28 May 2012
27 May 2012
Bridges news roundup
I'm off on holiday for a few days, so will leave you with a few links to bridge-related stuff spotted on the internet in recent weeks:
World's scariest bridges
No pumpkins or ghosts in sight, but still generally scary.
The Aizhai Suspension Bridge in China - in pictures
I especially like picture number 10.
Arganzuela Footbridge / Dominique Perrault Architecture
I'm still not sure what I think of this bridge.
Infinity Loop Bridge / 10 Design + Buro Happold
Or this one.
Forth Rail Bridge 360° views
But I know how much I like these photos!
World's scariest bridges
No pumpkins or ghosts in sight, but still generally scary.
The Aizhai Suspension Bridge in China - in pictures
I especially like picture number 10.
Arganzuela Footbridge / Dominique Perrault Architecture
I'm still not sure what I think of this bridge.
Infinity Loop Bridge / 10 Design + Buro Happold
Or this one.
Forth Rail Bridge 360° views
But I know how much I like these photos!
24 May 2012
River Wear bridge: an update
It seems that the NCE's news story featuring Simon Bourne's criticism of Sunderland's River Wear crossing (see my last post) has attracted quite a lot of comment. I guess that's why the magazine went for it in the first place, the chance to generate some controversy, column inches and pageviews.
First up are the comments on the NCE's website. At the time of writing there are 12 comments, which may not seem much, but is a positive bonanza by the website's normal standards. Civil engineers are normally true to their name, civil, and reluctant to voice strong opinions in a public forum.
The comments are fairly evenly divided between those pro and ante the bridge design. On the pro side are comments like these:
I found another comment profoundly depressing:
Here are some of the comments from those opposing the design:
This week's NCE continues to explore the story. In the letters page, architect Martin Knight writes:
Knight, of course, was the architect for England's other major current bridge scheme, the Mersey Gateway (pictured, click for full-size image). Like Sunderland's River Wear crossing, this is a cable-stayed bridge design, although far more conventional. The Mersey design is clearly a landmark design with a strong aesthetic vision, but founded on an assessment of what makes engineering sense, in this case towers with balanced cable-stays, and a truss deck which contributes significantly to the overall stability.
There's also a strong contrast in the two schemes' approach to procurement. The promoters of Mersey Gateway accept and encourage their bidders to depart from the original vision in the interests of saving money - the structure which will be built is unlikely to retain either the harp cable layout or the truss deck, although I imagine the single plane of cables has a fair chance of surviving. In Sunderland, the architect's vision appears to be sacrosanct, and must be built however difficult it may become.
This week's NCE also features an opinion piece, which mentions both these schemes and compares them to the eye-wateringly awful ArcelorMittal Orbit (pictured left). This resembles a giant corkscrew onto which a wino is vomiting jets of blood. It is half observation tower, half-giant sculpture, and it's that neither one-thing-nor-the-other quality which makes it such a failure. The economic case for the Orbit is simple: a very rich man is paying for it, so we don't have to, and whatever value it offers is therefore an easy win. The possibility that, carbuncle-like, it actively degrades the aesthetic value of everything around it, is surely too cynical to contemplate.
Other blogs are getting in on the act, with one making the not unreasonable point that debate over the merits of the River Wear bridge is ill-informed without a better understanding of its costs, and the costs of the alternatives, figures which are not easy to extract from publicly available information.
I'm aware that the bridge's promoter, Sunderland City Council, is pretty unimpressed with the NCE's coverage, stating that the scheme:
The document is riddled with flaws, foremost of which is that it calculates the cost-benefit ratio for the chosen landmark design, but doesn't actually compare it with what a conventional alternative would achieve. Many of the figures used are highly debatable, and a series of other landmark bridge case studies are presented, but offer at best anecdotal and certainly no quantitative evidence.
Nonetheless, I'd recommend the report to anyone interested in this debate, which surely needs to move on from the rather simplistic battle between engineering puritans and starry-eyed visionaries which we're faced with at present. There undoubtedly is a case to be made that a bridge as spectacular as Sunderland's will be justified if the additional benefits it brings are sufficiently large, and this would be true irrespective of its structural efficiency, which is not an issue of much interest outside the profession concerned.
The Sunderland promoters are unafraid to make the case that cheapest is not always best, that appearance remains important even in the middle of recession, and that value-generation is the important measure of viability. Anyone arguing against that needs to show not just that the alternatives are much cheaper (as they clearly are), but that the value lost is not significant, and that the end result isn't a blot on the landscape which future generations will look at with regret.
First up are the comments on the NCE's website. At the time of writing there are 12 comments, which may not seem much, but is a positive bonanza by the website's normal standards. Civil engineers are normally true to their name, civil, and reluctant to voice strong opinions in a public forum.
The comments are fairly evenly divided between those pro and ante the bridge design. On the pro side are comments like these:
"The bridge is elegant. Difficult to build -yes, but elegant. As a Bridge engineer myself, I can only envy those who will be taking on this design and build challenge."
"It looks exciting, and design is not just about cheapness. It is better to design to please many and offend some, than to design to not offend anyone."One comment seeks to justify the "iconic" nature of the Sunderland bridge by comparison to past engineering glories:
"The world is scattered with costly design examples, not only of bridges, that are striking, memorable and, by Mr Bourne's theory, should never have been built. A few to ponder are The Eiffel Tower, Sydney Opera House, The Forth Railway Bridge, etc."That one made me chuckle. The River Wear bridge adopts a design where the structural engineer has taken an entirely submissive attitude to their architect friend, essentially promising to make a vision work, at any cost, regardless of the Herculean structural feats required. Compare the Eiffel and Forth structures, where the engineer led the design and where the geometric form directly responds to the structural imperatives. The Sydney Opera House is, on the other hand, a better comparison: a completely unsuitable structural form, with an architect unwilling to compromise, where the work ran grossly behind programme and over budget. Today we can say this is justified by the enduring presence of an architectural masterpiece, but for the client at the time it was little short of a catastrophe.
I found another comment profoundly depressing:
"If you want, and can afford, to have an 'iconic' structure instead of a bog standard box girder bridge; Good, I'll design and supervise its construction. My brief is to make sure that it doesn't fall down or fail to meet building regulations. It is the promoter's responsibility to meet the cost!"This is a trait I hate to see in engineers - the view that we are simply the servants of the visions of those better suited, that we should only be the "calculators", in Le Corbusier's notorious phrase. That's not a world I would wish to work in - I'm happier in one where engineers can have their own vision, contribute actively to aesthetic, commercial and political discussions.
Here are some of the comments from those opposing the design:
"I always said that if I was a client I would sue the design team of my project if it won an RIBA award on the basis that it would be over-priced, difficult to build and probably expensive to maintain."
"I'm afraid I have to categorize it as an architect's whim. It is overly complicated and does not present a clear structural statement of what it is trying to achieve."
"... it fits into the architects inflated ego category and an extreme waste of public funds."
"A wonderful monument to architects sticking two fingers up at economic construction - and the councellors all too dazzled to call for sanity. I have no objection to landmark structures and abhor utilitarian design, but surely in these days of economic constraint it is better to get an economically designed project constructed ASAP - and have change left over for the next one - rather than splash out on expensive fantasies."As I've observed before, a distaste for extravagance is deeply ingrained in the engineering psyche. If a student over-designs the reinforcement in a beam section, they will fail their test - the working assumption throughout an engineering education is that economy is king, and the engineer's job is to minimise materials and cost. The possibility that what we design can have a value beyond the purely functional is rarely if ever acknowledged.
This week's NCE continues to explore the story. In the letters page, architect Martin Knight writes:
"Investment in infrastructure which is well designed, encourages growth, and reinforces social, environmental and economic sustainability is highly valuable."
Knight, of course, was the architect for England's other major current bridge scheme, the Mersey Gateway (pictured, click for full-size image). Like Sunderland's River Wear crossing, this is a cable-stayed bridge design, although far more conventional. The Mersey design is clearly a landmark design with a strong aesthetic vision, but founded on an assessment of what makes engineering sense, in this case towers with balanced cable-stays, and a truss deck which contributes significantly to the overall stability.There's also a strong contrast in the two schemes' approach to procurement. The promoters of Mersey Gateway accept and encourage their bidders to depart from the original vision in the interests of saving money - the structure which will be built is unlikely to retain either the harp cable layout or the truss deck, although I imagine the single plane of cables has a fair chance of surviving. In Sunderland, the architect's vision appears to be sacrosanct, and must be built however difficult it may become.
