If you're attending the conference, and want to read more about any of the bridges featured in the booklet, just google "Happy Pontist" and the name of the bridge, and you should find a link to the relevant post.
16 July 2014
Pontist in print
I'm currently visiting Footbridge 2014, and very pleased to note that the Happy Pontist is now in print for the first time. All the attendees at the event are being provided with a short guide to the pedestrian bridges of London in booklet form, with all the text and images provided from past blog posts here (edited for length!)
If you're attending the conference, and want to read more about any of the bridges featured in the booklet, just google "Happy Pontist" and the name of the bridge, and you should find a link to the relevant post.
If you're attending the conference, and want to read more about any of the bridges featured in the booklet, just google "Happy Pontist" and the name of the bridge, and you should find a link to the relevant post.
07 July 2014
Yorkshire Bridges: 2. Droppingwell Footbridge
The Wentbridge Viaduct is only one of a number of aesthetically forthright, highly ingenious bridges to be built by the West Riding County Council during the 1960s as the motorway network was progressively expanded, forming what is now parts of the M1 and M62.
Under the ambitious County Surveyor, Colonel Stuart Maynard Lovell, the bridge team at West Riding was led by Norman Buchi. Colleagues in the team included Bill Varley, Ron Bridle, Sri Sriskandan and F.A. ("Joe") Sims. The team was responsible for the introduction of a great deal of new computing technology into bridge design, as well as for some of the most imaginative bridge engineering going on anywhere in the country. Their design efforts were supported by close involvement in research and testing work, for example, on half-joints and concrete hinges. All the above named engineers went on to considerable seniority, some in the Department of Transport, and Sims and Bridle in particular have published various papers and contributed to books on the history of Britain's motorway development.
Nonetheless, I'd be surprised if many of my readers have heard of them, which is a shame, as they deserve to be recognised for the many interesting and in some cases spectacular bridges that they designed. I'm going to cover several in the course of this series of posts, but there are many more in the area which I haven't yet had time to get to. In particular, I'll highlight the Brodsworth and Sprotborough Footbridges, built for the Doncaster Bypass, which are magnificently slender, with a span to depth ratio of an incredible 94:1. Perhaps I can cover those another time.
Droppingwell Footbridge spans 52m across the M1 between Sheffield and Leeds. The land to the east of the motorway sits above the roadway; while on the west it sits below, and the bridge therefore has to facilitate a very large change in level. It does this first with a gradient of 1 in 10 (described at the time as "for the easy pushing of prams", but far steeper than would now be allowed), and then with a spiral ramp at its western end.
The bridge is a 3-pinned reinforced concrete arch, broadly of the Maillart Tavanasa Footbridge type, although with an elongated profile more closely resembling the Swanscombe Cutting design built perhaps a year or two earlier in Kent.
The arch sections are hollow where they are thick, and solid where they are slender. The arch leg at the western side splits in two, for lateral stability. The hinge at the crown of the arch is prestressed, to assist with lateral stability and to prevent the hinge opening up under wind load.
Considering the severe geometric constraints placed upon the bridge, that it manages to retain a degree of elegance is a commendable achievement. Viewed along the motorway, the profile of the bridge is very fine, although sadly for reasons of health and safety I was unable to photograph the bridge from this direction (you may wish to try Google Street View).
It's less successful in many other ways, however. The forked pier, with its stiff triangular form, lacks the curvature and flow of the bridge's elevation. This detail has been done better elsewhere, and I think it's the forces from the ramp that led to the fork being so wide in this instance.
The side spans are on half-joints, and look a little clumsy, especially at the junction with the ramp structure, where the soffit line kinks in elevation. This whole area is badly detailed.
The positioning of one of the ramp's four concrete piers between the forked legs of the main span is also visually quite awkward - the two elements needed to be kept apart.
The similar bridge at Swanscome is Listed, Grade II, whereas the Droppingwell Footbridge, along with the other bridges I'm about to feature, has no heritage status. This is a sad omission, as these bridges are unique and innovative, and if not always beautiful, they were clearly designed with a strong aesthetic sensibility.
