Welsh Bridges: 23. Dernol Footbridge, River Wye

This was the third of three bridges that I tried to visit on the River Wye in Powys in August this year.

The first was barely a ghost, and the second a literal washout. Would it be third time lucky?

I parked up in a layby on the A470, north-east of the bridge, and followed a public right of way through the fields and downhill towards the site of the bridge.

A bridge! A palpable bridge!

Alan Crow's book Bridges on the River Wye indicates that a suspension footbridge was built here in 1975 by the Newbridge-on-Wye firm N.R. Hope. From the description, it is the same bridge that can be seen today, except that all or much of the fabric may have been renewed. Powys County Council tell me that the bridge was "refurbished" in 2018.

The bridge is of essentially the same design as the now-destroyed Cwmcoch footbridge, constructed by the same builder in 1967.

Timber towers sit on concrete footings, and support a steel tube or roller over which the main cables pass. Additional stay cables are attached to the rear face of the towers.

The main cables are anchored to steel cantilever beams at each end of the bridge. Three parapet wires on each side of the bridge are also tensioned against these cantilevers, and the tower stay cables are spliced into the lower of these parapet wires.

The bridge floor comprises three timber planks running longitudinally, sitting on transverse timber members. Every third cross-member also provides the support for braced parapet posts.

Below the bridge decking, six parallel wires run longitudinally, and these provide substantial support to the deck along most of its length, as there are no vertical hangers as you would expect on a conventional suspension bridge. The deck wires are anchored to steel cross-members attached to the main anchor stanchions.

The main suspension cables only directly support the middle part of the deck. The two cables pass below grooved longitudinal timber members on each edge of the bridge.

In the absence of the under-deck wires, this would be quite an unstable arrangement, as the central portion would "rock" longitudinally under load. The deck wires are therefore essential both to prevent that rocking, and to support the deck between the points where the main cables connect.

I've included a few more photographs to illustrate both the details and the setting of this attractive, economical bridge, and a video to show how much it moves under a single person load.

Further information:

• Google maps
• Bridges on the River Wye, Alan Crow, 1995

Welsh Bridges: 22. Cwmcoch Footbrige, River Wye

This is the next crossing of the River Wye upstream from  the Cwmbach Footbridge discussed in my previous post.

It was built in 1967 by N.R. Hope of Newbridge-on-Wye, a "swing" suspension bridge of total length around 53m. A wooden beam bridge had previously spanned the river here, before being destroyed in the 1960 floods.

If you were unaware that this bridge existed, you would never look for it. I parked in the entrance to a farm track (although I can't recommend this) on the A470, and walked downhill. There is also space to park on the minor road on the opposite side of the river.

And indeed ... there was no bridge to find. At least, not one that you can still cross.

Powys County Council have no record of the bridge, suggesting this may have been a private bridge as there is no recorded public right of way.

However, Alan Crow's book Bridges on the River Wye, indicates that the bridge was built by the council, after an application to the Ministry of Agriculture and Fisheries for a grant to build a vehicular bridge was rejected.

Considering that the remains of the bridge are still visible, attached to the western abutment, I guess that this bridge may have been destroyed in the floods of February 2020.

Peering closely at the debris of the bridge, it can be seen that there was a timber walkway, three planks wide, supported on timber cross-members. Below these are a set of six parallel wires, an arrangement similar to the Invermark Footbridge in Scotland. The remains of suspension towers and cables are also just about visible.

I have included a picture of this bridge from Crow's book, to show it as it once was.

Further information:

• Google maps
• Bridges on the River Wye, Alan Crow, 1995

Welsh Bridges: 21. Cwmbach Footbridge, River Wye

Back in August 2021 (yes, time has been moving slowly here at Pontist Towers), I had the chance to look for three interesting footbridges along the River Wye in Powys. I had discovered all three in Alan Crow's excellent 1995 book Bridges on the River Wye.

Unfortunately, my search was less successful than I had hoped!

To reach the first of these spans, I parked in a layby on the A470 (see Google Maps link below), then crossed the road and headed downhill towards the river. Slightly further south, a public right of way is marked on the OS map, but no bridge is shown.

Crow's book records that a "swing" bridge was built here in 1962, to replace a structure that had been destroyed by a flood in 1960. This is the term "swing" bridge in its colloquial sense, referring to a slightly unstable suspension bridge. It was demolished in 1991 on safety grounds, and a new bridge was installed in August 1994 by Tysons plc, of Liverpool.

This was the bridge I had come to see: an arched beam bridge constructed in laminated timber, described by Crow as "one of the most attractive small bridges on the River Wye".

Sadly, there was no bridge to be found. If you look very closely at my main photo, perhaps you can spot the abutments.

The concrete abutments are still there, as are the remnants of steel hinges attached to them. Originally, there was third hinge at the crown of the arch as well.

It's an attractive river setting, and it was disappointing there was no bridge.

Powys County Council have told me that the bridge was removed for safety reasons in 2013, "as it had reached the end of its working life". There are no plans to replace it. I imagine the public right of way saw very little use before anyway.

Since there was little else to see here, I have included an image from Alan Crow's book showing the previous bridges.

Further information:

• Google maps
• Bridges on the River Wye, Alan Crow, 1995

Welsh Bridges: 20. Llantysilio Chain Bridge

This bridges goes by a number of names - Berwyn Chain Bridge may be equally as appropriate. Signs at the site just call it "The Chain Bridge". It is neither a Listed Building nor a Scheduled Monument, which will only be surprising if you incorrectly imagine that our heritage bodies are competent.

The first bridge across the River Dee at this location was the work of local man Exuperius Pickering, variously described as an entrepreneur or a "coalmaster". Pickering was looking for a way to transport his coal and other materials between the Llangollen Canal (1808) and Telford's recently improved London to Holyhead Road, without paying tolls to cross Llangollen Bridge. Conceived in 1814, his bridge was completed in 1818.

This was a period of rapid development in cable or chain-supported bridges within the United Kingdom. Granted, the Winch Bridge, an iron chain catenary structure, had been built over the River Tees in 1741. However, it was the early 19th century when cable and chain bridges took off, with stayed bridges in Galashiels (1816), King's Meadow Bridge (1817) and Dryburgh Abbey Bridge (1817, rebuilt as a suspension bridge in 1818), and the Union Chain Bridge (1820, suspension bridge). Things advanced rapidly enough for Robert Stevenson to present an article surveying these and other designs in 1821, as well as proposing his own bridge at Cramond, an underspanned suspension bridge, which was never built.

Pickering's bridge sits right in the middle of this chronology. Happily for posterity, drawings of the bridge were made by the French traveller Joseph-Michel Dutens (see below). These show the bridge to be an underspanned suspension bridge, with eyebar chains supporting the deck, and an additional tension rod below this, perhaps to enhance stability. The bridges I mentioned above were well-reported, and it's often stated that Stevenson was the first to propose an underspanned suspension bridge, and James Smith's Micklewood Bridge (1831) the first to be built. In reality, Pickering got there first, although how much of an improvement his structure was over a simple catenary bridge might be doubtful.

The first drawing by Dutens shows half of the bridge (it was a three-span structure), while the second drawing gives cross-sections and details of the chains. A dozen chains passed below the bridge deck to provide support.

In addition to the drawings, photographs of Pickering's bridge survive, although showing it enhanced on one side by a timber truss.

The bridge lasted remarkably well, until it became unsafe and was dismantled in 1870. In 1876, Henry Robertson, owner of Brymbo Ironworks, rebuilt the three spans and re-used the original chains, again adopting the underspanned system (photograph below). This one was destroyed in flooding in 1928.

Roberton's son rebuilt the bridge the following year, but this time with only a single pier in the river. The chains were re-used, but now to form a suspension bridge, with three suspension chains on each edge, and two stiffening chains connected along the deck underneath.

One tower sits on an outcrop of rock within the river, and the other on the river wall at the north edge. The river tower was protected by a large concrete pier, rendering the new bridge far less susceptible to flood damage.

The chains at the south end of the bridge were anchored into the ground, while at the north end they pass over the adjacent Chainbridge Hotel and were anchored into rock high above the canal. The deck chains were anchored into the ground using an adjustable tensioning system.

A pair of bars hang downwards from each chain link, and these are connected to a triangulated system of lower hangers. These in turn carry the lower deck chains and the timber deck.

The bridge was load-tested with 45 people when it opened, and lasted reasonably well, becoming gradually more dilapidated until being closed as unsafe in 1984. In 2014-15, it was completely refurbished, with all the metalwork carefully dismantled and then reinstated.

The works were completed by local firm Shemec Ltd to a design by consultants Ramboll. The engineers completed a careful structural assessment of the bridge, determining that even if corroded ironwork was replaced, it could not carry anywhere near modern loading requirements, being limited to 1.5 kPa of load. This equates to roughly 5 tonnes of load on the 24m main span, or around 60 people. Llangollen Town Council, who had taken over responsibility for the bridge, agreed that this was sufficient. Warning signs at the end of the bridge request that no more than ten people use it at once.

The reconstruction works are well documented in a paper by Ramboll and in photos on the Chain Bridge Project website. I'm not clear what proportion of the original metalwork was preserved and reused, but new pieces were fabricated in mild steel to match the existing details and dimensions wherever any piece could not be reused. All the chain pins had to be replaced. Nonetheless, in the rebuilt bridge it is claimed that these are the oldest bridge suspension chains in Britain to remain in use.

Prior to the refurbishment, there was no parapet remaining on the bridge. The reconstruction introduced a series of new parapet posts, a tensioned upper cable, and a mesh infill system. I'm not sure how well these match any parapet that had been there in the past, but I doubt the new system is compliant with normal modern standards.

Indeed it's interesting to compare the refurbishment work at Llantysilio with what was done at Brabyns Park Bridge in Marple, which I discussed in a recent post. The chain bridge project is an exemplary piece of conservation engineering, where even though the structure is not Listed, it has been treated with integrity and the original details preserved as closely as possible. The engineers sensibly recognised that compliance with modern standards would have been entirely inappropriate. By contrast, the Marple structure is Listed Grade II, but senseless attempts to impose modern standards on it have largely ruined its appearance (although thankfully not irreversibly).

The Llantysilio Chain Bridge is unique both in the complex history of its surviving structural fabric, and in its form and details. It is well worth visiting, in a setting within view of two other fine bridges, and with plenty more to see within walking distance.

Further information:

• Google maps
• Wikipedia
• Engineering Timelines
• Structurae
• Grace's Guide
• Coflein
• People's Collection Wales
• Chain Bridge Project
• The Chain Bridge Documentary (video)
• Plas Kynaston Canal Group
• The Refurbishment of Llangollen Chainbridge (Marginson / Matthews, Footbridge 2017)

Welsh Bridges: 19. Lôn Las Ogwen Footbridge

Not far from the Britannia and Menai Bridges, the dedicated pontist may happen upon this lesser-known footbridge.

It carries the Lôn Las Ogwen, a walking and cycling route, over the A4244 highway. The trail follows the line of the former Penrhyn Quarry Railway, which was closed in 1962.

The footbridge diverts from the original line of the railway, presumably to allow a small railway junkyard to be preserved on the south abutment of the original railway bridge.

I don't know who designed the bridge, possibly local consultancy YGC, but it was fabricated by D. Hughes Welding and Fabrications, and built by contractor Mulcair Ltd. At a guess, the main span probably doesn't exceed 20m.

At first glance, it's a steel arch bridge with a rather chunky looking parapet, decked out in the patriotic Welsh colours of green, white and red.

A second look makes clear that it is, as the fabricator says on their website, "a Vierendeel Construction with a Decorative Arch".

Opinions on this may vary. Some may note that it is just another in a long line of fake arch bridges, and hardly as egregious as some examples. Others may wonder if the emphasis on superficiality over substance combines with the colouring to act as a sly post-modern comment upon the inherent hollowness of nationalism.

I'm not sure I would go that far, but I can say that I don't like it.

Further information:

• Google maps
• D. Hughes Welding (including lots of construction photographs)

Welsh Bridges: 18. Britannia Bridge

Thomas Telford built two significant suspension bridges on the north Wales coastline: the Menai Suspension Bridge (1819-1826), and Conwy Suspension Bridge (1822-1826). These formed part of a significant and much-needed improvement to the nation's highways. However, they were completed just four years before George Stephenson's Liverpool and Manchester Railway would kick start a very different transport revolution.

Roughly two decades after Telford did so for roads, it was George's son Robert Stephenson's turn to bring the railways to north Wales and Anglesey. He built two revolutionary bridges to span the exact same stretches of water as Telford: the Conwy Railway Bridge (1846-1849), and the Britannia Bridge (1846-1850). And just as had been the case for Telford, Stephenson could not do it alone.

The bridge across the Menai Strait was the most challenging element in the Chester and Holyhead Railway, and decisions on how to span the Strait remained unresolved while designs progressed for other parts of the line. Some thought was given as to whether the Menai Suspension Bridge could be modified to carry trains, but the loads required for a railway far exceeded those imposed by the horsepower that initially crossed Telford's bridge.

As in Telford's time, consideration turned to building a new arch bridge, but the Admiralty insisted on the provision of full clearance for high-masted ships across the full width of the Menai. Having settled on an alignment that made use of Britannia Rock in the middle of the channel, Stephenson proposed a flat span structure, with girders supported from above by suspension chains. The bridge towers were designed and then constructed tall enough to support such chains, although in the end they were never installed.

It seems that Stephenson conceived initially of a suspension bridge, and then sought a way in which it could be made sufficiently stiff to carry railway loads. He turned to William Fairbairn to investigate the feasibility of tubular stiffening girders, through which the railway tracks could run. Fairbairn rapidly came to the conclusion that the suspension chains would be too flexible, and should be dispensed with, but the less confident Stephenson kept provision for the chains until the bridge was complete.

Fairbairn undertook many experiments on tubular cross-sections, and in turn involved the mathematician Eaton Hodgkinson to analyse the experimental results. Stephenson's preferred girder design was for an elliptical cross-section, but Fairbairn soon determined that a rectangular section was more efficient. It rapidly became clear that buckling of the top flange of the girder was the key issue, a problem that was resolved by adopting a cellular upper flange to the girder, initially comprising two hollow circular tubes joined together, and later evolving into multiple cells side-by-side. 

Fairbairn constructed a 75ft span model tubular girder to resolve the final details of the rectangular tube design. The side walls required internal stiffening, and in the final design both the top and bottom flanges were made cellular. Although Fairbairn's experiments had been on single spans, the bridge was built as a continuous girder, giving it additional strength and stiffness.

Some of the other key participants in the project included Stephenson's assistant Edwin Clark, and Fairbairn's assistant Mr Blair, who was largely responsible for producing all the bridge's design drawings. After Fairbairn and Stephenson fell out in a dispute over recognition as being the true designer of the bridge, it was Clark who wrote the account setting out Stephenson's side of the story. Fairbairn published his own, and it seems generally to be regarded as the more honest version.

Credit is also due to architect Francis Thompson, who designed the masonry elements in a vaguely Egyptian style, as well as several other works along the railway. Thompson later worked again with Stephenson on Victoria Bridge, Montreal, another tubular bridge, as part of the Grand Trunk Railway in Canada.

Four sculptural lions were installed, one at each corner of Britannia, Bridge,sculpted by John Thomas, who also worked on the Palace of Westminster.

Hodgkinson had also fallen out with Fairbairn, essentially over the latter's willingness to extrapolate the results of his experimental work in the absence of a justifying mathematical theory. Around this time, Fairbairn began building many girder bridges with tubular (box) girders, but suitable theory was only just becoming available to practicing engineers. The sheer scale of the Britannia structure went well beyond what had been attempted previously - just as Telford's Menai Bridge had done a quarter of a century before.

The project innovated in many ways. The extensive reliance on wrought iron was pioneering, and the span was exceptional for a flat-span bridge. The range of experimental work relied upon was impressive, as was the idea for the cellular construction. Even the erection of the bridge required major innovation, with the girders lifted into place by jacking upwards with massive hydraulic jacks. The slots for the jacking process remain visible on the towers, and part of one jack can still be seen near the bridge on its south-west side.

Two million rivets were reported to be used, with workers having to squirm through the box cells to install many of them. This, more than anything else, determined the size of the cells used.

On 24 May 1847, while construction of the Britannia Bridge progressed, one of Stephenson's other railway bridges collapsed, killing five people. The bridge over the River Dee near Chester was constructed of three cast iron girder sections connected with wrought-iron link bars. It was a popular design at that moment of time, with at least thirty-four built prior to the Dee failure. Fairbairn had proposed in 1846 that Stephenson should use a wrought-iron tubular girder bridge across the Dee, but had been turned down.

This incident exposed Stephenson's lack of expertise as a structural engineer, and Fairbairn's views prevailed both at Britannia and more widely - he was involved in over 100 more tubular girder bridges (albeit predominantly with the girders sitting beside the tracks, rather than containing the tracks) within a 5 year period.

While the tubular girder was successful in the short-term for short and medium span bridges, it was not the optimal solution for larger structures, and the Britannia Bridge design would prove a dead-end. Before long, various forms of lattice-girder and truss bridges took over, although early lattice-girder railway bridges experienced their own problems. For more detail I can wholeheartedly recommend John Rapley's and Richard Byrom's books (see list of references below), both of which are excellent.

Britannia Bridge was bold, if not entirely beautiful, but I think there is a great deal to admire in its simplicity of line. It lasted 120 years until, on 23rd May 1970, a fire broke out, irreparably damaging the bridge's two tubular girders.

The replacement bridge seen today was built between 1971 and 1974, with two main truss arch spans over the Menai Strait. Both Telford and Stephenson had considered arch bridges, and finally the navigational restrictions that had forced both into bolder and more innovative designs were no longer an issue.

The form of the present-day bridge, designed by Husband and Co. (merged into Mott, Hay and Anderson in 1989, now Mott MacDonald), owes a great deal to the challenges of safely dismantling the damaged tubular girders, as well as to the need to reinstate a railway line as quickly as possible. 10,500 tons of metalwork had to be removed, forming a load well in excess of the railway traffic that the replacement structure would carry, and the arches were therefore designed and sized primarily to act as support to the demolition operation. Once the tubes were safely and temporarily supported, railway services were reopened through one of the damaged tubes in January 1972. The tubes themselves were cut into short sections, and then hauled off the end of the bridge using small locomotives.

The bases of the towers were extended with small concrete skewbacks to carry stainless steel pins, which carry the entire load of the new bridge. The steelwork for the new arches was assembled by Cleveland Bridge four miles from the bridge, at Port Dinorwic, and floated into place on barges.

The spans were cantilevered outwards from the central tower, with adjustable tie bars passing through the tower to provide temporary support. Lifting gantries moved along the upper chord of the arch truss to lift each new truss unit into place, as can be seen in the construction photograph below (taken from a souvenir booklet about the bridge).

During construction, the arches each briefly formed a three-pinned arch before pre-load was jacked into the upper member to transform the whole system into a two-pinned arch.

Because the arches had capacity well in excess of railway loading, this created the opportunity to add a second deck to carry highway loading, and openings in the towers were enlarged to facilitate this. The railway bridge was finished in 1974 (albeit with only one deck carrying services, as railway traffic was much diminished), and the road deck eventually completed by Fairclough Civil Engineering and Fairfield Mabey in 1980.

The steelwork for the new railway bridge weighed less than half of Stephenson and Fairbairn's original wrought iron bridge, only 4,961 tons, although the road bridge (which is nearly twice as long as the rail bridge) incorporates another 4,338 tons of steel.

Although it is often noted that the ordinary observer prefers an arch bridge over any alternative, the modern bridge is, to my eyes, less loveable than the original. Partly this is because of the sheer quantity of truss bracing, and partly that the visual relationship between road and rail decks is uncomfortable. I think this is partly due to the sheer depth of the edge beams at railway level.

On the plus side, the history of the bridge is there to be seen. The excess tower height originally intended to carry suspension chains contributes to the support of the road deck and punctuates the span in a pleasing way (compare Sydney Harbour Bridge). The form of the arches betrays their origin as falsework for a demolition process. The preserved cross-section of  tubular girder (accessible via a path leading to the south-west corner of the bridge) is well worth visiting. The masonry still looks excellent today, and the bridge's best-kept secret, the "cathedral" vaults at each end, are still intact albeit normally inaccessible.

Further information:

• Google maps
• Wikipedia
• Structurae (1850)
• Structurae (1972)
• British Listed Buildings
• Coflein
• Engineering Timelines (1850)
• Engineering Timelines (1974)
• Grace's Guide
• Menai Heritage
• William Fairbairn: The Experimental Engineer (Byrom, 2017)
• A Walking Guide to the Menai Strait Bridges (Daimond and Kovach, 2015)
• Conway and Britannia Tubular Bridges: Stephenson's Team (Barr, ICE Engineering History and Heritage, 2010)
• Building Bridges and Boundaries: The Lattice and the Tube, 1820–1860 (Dreicer, Technology and Culture, 2010)
• The Britannia and other Tubular Bridges (Rapley, 2003)
• The Britannia And Conwy Tubular Bridges Of Robert Stephenson (Chilton, IASS Symposium, 2000)
• Britannia Bridge: from concept to construction (Ryall, ICE Civil Engineering, 1999)
• Design Paradigms: Case histories of error and judgement in engineering (Petroski, 1994)
• Britannia Bridge: The Generation and Diffusion of Technological Knowledge (Rosenberg and Vincenti, 1978)
• The Britannia Bridge (Husband and Co.? souvenir booklet, 1978?)
• Reconstruction of the Britannia Bridge (Husband and Husband, ICE Proceedings, 1975)
• Joint Report on a Fire in the Britannia Tubular Bridge, Menai Straits on Saturday May 23rd 1970 (Caernarvonshire and Anglesey Fire Brigades, 1970)
• Tubular and other Iron Girder Bridges Particularly Describing the Britannia and Conway Tubular Bridges (Drysdale Dempsey, 1850)
• The Britannia and Conway Tubular Bridges Vol. 2 (Clark, 1850)
• The Britannia and Conway Tubular Bridges Vol. 1 (Clark, 1850)
• General Description of the Britannia and Conway Tubular Bridges (Clark, 1849)
• An account of the construction of the Britannia and Conway tubular bridges (Fairbairn, 1849)