Other blogs are getting in on the act, with one making the not unreasonable point that debate over the merits of the River Wear bridge is ill-informed without a better understanding of its costs, and the costs of the alternatives, figures which are not easy to extract from publicly available information.
I'm aware that the bridge's promoter, Sunderland City Council, is pretty unimpressed with the NCE's coverage, stating that the scheme:
"has been rigorously designed, costed, admired and backed within the industry and profession, including the Institution for Civil Engineers which presented the project with a CEEQUAL award ... The project is functional and symbolic, and its regeneration benefits were recognised by the Department for Transport in its decision to award funding."Sunderland's website for the project reproduces much of the relevant documentation. The most interesting in the context of the present debate are the reports which attempt to put a value on the choice of a bridge with striking rather than commonplace aesthetics. These calculate that the landmark bridge design offers £33m of economic benefits which wouldn't apply to a conventional girder bridge design, most of that in generation of employment.
The document is riddled with flaws, foremost of which is that it calculates the cost-benefit ratio for the chosen landmark design, but doesn't actually compare it with what a conventional alternative would achieve. Many of the figures used are highly debatable, and a series of other landmark bridge case studies are presented, but offer at best anecdotal and certainly no quantitative evidence.
Nonetheless, I'd recommend the report to anyone interested in this debate, which surely needs to move on from the rather simplistic battle between engineering puritans and starry-eyed visionaries which we're faced with at present. There undoubtedly is a case to be made that a bridge as spectacular as Sunderland's will be justified if the additional benefits it brings are sufficiently large, and this would be true irrespective of its structural efficiency, which is not an issue of much interest outside the profession concerned.
The Sunderland promoters are unafraid to make the case that cheapest is not always best, that appearance remains important even in the middle of recession, and that value-generation is the important measure of viability. Anyone arguing against that needs to show not just that the alternatives are much cheaper (as they clearly are), but that the value lost is not significant, and that the end result isn't a blot on the landscape which future generations will look at with regret.
17 May 2012
Engineers say £118M Wear bridge is waste of public money
New Civil Engineer magazine has this week found one bridge engineer willing to break cover and criticise Sunderland's River Wear Crossing, an astoundingly bold, astoundingly inefficient cable-stayed design (pictured below) that I've featured on many occasions on this blog.
Simon Bourne is the chap willing to put his head above the parapet. He's described as an "independent bridge expert", although until very recently he was head of bridges at Benaim, part of URS. Presumably with independence comes the ability to speak one's mind much more freely. Benaim were the quintessential contractor's bridge designer, expert at means of construction and ways of building bridges more efficiently.
Bourne describes the bridge as "a gross misuse of public money in a time of austerity", and has written to the government to tell them so. NCE report two other engineers chiming in with support, one noting its complexity makes it "completely unnecessary for a span of this length", and the other "just can't believe it's got this far". I'm always suspicious of anonymous quotes, but these echo views I have heard expressed privately by any number of highly experienced bridge designers, including those often associated with landmark bridges.
Bourne's criticisms of the bridge in terms of its efficiency as a structure are entirely reasonable, although long-time readers of the Pontist will find nothing new there as I've critiqued the bridge in far more detail in the past.
I'm far less convinced by his argument that the expense isn't justified by the economic value that a landmark bridge will trigger. He says that comparisons with the Gateshead Millennium Bridge are invalid because it is a city centre bridge, while the Wear Crossing sits in an industrial estate. I can't see how that's relevant - the Wear bridge will be visible from miles around and its value is as a statement of intent, a trigger for investment which creates value through employment. It's not intended primarily as a visitor destination in its own right. Whether that investment will materialise may quite reasonably be doubted, but I don't imagine Bourne can claim to be an expert in the economics of regeneration. I suspect that the real reason he takes this view is simply the traditional, highly conservative bridge engineer's belief that extravagance is essentially immoral, a narrowly puritan perspective which prevents engineers from contributing more meaningfully into wider cultural discussions on the value of aesthetics.
I've discussed in the past how rarely bridge engineers are willing to publicly criticise the work of their peers in the industry. In that respect, Bourne's intervention is a positive thing, as there is far too little debate on the merits of designs such as this. The NCE quote Sunderland's River Wear project director as saying the bridge has been "rigorously designed, costed, admired and backed within the industry and profession", a statement of such deep complacency that it positively demands more critics to clamber out of the woodwork, brandishing angry manifestoes reading "form follows function" and so on.