Further information:
Under the ambitious County Surveyor, Colonel Stuart Maynard Lovell, the bridge team at West Riding was led by Norman Buchi. Colleagues in the team included Bill Varley, Ron Bridle, Sri Sriskandan and F.A. ("Joe") Sims. The team was responsible for the introduction of a great deal of new computing technology into bridge design, as well as for some of the most imaginative bridge engineering going on anywhere in the country. Their design efforts were supported by close involvement in research and testing work, for example, on half-joints and concrete hinges. All the above named engineers went on to considerable seniority, some in the Department of Transport, and Sims and Bridle in particular have published various papers and contributed to books on the history of Britain's motorway development.
Nonetheless, I'd be surprised if many of my readers have heard of them, which is a shame, as they deserve to be recognised for the many interesting and in some cases spectacular bridges that they designed. I'm going to cover several in the course of this series of posts, but there are many more in the area which I haven't yet had time to get to. In particular, I'll highlight the Brodsworth and Sprotborough Footbridges, built for the Doncaster Bypass, which are magnificently slender, with a span to depth ratio of an incredible 94:1. Perhaps I can cover those another time.
Droppingwell Footbridge spans 52m across the M1 between Sheffield and Leeds. The land to the east of the motorway sits above the roadway; while on the west it sits below, and the bridge therefore has to facilitate a very large change in level. It does this first with a gradient of 1 in 10 (described at the time as "for the easy pushing of prams", but far steeper than would now be allowed), and then with a spiral ramp at its western end.
The bridge is a 3-pinned reinforced concrete arch, broadly of the Maillart Tavanasa Footbridge type, although with an elongated profile more closely resembling the Swanscombe Cutting design built perhaps a year or two earlier in Kent.
The arch sections are hollow where they are thick, and solid where they are slender. The arch leg at the western side splits in two, for lateral stability. The hinge at the crown of the arch is prestressed, to assist with lateral stability and to prevent the hinge opening up under wind load.
Considering the severe geometric constraints placed upon the bridge, that it manages to retain a degree of elegance is a commendable achievement. Viewed along the motorway, the profile of the bridge is very fine, although sadly for reasons of health and safety I was unable to photograph the bridge from this direction (you may wish to try Google Street View).
It's less successful in many other ways, however. The forked pier, with its stiff triangular form, lacks the curvature and flow of the bridge's elevation. This detail has been done better elsewhere, and I think it's the forces from the ramp that led to the fork being so wide in this instance.
The side spans are on half-joints, and look a little clumsy, especially at the junction with the ramp structure, where the soffit line kinks in elevation. This whole area is badly detailed.
The positioning of one of the ramp's four concrete piers between the forked legs of the main span is also visually quite awkward - the two elements needed to be kept apart.
The similar bridge at Swanscome is Listed, Grade II, whereas the Droppingwell Footbridge, along with the other bridges I'm about to feature, has no heritage status. This is a sad omission, as these bridges are unique and innovative, and if not always beautiful, they were clearly designed with a strong aesthetic sensibility.
Further information:
- Google maps / Bing maps
- The Motorway Archive
- Bridges on the Ashton - Sheffield - Leeds Motorway (Concrete Quarterly, No.80, 1969)
- An Encyclopaedia of Britain's Bridges (McFetrich, 2010)
04 July 2014
Yorkshire Bridges: 1. Wentbridge Viaduct
I spent a day earlier this year in the area between Sheffield and Leeds, touring a number of very interesting bridges, principally under and over the local motorways. I've been to some interesting modern bridges in Yorkshire before (the York Millennium Bridge and the Castleford Footbridge), but this was a chance to take more of a trip back in time, roughly half a century to be precise.
Wentbridge Viaduct was built as part of improvements to the A1 in 1961. It cost £320,000 when built, and has a central span of 94m (between the pier bases). Design is usuallly credited to the West Riding County Council's County Engineer, S. Maynard Lovell, although the lead designer was more probably F.A. Sims.
The deck is of post-tensioned concrete, with six box cells, and sits on inclined cellular reinforced concrete legs which are pinned at top and bottom, with precast concrete hinges. The prestressing cables are external to the box webs, and are continuous over the full 143m length of the deck. This was the first use of external prestressing in the UK. Additional internal cables are provided in the top slab over the piers, and vertical prestressing is used to counteract shear in the side spans.
