have time to find a good viewpoint.

The bridge was designed by Whitbybird and Wilkinson Eyre and spans the main hall in London's Science Museum, providing access to the Challenge of Materials gallery.

The bridge is intended to use the bare minimum of materials, with 16m spanned by 8.5 tonnes of glass and steel. The deck comprises 828 glass plates placed on edge, the ideal arrangement to resist the compressive stresses introduced by the cable-stay support arrangement. The balustrades are in the form of glass plates.

The "stays" are a closely spaced fan of stainless steel wires, each a mere 1.58mm thick. Each wire runs from a support fan bolted to the building's columns, down to the deck, across the underside, and then back to another support fan. Tie-down cables below the deck ensure rigidity and prevent vibrations.

From several angles, as will be apparent from the photographs, the ultra-slender wires almost become invisible, leaving only an evanescent presence in the air. From other perspectives, the light glints off the wires, and elements of their cat's cradle geometry are revealed. This is my favourite aspect of the design: the supporting wires define a wider space for the walkway to rest within, but the effect is subtle rather than intrusive.

The glass floor and sides, which you might expect to give a sense of precariousness, are surprisingly unexciting: the sheer rigidity of the structure eliminates any sense of peril.

Further information:

02 December 2011

Bridges news roundup

It's time for another very quick roundup of bridge-related news.

Hopefully in the near future I'll get some more spare time, and I plan to post about a series of footbridges in the Paddington area of London which I visited recently.

Walk of Faith: Glass Pavement for Tourists Built on 4,690ft Mountain in China
That's another place added to my future holidays list.

Shrewsbury footbridge to undergo major refurbishment
£450,000 to refurbish a bridge which originally cost £2,600. It would certainly be a shame not to preserve this fine bridge, but you could probably build an entirely new bridge for that sum.

9 Beautiful Pedestrian Bridges Not Designed by Santiago Calatrava
The design credits generally omit the engineer, and several of these actually look rather horrible, but it's nice to see lesser known structures getting some recognition.

Peace Bridge placed over Bow River
The long-running saga of Calatrava's Calgary bridge seems to be approaching an end. Some people still find time to grumble.

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!

26 October 2011

Merseyside Bridges: 7. Silver Jubilee Bridge

Righty-ho, from Liverpool, this tour heads inland, following the course of the River Mersey, although with a sharp detour coming up soon.

In Liverpool, there are no bridges across the river, only tunnels below it. Heading east, the next opportunity to cross comes between Runcorn and Widnes. The River was first spanned here in 1868 by William Baker's Runcorn Railway Bridge, with three 93m spans, bridged by wrought iron lattice girders. It's visible in several of the photographs.

The second bridge here was the Widnes-Runcorn Transporter Bridge, opened in 1905. This was the first (and longest) of the only four transporter bridges to be built in Britain. In July 1961, the bridge now known as the Silver Jubilee Bridge was completed (originally the Runcorn-Widnes Bridge), with the transporter bridge being demolished immediately thereafter.

This was designed by Mott Hay and Anderson, at almost exactly the same time as they designed Tamar Bridge. As I noted a month ago, the two bridges make for an interesting comparison. The Runcorn-Widnes Bridge spans 330m, while the Tamar Bridge spans 335m. Both structures are in close proximity to a historic railway viaduct, but very different solutions were chosen.

Three-span and five-span truss designs were considered for the Runcorn-Widnes crossing, but rejected in favour of a suspension bridge design. However, wind tunnel tests showed that eddies in the wind caused by the proximity of the Runcorn Railway Bridge could destabilise the suspension bridge, and it had to be redesigned with an unusually stiff deck to prevent this. Nonetheless, the client eventually preferred a steel arch bridge option. At Tamar, the opposite conclusion was drawn when it was shown that the height of the road bridge deck relative to the railway bridge sufficiently reduced the effect of wind eddies.

The arch design was not without its problems. A two-pinned arch similar to the Sydney Harbour Bridge was shown to have a natural frequency of vibration not dissimilar to the rejected suspension bridge option. Making the bridge continuous with its side-spans altered the frequency sufficiently to resolve this concern, and offered the added benefit that the arch could easily be built by cantilevering from either side. Two very similar bridges built in the 1960s were the Bridge of the Americas (344m span, 1962) and Laviolette Bridge (335m span, 1967).

One aspect of the Silver Jubilee Bridge's construction which was perhaps surprising for the period was that it was built from riveted steel, a method essentially then obsolete. It was certainly one of the last, if not the last, large bridge in the UK to be riveted.

Any attempt to judge the aesthetics of a bridge like this might be considered irrelevant. The form and appearance are determined almost entirely by considerations of load-carrying function and of the demands of construction. Nonetheless, the not dissimilar Sydney Harbour Bridge and Tyne Bridge (the latter also designed by Mott Hay and Anderson) have both become city icons in spite of their industrial appearance. The Silver Jubilee Bridge is less iconic but much of that is down to context: power stations and chemical plants define much of the landscape of Runcorn and Widnes, and the Silver Jubilee Bridge seems an inevitable consequence of this rather than something which stands alone.

Its closeness to the railway bridge renders it difficult to see in isolation, as will be apparent from the photos of the main elevation. I find the choice of colour unconvincing as well, it's a little sickly and not really in keeping with a bridge where its sheer scale means it dominates the largely flat landscape. A difficult bridge to love.

Further information: