18 July 2018

Welsh Bridges: 15. Devil's Bridge

So, Devil's Bridge, we meet again.

Well this is a different Devil's Bridge to last time. Two down, many more still to go!

Devil's Bridge (or in the local tongue, Pontarfynach) is one of the better known tourist attractions in the Aberystwyth area. Anyone can cross the bridge, as it carries a public road, but to see it properly requires payment to access private land. In addition to the bridge, this gives access to a very scenic woodland walk, and is well worth the price of admission.

The legend that gives this bridge its name is the same or similar to most other Devil's Bridge tales: an old woman's cow somehow crossed over the river Mynach, and she couldn't get it back. The Devil offered to build a bridge in return for the first living soul to pass across it, and the old woman agreed. She threw some bread across the bridge, and her dog ran after it. The Devil, having expected a higher price, had to be satisfied with the dog.

What I learn from this legend is that the old woman was pretty smart to give up the dog in return for getting her cow back. It must have been a pretty impressive cow to have jumped across the River Mynach gorge before the first bridge was built.

There are three bridges here, each one built above its predecessors. It goes one better than Rumbling Bridge, in Scotland, in that respect.

The lowets span is medieval, generally though to date back at least to 1188, and comprises a pointed masonry arch sitting astride a remarkably deep cleft in the local rock, which contains the River Mynach (Afon Mynach). The most widely-held view seems to be that it was built by the monks of Strata Florida Abbey.

In Gwyndaf Breese's book on Welsh bridges, he suggests that the bridge reported by a traveller in 1188 was a "rickety wooden bridge", and that the stone span may have been built a century later.

Other than for its age and situation, it is relatively unremarkable. The arch barrel consists of thin bands of stone, springing directly from the rock. The profile of the barrel is noticeably distorted and unsymmetrical.

Breese quotes Jervoise in suggesting that the lowest bridge was widened or rebuilt at some stage. Jervoise's comment was that pointed arches were not in use in the 12th century, and the span must therefore have been a later reconstruction. I'm no expert but I don't think that is conclusive: the pointed spans in the medieval Exe Bridge are believed to be original, so why not here?

In 1753, a second bridge was constructed, a segmental stone arch spanning 32 feet. This bridge was later modified, with the height of its spandrel walls increased in 1814 to reduce the steepness of the highway approaches. It looks like you can see evidence of this in the banding of the horizontal stones in the spandrel walls. The ornate cast iron parapets were added at this time.

Buttresses at either ends of this span appear to have been added later. At one end of the bridge, the arch appears to spring from a higher point than at the other. Instead, it seems that the original springing is hidden within the masonry buttress, which continues under the arch barrel.

The third bridge was built in 1901 to a design by the County Surveyor Roderick Lloyd. Masonry abutments were built up to support steel lattice girders spanning 60 feet and carrying a 20-foot wide roadway.

This was substantially modified in 1971, when the castellated plate girders visible today were inserted along with a concrete deck slab. It looks to me that the parapets were designed to retain the appearance of the lattice girders, but clearly what's there now could not have been the original spanning trusses as there are no upper or lower chord members.

The newer girders are supported at one end on the abutment of the middle bridge, and at the other end on steel portal frames carrying the load to either side of that span. This was obviously an unfortunate period in the life of Devil's Bridge: the position, appearance and level of the new girders were all entirely unsympathetic to this span's predecessors.

Further information:

15 July 2018

Welsh Bridges: 14. Llandeilo Bridge

Completed in 1848, the mighty Llandeilo Bridge is one of the largest masonry arch spans in the United Kingdom. By my count it takes third place behind Grosvenor Bridge (61m / 200 ft, 1832) and Ballochmyle Viaduct (55m / 181 ft, 1848), making it the longest masonry span in Wales. (I'm happy to be told otherwise if there are any bridge spods out there somewhere.)

The Grade II* Listed bridge is said to span either 143 ft or 145 ft (take your pick!), and to be 26 ft or 10.1m wide (again, you choose!).

However tall and proud it may stand today, it had a difficult beginning.

The first bridge on this site was a seven-span arch bridge, which partially collapsed in 1795. The failed centre spans were replaced with a timber structure.

According to some accounts, the bridge was replaced in the early 1800s by a narrow three-arched bridge, which proved too narrow for traffic, although not all histories seem to agree (for the most thorough story of the bridge's past, see Llandeilo Past and Present).

County bridge surveyor William Williams was appointed to design a replacement in 1843, estimating the cost of his design to be £10,000. Builder Morgan Morgan was appointed on a contract price of £5,870.