Unfortunately, Bourne's complaint also falls into the too little, too late type of criticism which comes at a stage in the project where it is largely pointless. It has been clear throughout the River Wear saga that the promoters are wedded so deeply to their vision that they are happy to ignore opinion, whether expert or non-expert. I recall in particular their reliance on a public survey expressing support for a design, when a much larger survey showed that at least half wanted to avoid expense and favoured a "tried and tested" solution instead. Sunderland have had the design reviewed by experts who agree that it can be built. The most serious test of its feasibility will come when contractors' tenders are finally submitted, and we discover whether they share the client's expectations as to what it will cost.
Simon Bourne is the chap willing to put his head above the parapet. He's described as an "independent bridge expert", although until very recently he was head of bridges at Benaim, part of URS. Presumably with independence comes the ability to speak one's mind much more freely. Benaim were the quintessential contractor's bridge designer, expert at means of construction and ways of building bridges more efficiently.
Bourne describes the bridge as "a gross misuse of public money in a time of austerity", and has written to the government to tell them so. NCE report two other engineers chiming in with support, one noting its complexity makes it "completely unnecessary for a span of this length", and the other "just can't believe it's got this far". I'm always suspicious of anonymous quotes, but these echo views I have heard expressed privately by any number of highly experienced bridge designers, including those often associated with landmark bridges.
Bourne's criticisms of the bridge in terms of its efficiency as a structure are entirely reasonable, although long-time readers of the Pontist will find nothing new there as I've critiqued the bridge in far more detail in the past.
I'm far less convinced by his argument that the expense isn't justified by the economic value that a landmark bridge will trigger. He says that comparisons with the Gateshead Millennium Bridge are invalid because it is a city centre bridge, while the Wear Crossing sits in an industrial estate. I can't see how that's relevant - the Wear bridge will be visible from miles around and its value is as a statement of intent, a trigger for investment which creates value through employment. It's not intended primarily as a visitor destination in its own right. Whether that investment will materialise may quite reasonably be doubted, but I don't imagine Bourne can claim to be an expert in the economics of regeneration. I suspect that the real reason he takes this view is simply the traditional, highly conservative bridge engineer's belief that extravagance is essentially immoral, a narrowly puritan perspective which prevents engineers from contributing more meaningfully into wider cultural discussions on the value of aesthetics.
I've discussed in the past how rarely bridge engineers are willing to publicly criticise the work of their peers in the industry. In that respect, Bourne's intervention is a positive thing, as there is far too little debate on the merits of designs such as this. The NCE quote Sunderland's River Wear project director as saying the bridge has been "rigorously designed, costed, admired and backed within the industry and profession", a statement of such deep complacency that it positively demands more critics to clamber out of the woodwork, brandishing angry manifestoes reading "form follows function" and so on.
Unfortunately, Bourne's complaint also falls into the too little, too late type of criticism which comes at a stage in the project where it is largely pointless. It has been clear throughout the River Wear saga that the promoters are wedded so deeply to their vision that they are happy to ignore opinion, whether expert or non-expert. I recall in particular their reliance on a public survey expressing support for a design, when a much larger survey showed that at least half wanted to avoid expense and favoured a "tried and tested" solution instead. Sunderland have had the design reviewed by experts who agree that it can be built. The most serious test of its feasibility will come when contractors' tenders are finally submitted, and we discover whether they share the client's expectations as to what it will cost.
16 May 2012
Kent Bridges: 5. Eureka Skyway
This footbridge is a very marked contrast to the one at Swanscombe Cutting that I covered recently.
First, it has a silly name, the "Eureka Skyway". I was just going to call it the M20 Ashford Footbridge, as that's what it crosses and where it is, but I've decided to use its official name for the blog post title. It links two retail parks either side of the motorway, and also acts as a gateway to Ashford itself, an indication to motorway users that perhaps something of significance can be found here.
The £8m bridge was designed by Nicol Russell Studios with Jacobs, and built by BAM Nuttall. It was installed in May 2011 and opened in September 2011. There were rumours before it opened that it suffered from "wobble", but I could feel no evidence of that when I visited earlier this year.