The bridge was recognised for its significance at the time it was built. It was one of only two British bridges selected to feature in the Twentieth Century Engineering exhibition at New York's Museum of Modern Art (the other was the Hammersmith Flyover). The image on the left was taken shortly after the bridge was built, and shows it at its majestic best.
Concrete Quarterly was almost rhapsodic:
Since 1998, its significance has been recognised by its status as a heritage-protected Listed Building, a rare accolade for such a modern structure.
It's difficult to see a bridge like this being built again, certainly in Britain. Much of the cost will have been in the temporary works, particularly the forest of scaffolding required, which was pictured in Modern British Bridges, and is shown here. Any bridge of this scale would almost certainly now be built in steel, and probably on much cheaper vertical concrete piers, regardless of the aesthetic merits.
I didn't visit on the brightest day, and growth of vegetation in the river valley over time has made the bridge hard to photograph in its entirety. The bridge is massive in size and presence, but delightful in its stark lack of embellishment. Only a subtle curvature to the underside of the deck, and the minimal taper of the pier legs, give any sense that the bridge is more than a series of giant, monolithic paving slabs propped against each other.
A caged walkway has been added to the east face of the bridge, with very little care taken over its appearance. In parts, it projects below the deck soffit, so it can be seen from the other side of the bridge, spoiling the deck's clear lines. I guess this must have been installed before the bridge was Listed, because I can't see how it would be an acceptable addition to a Listed Building.
Further information:
Wentbridge Viaduct was built as part of improvements to the A1 in 1961. It cost £320,000 when built, and has a central span of 94m (between the pier bases). Design is usuallly credited to the West Riding County Council's County Engineer, S. Maynard Lovell, although the lead designer was more probably F.A. Sims.
The deck is of post-tensioned concrete, with six box cells, and sits on inclined cellular reinforced concrete legs which are pinned at top and bottom, with precast concrete hinges. The prestressing cables are external to the box webs, and are continuous over the full 143m length of the deck. This was the first use of external prestressing in the UK. Additional internal cables are provided in the top slab over the piers, and vertical prestressing is used to counteract shear in the side spans.The bridge was recognised for its significance at the time it was built. It was one of only two British bridges selected to feature in the Twentieth Century Engineering exhibition at New York's Museum of Modern Art (the other was the Hammersmith Flyover). The image on the left was taken shortly after the bridge was built, and shows it at its majestic best.Concrete Quarterly was almost rhapsodic:
"At last Britain has a bridge to show which, by virtue of its dramatic impact, its taut, pared-down elegance, its sheer size, can hold its own with any of the great bridges on the Continent - a bridge of a Maillart-like excitement, that even the Autostrada del sole cannot better".
Since 1998, its significance has been recognised by its status as a heritage-protected Listed Building, a rare accolade for such a modern structure.
It's difficult to see a bridge like this being built again, certainly in Britain. Much of the cost will have been in the temporary works, particularly the forest of scaffolding required, which was pictured in Modern British Bridges, and is shown here. Any bridge of this scale would almost certainly now be built in steel, and probably on much cheaper vertical concrete piers, regardless of the aesthetic merits.
Further information:
- Google maps / Bing maps
- Structurae
- British Listed Buildings
- The Motorway Archive (contains considerable detail on design and construction)
- The Wentbridge Viaduct (Concrete Quarterly No. 52, 1962)
- Twentieth Century Engineering
(MoMA, 1964)
- Modern British Bridges (Henry and Jerome, 1965)
- An Encyclopaedia of Britain's Bridges (McFetrich, 2010)
30 June 2014
Bridges news roundup
I'm preparing some posts on groups of bridges I visited earlier this year in Yorkshire and in Lancashire, as well as on the Foryd Harbour Bridge. Meanwhile ...
Footbridge 2014
The Happy Pontist will be attending this conference from July 16-18 in London. The final programme has been released, and looks really interesting, so I'm looking forward to it.
Tees Transporter Bridge
A new book is being published on this bridge, and you can read a short sample online. I visited the bridge last year.
Taminabruecke
Very thorough website telling you more than you could possibly want to know about what will apparently be Europe's longest arch bridge, currently under construction.