The cash ran out while Morgan was still constructing the bridge foundations, and work was further set back when a flood destroyed part of the works. Edward Haycock took over the scheme, completing the bridge in 1848 with a total expense of £22,000.

Despite these difficulties, it was a huge engineering achievement. The causeway to the south is a substantial structure in its own right, giving a total length of 111m for both causeway and arch. The town of Llandeilo sits some height above the River Towy and its flood plain, and a lengthy ramp was required to allow traffic to enter the town at a suitable level.

The causeway is pierced by a smaller cattle creep, span which like the main span has an elliptical profile. On the main arch, this choice was driven by the span dimensions and height of the roadway. The cattle creep arch is described elsewhere as a flood arch, but I doubt it adds greatly to the bridge's flood capacity.

The main arch is described as having long, thin voussoirs, but as can be seen close up, they are in fact made up of short(ish) stones with thin ashlar joints.

Looking at the bridge today, it's impossible not to be impressed by the sheer ambition of this small town and those involved in building the bridge. It's difficult to know whether they could have fully appreciated the nature of the task they were taking on. It's interesting to think what would have happened if the entire project had been abandoned after Morgan's failure.

Further information:

11 July 2018

Welsh Bridges: 13. Llandeilo Suspension Bridge

This is the first of two bridges spanning the River Towy that I visited in Llandeilo, a small village in Carmarthenshire.

I've found very little hard information on this bridge. The Bridgemeister website lists it as built in 1911. The same date appears elsewhere, along with an indication that the bridge was built to commemorate the coronation of King George V.

It was originally named the King's Bridge, although nicknamed the Swing Bridge in more recent times (as is the case with many suspension footbridges).

The Buildings of Wales: Carmarthenshire and Ceredigion agrees that the bridge was built for the coronation in 1911, gives the span as 144 ft (44 m), and states the bridge was built by David Rowell and Co. That makes it the third and final Rowell bridge I visited on this trip, the others being Llanstephan (1922) and Elan Village (1904).

This is the least well-preserved of the trio (which is saying something given the condition of the bridge at Elan Village).

It's not entirely clear how much of the original structure remains. The cables look like they may be original, but the hangers and parapets definitely are not. I didn't get a close look at the bridge deck.

The original steel lattice towers, of a design typical of Rowell's bridges (compare the Howley bridge), have been almost entirely encased in concrete, with just part of the upper cross-bracing left exposed, along with the tower finials (another classic Rowell visual signature).

The concrete has preserved the bridge for a time, but at a cost: future replacement of the cables is now made very difficult, and eventually the day will come when the only option is to replace the entire structure.

Further reading:

09 July 2018

Network Rail Footbridge Design Ideas Competition

The UK's national rail network owner, Network Rail, recently announced an international Footbridge Design Ideas Competition. The competition is being administered by the RIBA Competitions office, and seeks architects or engineers (or teams of both) to present their ideas no later than Tuesday 18th September.

The organisers ask for ideas which are "innovative, challenge presumptions and raise expectations for the quality of future designs". I get the impression there is a perception that design quality in the UK railway network is often poor (I think this is true), and an ambition to see if anything better can be suggested.

Entrants must be either professionally qualified, or students. There is a nominal registration fee of £50 for professionals or £25 for students, presumably to minimise the number of contributions from complete jokers. Submission requirements are not especially onerous, being two A2-sized digital layouts, three images, and a declaration form.

There is only one prize, a "Design Fund" worth £20,000 which is expected to be awarded to the best entry, although could presumably be split. There is no potential design contract being dangled, with Network Rail essentially purchasing the ideas to use as they see fit with no further input from the winning competitor.

For the promoter, this is a cheap way of generating fresh and imaginative proposals. For competitors, it's a chance to freshen up their creative muscles, enjoy the pleasure of collaboration, and hope for a bit of positive publicity.

What's notable about this contest is that it is not Network Rail's first attempt to improve design quality in this area. They held a shortlisted design competition a few years ago, but I don't think much came of it.

They are also, in parallel with the new competition, looking to procure a consultant designer to develop a new footbridge design, again with a substantial architectural contribution. This is not announced publicly, only to Network Rail's pre-registered suppliers. The successful designer will be appointed through a conventional price / quality bid, and given the opportunity to work with the client and relevant stakeholders to come up with a new design or range of complementary designs.

It doesn't feel very joined up, but represents well the issues that large public bodies can have with procurement of creative design. Open competitions tend to generate more imaginative ideas, but may highlight teams who the client would find difficult to work with in follow-on stages; more direct procurement will lead to more predictable outcomes but gives the advantages of control. I think there is no perfect answer to this dilemma.