Three immediate precedents come to mind when viewing the Ashford bridge: Lancaster's Lune Millennium Bridge (2001), and Newport's City Footbridge (2006) are two. Closer to Ashford, Maidstone's Lockmeadow Footbridge (1999) has a similar form. All four bridges share a resemblance to a giant crane, with twin masts tied together with cables, and a deck supported from cable stays. There are some structural advantages to this arrangement, chiefly that the angle of the cables supporting the deck is steeper, and hence they provide a stiffer and more efficient support. A significant disadvantage is that maintenance is more difficult, as cables are required with no low-level termination, making it more difficult to adjust or replace them in the future.
I saw preliminary visuals for this bridge some time ago, and you can find them on the architect's website. I thought it looked quite nice, appealing in its height and slenderness. In real life, I found the sheer scale of the bridge to be much more difficult to accept. The bridge deck is roughly 100m long, with a 67m clear span. The masts are 38m high. It's nowhere near as big as the Newport bridge (70m tall, 145m main span), but it's still a very large structure to span a motorway. The span length is driven by the presence of two motorway slip roads, and a watercourse, forcing the bridge to be much larger than is normally required on motorways.
The bridge looks attractive enough when viewed from the motorway, but as a pedestrian it is quite overpowering. The masts tower far above you, and their angle of inclination makes them loom in a way that I don't recall experiencing with vertical masts.
What seems like a simple enough cable system when viewed in elevation becomes not only complex but positively confusing from most other perspectives. This is true of the forestays, but doubly so for the backstays. As well as tying the masts back to anchorage foundations, these also support a secondary deck, which curves below the main deck and provides part of a shallow gradient ramp access to the bridge's north end. The end result is a web of cables with little apparent visual order.
In the main span, the confusion is partly caused by the presence of a set of tie-down cables, two on each side of the deck, which anchor the foremast to the foundations. These are an odd presence on an asymmetric cable-stayed bridge, where normally the anchor cables are only required at the "rear end" of the bridge. Here, I guess the main deck is insufficiently heavy to hold the main masts in place on its own, or they are needed because the masts are offset to the side of the deck, or they minimise movement and vibration in the masts. Compare the Swansea Sail Bridge as an example of a cable-stayed bridge with an offset mast which doesn't need to be tied down in this way (and where the mast has been more artfully shaped).
Below deck, the oddest feature is a Y-shaped strut which holds the deck in place, presumably to restrain either vertical or lateral movement, or both. It has a sort of "tacked-on" feeling. My initial thought was that a corbel from the masts would have been better, but it's not clear which mast you would add the corbel too, and what the effect on structural behaviour would have been. The view from below also highlights the somewhat rudimentary details where the cables are anchored to the deck.
The approach ramps at each end of the bridge are also very heavy, largely because of their height above the surrounding ground. There's a stone-walled ramp at the south end, and an earth mound at the north end, into which stairs and a spiral ramp are cut. Growth of landscaping over time will help soften these, but they weigh the bridge down rather than allowing its slenderness to float free.
Overall, it's a bridge which had the potential to be great, but which is let down by the awkward resolution of many of the details. Notwithstanding the funder's desire for a gateway structure, I found the sheer scale to be oppressive. The bridge at Swanscombe spanned the motorway with delicacy and modesty - the one at Ashford has neither.
Further information:
First, it has a silly name, the "Eureka Skyway". I was just going to call it the M20 Ashford Footbridge, as that's what it crosses and where it is, but I've decided to use its official name for the blog post title. It links two retail parks either side of the motorway, and also acts as a gateway to Ashford itself, an indication to motorway users that perhaps something of significance can be found here.
The £8m bridge was designed by Nicol Russell Studios with Jacobs, and built by BAM Nuttall. It was installed in May 2011 and opened in September 2011. There were rumours before it opened that it suffered from "wobble", but I could feel no evidence of that when I visited earlier this year.
Three immediate precedents come to mind when viewing the Ashford bridge: Lancaster's Lune Millennium Bridge (2001), and Newport's City Footbridge (2006) are two. Closer to Ashford, Maidstone's Lockmeadow Footbridge (1999) has a similar form. All four bridges share a resemblance to a giant crane, with twin masts tied together with cables, and a deck supported from cable stays. There are some structural advantages to this arrangement, chiefly that the angle of the cables supporting the deck is steeper, and hence they provide a stiffer and more efficient support. A significant disadvantage is that maintenance is more difficult, as cables are required with no low-level termination, making it more difficult to adjust or replace them in the future.