New St Elmo Breakwater Footbridge
Of various bridges showcased on architectural websites recently, this one stood out for me as one of the most interesting.
London's Garden Bridge: "It feels like we're trying to pull off a crime"
It feels like that because you are trying to pull off a crime. Please hand yourself into the authorities immediately! The planning application is available online, and reveals the bridge to be a steel truss wrapped in ultra-expensive bronzed steel, rather than the pair of concrete flower-pots that I had expected. Please place some cushions on the floor around your chair before you read it.
Footbridge 2014
The Happy Pontist will be attending this conference from July 16-18 in London. The final programme has been released, and looks really interesting, so I'm looking forward to it.
Tees Transporter Bridge
A new book is being published on this bridge, and you can read a short sample online. I visited the bridge last year.
Taminabruecke
Very thorough website telling you more than you could possibly want to know about what will apparently be Europe's longest arch bridge, currently under construction.
New St Elmo Breakwater Footbridge
Of various bridges showcased on architectural websites recently, this one stood out for me as one of the most interesting.
London's Garden Bridge: "It feels like we're trying to pull off a crime"
It feels like that because you are trying to pull off a crime. Please hand yourself into the authorities immediately! The planning application is available online, and reveals the bridge to be a steel truss wrapped in ultra-expensive bronzed steel, rather than the pair of concrete flower-pots that I had expected. Please place some cushions on the floor around your chair before you read it.
26 June 2014
Australian Bridges: 3. Sydney Harbour Bridge
What can I possibly add to the volumes that have already been written about the Sydney Harbour Bridge?
Well, I have selected nine photos from when I visited, so here are nine thoughts (in no particular order, and not especially related to each photo!)
Completed in 1932, spanning 503m, Sydney Harbour Bridge was never the longest arch bridge in the world, but it was by far the widest and heaviest. It was pipped to the record by the Bayonne Bridge, opened in 1931, and which spans 25 inches longer. However, Sydney's bridge is probably still the most iconic of the world's long span arch bridges, situated in a beautiful harbour, and forming a key landmark visible from much of the surrounding city.
Cantilever construction of large arch bridges was a common method at the time (and since), although now it would be recognised as highly inefficient (a cable-stay bridge uses its temporary erection cables as part of its permanent structure, so is almost always more efficient, as is a suspension bridge at longer spans). Indeed, other engineers commented on this at the time. David Steinman, who had submitted a design for a losing tenderer, considered that a suspension bridge should have been cheaper, and noted that Dorman Long were reported to have lost a million dollars on the contract, suggesting the arch option was under-priced.
Steinman also commented that the heavy temporary cables would have been sufficient to build a suspension bridge. Ironically, some of the temporary cables were indeed used for precisely that purpose, to build the Walter Taylor Bridge at Indooroopilly in Brisbane in 1936. Again ironically, this was one of the very few bridges in the world to copy Steinman's "Florianopolis" bridge design, an unusual trussed suspension bridge which never gained wide favour.
The design of the Sydney Harbour Bridge was controversial. A huge disagreement arose between J.J.C. Bradfield, who had conceived the bridge, and Dorman Long's designer Sir Ralph Freeman, who was responsible for the detailed design and for all the construction drawings. Bradfield claimed to be the designer, and went so far as to entirely omit any mention of Freeman in various articles, including his technical paper for the Proceedings of the ICE. Freeman, in turn, campaigned in the Sydney press for his role to be recognised, with the full support of Dorman Long. The contractor even threatened to sue the New South Wales government if Freeman's role wasn't properly credited on a plaque to be placed on the bridge.
The entire controversy is discussed in great detail in Peter Lalor's excellent book, The Bridge. In the discussion in the ICE Proceedings on the various technical papers by Bradfield and Freeman, Oscar Faber is recorded making the point that Freeman appeared to have received too little credit. Faber and several others also spoke against the design of the bridge pylons, which were regarded by many as an unnecessary extravagance.
The bridge must be one of few large bridges in the world where the public can gain access to climb the structure, albeit for a hefty fee. BridgeClimb is one of the most popular tourist highlights in Sydney, and I took advantage of it when I visited. It must make considerable profit for both the bridge owner and for BridgeClimb, as when I was there, at the height of the Australian summer, groups of visitors could be seen walking up the bridge on a very regular basis.