The bridge looks attractive enough when viewed from the motorway, but as a pedestrian it is quite overpowering. The masts tower far above you, and their angle of inclination makes them loom in a way that I don't recall experiencing with vertical masts.
Overall, it's a bridge which had the potential to be great, but which is let down by the awkward resolution of many of the details. Notwithstanding the funder's desire for a gateway structure, I found the sheer scale to be oppressive. The bridge at Swanscombe spanned the motorway with delicacy and modesty - the one at Ashford has neither.
Further information:
- Google maps / Bing maps (both views pre-date construction of the bridge)
- Construction video
03 May 2012
Kent Bridges: 4. Swanscombe Cutting Footbridge
I visited Kent earlier this year, and took in a couple of bridges while there. Both are pedestrian bridges spanning over a major highway, but very different in conception.
The first dates from 1965, and spans the A2 highway, which follows the route of the ancient Watling Street. Approached from the east, the road rises up across the landscape before slicing through the hillside in the Swanscombe Cutting, and it's not dissimilar approached from the west. The cutting is therefore at the high point of the road, and the bridge that spans it is a key marker on the route. As a note on the Concrete Bridge Development Group website states, "the scene is set for both drama and beauty".
The bridge was designed under the direction of H Bowdler, County Surveyor of Kent, and J A Bergg, County Bridge Engineer. It was built by Leonard Fairclough Construction at a cost of £17,000. In Bergg's paper covering a number of bridges built in Kent in the 1960s, he reports the cost to equate to £116 per square metre of deck.
The structure spans 48.77m, being 2.3m wide between railings, and 89.5m long in total. The arch rises 7.83m. Triangular abutments are buried in the cutting slopes, approximately 8m tall, which provide combined support to both the arch and the deck arms.
The bridge was Listed Grade II in 1998. I'd be interested to know whether any other bridges built this recently in the UK have been given similar protected status! It certainly seems remarkable, but then, it is clearly a remarkable bridge.
Structurally, the bridge is a descendant of Robert Maillart's 1906 Tavanasa Bridge, a three-pinned concrete arch, with propped cantilevers for the bridge deck. All parts of the bridge taper towards a hinge - the arch members to hinges at their base and crown, and the deck to the support at their ends. At the thinnest points the bridge is a mere 0.36m thick.
The bridge at Swanscombe is far more slender than even Maillart could achieve. It represents what Maillart could have done had he collaborated in later life with his contemporary, Eugène Freyssinet. Maillart worked only with reinforced concrete, but it's Freyssinet's pioneering of prestressed concrete which ultimately allowed the bridge at Swanscombe to be so slender. The deck and arch at Swanscombe were both prestressed using the CCL system.
The main deck has a roughly 1m wide central prestressed spine beam, with the deck slab cantilevering either side. The deck is so thin at the arch crown that it entirely disappears into shadow below the deck cantilevers, an effect which is visually a little disconcerting as the visual continuity of the arch soffit can be lost.
It's not an easy bridge to photograph, thanks to the extensive tree growth along the cutting slopes. There's a photo in Bridges of Britain which shows the bridge with bare slopes, which really shows the structure at its best. I can't help but think that if this bridge spanned across a Swiss gorge rather than an English dual carriageway, it would be far better known, and properly recognised in the bridges literature. As it is, even Henry and Jerome's Modern British Bridges, published in 1965 and intended to recognise the best modern bridges of its time, fails to include it.
Perhaps the only criticism I would offer of this bridge is a personal one. The parapets provide an effectively minimalist solution, largely eliminating their visual impact on the bridge's elevation. However, they did little to calm my sense of vertigo and exposure while crossing the bridge. How much of this was the normal experience of crossing a busy main road, and how much due to the bridge's setting, I couldn't tell.
Swanscombe Cutting Footbridge is far from the only ultra-slender concrete footbridge built in Kent under Bergg's direction, and hopefully I will get a chance to visit some of the others in the future.
Further information:
The first dates from 1965, and spans the A2 highway, which follows the route of the ancient Watling Street. Approached from the east, the road rises up across the landscape before slicing through the hillside in the Swanscombe Cutting, and it's not dissimilar approached from the west. The cutting is therefore at the high point of the road, and the bridge that spans it is a key marker on the route. As a note on the Concrete Bridge Development Group website states, "the scene is set for both drama and beauty".