The BridgeClimb experience is very well organised: visitors are thoroughly briefed on what to expect, and even get to climb a trial set of staircases before being allowed onto the bridge itself. The safety arrangements are impressive, with "climbers" hooked onto a static line constantly from the beginning of their experience to the end. The scariest part of the experience is passing across the approach spans towards the pylon, where the proximity of the roadway directly below the catwalk is a little disconcerting. Once past the main pylons and onto the main span, the climb is straightforward, and not at all terrifying. The large width of the main arch chords is such that you never feel close enough to an edge to feel at all worried.
For anyone wanting to know more about the bridge, I can recommend Peter Lalor's book (see below), which is from the perspective of a journalist, concentrating on the human-interest stories, and Peter Spearritt's (also see below), from the perspective of a historian, very well illustrated and placing the bridge narrative in more of a social context.
Further information:
Well, I have selected nine photos from when I visited, so here are nine thoughts (in no particular order, and not especially related to each photo!)
Completed in 1932, spanning 503m, Sydney Harbour Bridge was never the longest arch bridge in the world, but it was by far the widest and heaviest. It was pipped to the record by the Bayonne Bridge, opened in 1931, and which spans 25 inches longer. However, Sydney's bridge is probably still the most iconic of the world's long span arch bridges, situated in a beautiful harbour, and forming a key landmark visible from much of the surrounding city.
Sydney's chief engineer, J.J.C. Bradfield, had originally proposed a cantilever bridge for the site. A tour of bridges around the world convinced him that a steel arch might be more efficient, and he prepared alternative plans based on the Hell Gate Bridge in New York. Both bridges are two-pinned trussed arches, with monolithic and essentially non-structural pylons abutting the steel arch.
There are subtle differences. The Hell Gate Bridge has a more visible reverse curvature towards the upper ends of the arch, and its top chord disappears within the bridge pylons, rather than stopping short as is the case in Sydney. The upper chord carries no significant force at its ends, so the Sydney Bridge is more honest in this respect, but I think it looks quite odd.
A further difference between Hell Gate Bridge and the Sydney Harbour Bridge is in the approach spans. Hell Gate has girder spans of similar depth to the main span, with a continuity of road deck line which the pylons don't greatly interrupt. Sydney's bridge has very deep deck trusses for the approach spans, and its pylons serve a useful purpose in distracting the eye from the very different structural forms on either side.
The bridge arch is two-pinned, quite literally supported on a pin at each end. The entire weight of the bridge and the road, rail and pedestrian traffic that it carries is passed through the bottom chord into these pins. What then, is the point of the upper chord of the arch truss? The deep trusses are needed to stiffen the arch against buckling, particularly from unevenly distributed loading. However, they could converge to a point at the ends, a "crescent arch", as was the case for the much smaller Tyne Bridge, built by the same contractor, Dorman Long, in 1928.
I think the reason for the difference lies in the method of construction. Sydney Harbour Bridge was built by cantilevering from each bank of the harbour, with the steelwork tied back by massive arrays of temporary cables anchored into bedrock.
In its temporary state, the bridge doesn't behave as an arch at all, but as a cantilever bridge of the type that Bradfield eventually rejected. As such, it relies on its strength in bending to stand up, and this is why the arch truss is so deep towards its ends. It had to resist enormous bending during construction, even if this meant that the top chord at the ends became largely redundant in service. While a crescent arch might be strong enough to be built by cantilevering at smaller spans, it would simply not be deep enough and strong enough for a span of this magnitude.
The entire form of the iconic Sydney Harbour Bridge is therefore essentially a relic of its process of making.
The debate on the type of arch which should have been used was rehearsed by other engineers at the time, and I'd thoroughly recommend the discussions of the papers presented to the ICE. These cover a wider variety of procurement and technical issues which remain relevant today.
The debate on the type of arch which should have been used was rehearsed by other engineers at the time, and I'd thoroughly recommend the discussions of the papers presented to the ICE. These cover a wider variety of procurement and technical issues which remain relevant today.