The bridge was designed under the direction of H Bowdler, County Surveyor of Kent, and J A Bergg, County Bridge Engineer. It was built by Leonard Fairclough Construction at a cost of £17,000. In Bergg's paper covering a number of bridges built in Kent in the 1960s, he reports the cost to equate to £116 per square metre of deck.
The structure spans 48.77m, being 2.3m wide between railings, and 89.5m long in total. The arch rises 7.83m. Triangular abutments are buried in the cutting slopes, approximately 8m tall, which provide combined support to both the arch and the deck arms.
The bridge was Listed Grade II in 1998. I'd be interested to know whether any other bridges built this recently in the UK have been given similar protected status! It certainly seems remarkable, but then, it is clearly a remarkable bridge.
Structurally, the bridge is a descendant of Robert Maillart's 1906 Tavanasa Bridge, a three-pinned concrete arch, with propped cantilevers for the bridge deck. All parts of the bridge taper towards a hinge - the arch members to hinges at their base and crown, and the deck to the support at their ends. At the thinnest points the bridge is a mere 0.36m thick.
The bridge at Swanscombe is far more slender than even Maillart could achieve. It represents what Maillart could have done had he collaborated in later life with his contemporary, Eugène Freyssinet. Maillart worked only with reinforced concrete, but it's Freyssinet's pioneering of prestressed concrete which ultimately allowed the bridge at Swanscombe to be so slender. The deck and arch at Swanscombe were both prestressed using the CCL system.The main deck has a roughly 1m wide central prestressed spine beam, with the deck slab cantilevering either side. The deck is so thin at the arch crown that it entirely disappears into shadow below the deck cantilevers, an effect which is visually a little disconcerting as the visual continuity of the arch soffit can be lost.
It's not an easy bridge to photograph, thanks to the extensive tree growth along the cutting slopes. There's a photo in Bridges of Britain which shows the bridge with bare slopes, which really shows the structure at its best. I can't help but think that if this bridge spanned across a Swiss gorge rather than an English dual carriageway, it would be far better known, and properly recognised in the bridges literature. As it is, even Henry and Jerome's Modern British Bridges, published in 1965 and intended to recognise the best modern bridges of its time, fails to include it.
Perhaps the only criticism I would offer of this bridge is a personal one. The parapets provide an effectively minimalist solution, largely eliminating their visual impact on the bridge's elevation. However, they did little to calm my sense of vertigo and exposure while crossing the bridge. How much of this was the normal experience of crossing a busy main road, and how much due to the bridge's setting, I couldn't tell.Swanscombe Cutting Footbridge is far from the only ultra-slender concrete footbridge built in Kent under Bergg's direction, and hopefully I will get a chance to visit some of the others in the future.
Further information:
- Google maps / Google street view / Bing maps
- British Listed Buildings
- Eighty Highway Bridges in Kent (Bergg, ICE Proceedings, 1973)
- Bridges of Britain (de Maré, 1975)
02 May 2012
China Bridges Trip 2012
Here's one for the wealthy (and slightly nutty) of you.
Erik Sakowski, creator of the truly mighty highestbridges.com website, is organising a 3-week tour of bridges along China's Yangtze river. The itinerary will take in 5 of the world's 10 longest suspension bridges, 5 of the world's 10 longest cable-stayed bridges, 5 of the world's 10 longest arch bridges, and many more superlative-loaded treats for the ambitious pontist.
It's a genuine snip at an all-inclusive US$3,950 (excluding air fares to and from Shanghai), and you just need to read the preview to see how amazing it will be. If that's not convincing enough, spend a good hour looking over Eric's mouth-watering photos from his 2011 China bridge trip. Hopefully he won't mind that I've borrowed one of his 2011 photos below to give you a flavour.
Erik Sakowski, creator of the truly mighty highestbridges.com website, is organising a 3-week tour of bridges along China's Yangtze river. The itinerary will take in 5 of the world's 10 longest suspension bridges, 5 of the world's 10 longest cable-stayed bridges, 5 of the world's 10 longest arch bridges, and many more superlative-loaded treats for the ambitious pontist.
It's a genuine snip at an all-inclusive US$3,950 (excluding air fares to and from Shanghai), and you just need to read the preview to see how amazing it will be. If that's not convincing enough, spend a good hour looking over Eric's mouth-watering photos from his 2011 China bridge trip. Hopefully he won't mind that I've borrowed one of his 2011 photos below to give you a flavour.
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