Steinman also commented that the heavy temporary cables would have been sufficient to build a suspension bridge. Ironically, some of the temporary cables were indeed used for precisely that purpose, to build the Walter Taylor Bridge at Indooroopilly in Brisbane in 1936. Again ironically, this was one of the very few bridges in the world to copy Steinman's "Florianopolis" bridge design, an unusual trussed suspension bridge which never gained wide favour.
The design of the Sydney Harbour Bridge was controversial. A huge disagreement arose between J.J.C. Bradfield, who had conceived the bridge, and Dorman Long's designer Sir Ralph Freeman, who was responsible for the detailed design and for all the construction drawings. Bradfield claimed to be the designer, and went so far as to entirely omit any mention of Freeman in various articles, including his technical paper for the Proceedings of the ICE. Freeman, in turn, campaigned in the Sydney press for his role to be recognised, with the full support of Dorman Long. The contractor even threatened to sue the New South Wales government if Freeman's role wasn't properly credited on a plaque to be placed on the bridge.
The entire controversy is discussed in great detail in Peter Lalor's excellent book, The Bridge. In the discussion in the ICE Proceedings on the various technical papers by Bradfield and Freeman, Oscar Faber is recorded making the point that Freeman appeared to have received too little credit. Faber and several others also spoke against the design of the bridge pylons, which were regarded by many as an unnecessary extravagance.
The bridge must be one of few large bridges in the world where the public can gain access to climb the structure, albeit for a hefty fee. BridgeClimb is one of the most popular tourist highlights in Sydney, and I took advantage of it when I visited. It must make considerable profit for both the bridge owner and for BridgeClimb, as when I was there, at the height of the Australian summer, groups of visitors could be seen walking up the bridge on a very regular basis.
The BridgeClimb experience is very well organised: visitors are thoroughly briefed on what to expect, and even get to climb a trial set of staircases before being allowed onto the bridge itself. The safety arrangements are impressive, with "climbers" hooked onto a static line constantly from the beginning of their experience to the end. The scariest part of the experience is passing across the approach spans towards the pylon, where the proximity of the roadway directly below the catwalk is a little disconcerting. Once past the main pylons and onto the main span, the climb is straightforward, and not at all terrifying. The large width of the main arch chords is such that you never feel close enough to an edge to feel at all worried.
For anyone wanting to know more about the bridge, I can recommend Peter Lalor's book (see below), which is from the perspective of a journalist, concentrating on the human-interest stories, and Peter Spearritt's (also see below), from the perspective of a historian, very well illustrated and placing the bridge narrative in more of a social context.
Further information:
- Google maps / Bing maps
- Wikipedia
- Structurae
- BridgeClimb
- Sydney Harbour Bridge: New South Wales government website
- Sydney Harbour Bridge Report on Tenders (Bradfield, 1924)
- The Sydney Harbour Bridge and Approaches (Bradfield, Proc. ICE, 1934)
- Sydney Harbour Bridge: Design of the Structure and Foundations (Freeman, Proc. ICE, 1934)
- Sydney Harbour Bridge: Calculations for the Steel Superstructure (Pain and Roberts, Proc. ICE, 1934)
- Sydney Harbour Bridge: Manufacture of the Structural Steelwork and Erection of the Bridge (Freeman and Ennis, Proc. ICE, 1934)
- Bridges and Their Builders
(Steinman, 1941)
- The Bridge
(Lalor, 2005)
- The Sydney Harbour Bridge: A Life
(Spearritt, 2012)
23 June 2014
Australian Bridges: 2. Illawarra Fly Treetop Walk
This isn't the first time I've featured an elevated walkway in this blog about bridges. It's not really a bridge, because it doesn't cross a particular obstacle.
Located above an escarpment some way to the south of Wollongong, New South Wales, the walkway provides views both into the rainforest canopy, and down from the escarpment across the lowlands towards the ocean. It's only part of a rainforest trail, but it stretches some 500m long, 25m above the ground. Completed in 2008, it cost about AUS$6.5m.
Despite the reported cost, it looks like it has been built on the cheap, with a mesh floor, open parapets, and somewhat industrial trusses below the walkway. It seems almost designed to enhance vertigo rather than to provide reassurance. There are two sections, one near each end of the walkway, which are cable-stayed cantilevers, and which sway a little alarmingly when loaded.
As if those weren't enough to deter the faint-hearted, a central tower rises to an even higher viewing platform, reached by a spiral staircase. When I was there, I saw at least one visitor unwilling to attempt the climb, and I didn't blame them.
Nonetheless, the walkway offers some fantastic views, not only across the landscape but also of the forest canopy, and down to the undergrowth, dominated by gigantic ferns.
Further Information:
Located above an escarpment some way to the south of Wollongong, New South Wales, the walkway provides views both into the rainforest canopy, and down from the escarpment across the lowlands towards the ocean. It's only part of a rainforest trail, but it stretches some 500m long, 25m above the ground. Completed in 2008, it cost about AUS$6.5m.
Despite the reported cost, it looks like it has been built on the cheap, with a mesh floor, open parapets, and somewhat industrial trusses below the walkway. It seems almost designed to enhance vertigo rather than to provide reassurance. There are two sections, one near each end of the walkway, which are cable-stayed cantilevers, and which sway a little alarmingly when loaded.
As if those weren't enough to deter the faint-hearted, a central tower rises to an even higher viewing platform, reached by a spiral staircase. When I was there, I saw at least one visitor unwilling to attempt the climb, and I didn't blame them.
Nonetheless, the walkway offers some fantastic views, not only across the landscape but also of the forest canopy, and down to the undergrowth, dominated by gigantic ferns.
19 June 2014
Australian Bridges: 1. Sea Cliff Bridge
I spent some time in Australia a few months ago, and while I didn't get time to visit many bridges, I did see a couple which are worth featuring here.
The Sea Cliff Bridge is on the coast of New South Wales, north of Wollongong. It was built in 2005 to replace a section of highway repeatedly affected by landslides and rockfalls. The bridge is 665m long, and represents a significant engineering achievement, not only for having been built at such an exposed location.
The southernmost section of the bridge is in the form of a post-tensioned balanced cantilever bridge, with 108m main spans. This is built on a varying curve in plan, meaning that every 5m long concrete box segment was different in curvature, angles, as well as in depth, with the spans varying from 6m deep over the piers to 2.5m at midspan.
The northern section of the bridge, approximately 200m long, has much shorter spans of 30m, and a constantly curved radius in plan of 150m. The form of the deck is different, comprising twin curved concrete beams, of constant depth, matching the depth of the balanced cantilever bridge where they join so that in elevation they appear to be continuous structure. The north part of the bridge was installed by incremental launching.
The combination of the two construction methodologies is unusual, and I'm impressed by the careful construction control that will have been required.
Despite the spectacular setting, the bridge is not especially exciting to drive over, as views off the highway are largely obscured by precast concrete barriers. It is better appreciated by walking down to the shoreline below. As well as the bridge itself, there are some impressively eroded rocks to see.
Further information:
The Sea Cliff Bridge is on the coast of New South Wales, north of Wollongong. It was built in 2005 to replace a section of highway repeatedly affected by landslides and rockfalls. The bridge is 665m long, and represents a significant engineering achievement, not only for having been built at such an exposed location.
The southernmost section of the bridge is in the form of a post-tensioned balanced cantilever bridge, with 108m main spans. This is built on a varying curve in plan, meaning that every 5m long concrete box segment was different in curvature, angles, as well as in depth, with the spans varying from 6m deep over the piers to 2.5m at midspan.
The northern section of the bridge, approximately 200m long, has much shorter spans of 30m, and a constantly curved radius in plan of 150m. The form of the deck is different, comprising twin curved concrete beams, of constant depth, matching the depth of the balanced cantilever bridge where they join so that in elevation they appear to be continuous structure. The north part of the bridge was installed by incremental launching.
The combination of the two construction methodologies is unusual, and I'm impressed by the careful construction control that will have been required.
Despite the spectacular setting, the bridge is not especially exciting to drive over, as views off the highway are largely obscured by precast concrete barriers. It is better appreciated by walking down to the shoreline below. As well as the bridge itself, there are some impressively eroded rocks to see.
Further information:
- Google maps / Bing maps
- Wikipedia
- seacliffbridge.com
- Structurae
- Sea Cliff Bridge, C+A Magazine, Issue 6
- Timelapse video of construction
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