08 July 2018

Welsh Bridges: 12. Dolauhirion Bridge

One of the best known Welsh Bridges is Pontypridd Bridge, designed by William Edwards, and completed in 1756 (after a series of false starts). This tall arch with its circular openings near the ends of the arch is so closely associated with Edwards that it is, apparently, sometimes just referred to as the William Edwards Bridge.

Edwards was not a "born" bridge engineer, having been first a clergyman and then becoming a self-taught mason and engineer. In addition to the Pontypridd Bridge, he completed a series of other bridges, including the Dolauhirion Bridge depicted here, from my recent visit. It was completed in 1773 by William's son Thomas.

You can easily drive over this Grade I Listed bridge (and Scheduled Ancient Monument) without really knowing it is there. It is smaller and less impressive than its Pontypridd cousin, but also has circular openings in the spandrels, and is very well suited to its place on the River Towy. The prolific chronicler of ancient bridges, Edwin Jervoise, described it as "the finest bridge over the upper part of the Towy".

Different sources report the span as variously 30m or 25.6m (84 feet). The circular holes (oculi) are  reported to be 2.4m (8 feet) in diameter.

Both the arch barrel and the oculi are formed from a single layer of large stones, while the spandrel walls are faced using less well-ordered and much thinner stones.

At some point in its history, the bridge was strengthened with a reinforced concrete saddle. I don't know whether the bridge was waterproofed at this time, but the underside of the arch barrel is thickly encrusted in mineral deposits resulting from many years of water seepage.

This is not by any standard a magnificent, monumental bridge; it is relatively understated, but attractive and appropriate for the location. Given the extent of surrounding vegetation, the oculi do little to lighten the overall appearance.

Seen from below, the bridge's lateral slenderness is the most obvious and impressive feature.

Further information:

04 July 2018

Welsh Bridges: 11. Llanstephan Bridge

This is the second bridge I visited on my Welsh trip which was designed and built by David Rowell and Co, the first having been the bridge at Elan Village.

Rowell and Co were frequent builders of suspension bridges in the early part of the twentieth century, mostly in the UK but with some overseas examples. Most of these were intended as footbridges, and the example at Llanstephan is a rare surviving Rowell bridge which was built to carry vehicles, and which still does so.

It is very narrow, only just wide enough to pass through the towers without clipping wing mirrors, and plaques at either end of the bridge state that it is suitable for only one vehicle at a time, with a maximum speed of 4 mph. The plaques state the bridge was built to carry vehicles up to 5 tons in weight, but this is enforced now as a 3 tonne restriction.

The bridge was built in 1922, and it carries a minor side road. It spans 81m across the River Wye, and is 2.4m wide. The bridge deck is supported by rivet steel lattice trusses on each side, and each truss is supported from a pair of steel suspension cables.

The main cable anchorages are clearly not original, and the cables look in excellent condition. The bridge was reported as closed in 1994 pending replacement of corroded cables, so I assume this took place in the mid-1990s.

The upper clamps connecting the hanger bars to the main cables are certainly not original, and are bulky and clumsy compared to the original clamps, which can be seen in a historic photo.

The bridge decking consists of transverse timber planks with a non-slip topping, and these are likely to be the element which limits the bridge's carrying capacity: I definitely wouldn't want to drive a heavy vehicle across here, or a light vehicle at a higher speed.

Further information:

01 July 2018

Welsh Bridges: 10. Doldowlod Bridge

This is a very unusual pedestrian bridge hidden on a private estate, and I think it is not very well known. I discovered it thanks to one of the Civil Engineering Heritage books. Please be aware that it is on private land, not a public right of way, so there is no right of access to the bridge without the permission of the landowner.

The Doldowlod Estate was purchased and developed by the famous engineer James Watt near the beginning of the 19th century, and it remains owned by his descendants, the Gibson-Watt family. Doldowlod Hall was developed by Watt's family in the mid 19th-century, but it's not entirely clear when this bridge was built on the estate, carrying a path across the River Wye.

McQuillan includes this bridge in his paper on brewer-turned-bridge-engineer James Dredge, but refers to it as Llanwrthyl Bridge, or Ystrad Bridge, stating that it was built around 1867 by Dredge's son William. Doldowlod is around 3.5km from Llanwrthyl, and Ystrad is the name of a farmhouse on the west bank of the river close to the bridge.

In Civil Engineering Heritage, Roger Cragg is silent on the designer but indicates the bridge was built around 1880, with the ironwork cast at Llanidloes Railway Foundry. Alan Crow, in the excellent book Bridges on the River Wye, says the same. This date seems to have been estimated by the bridge owner when the structure was visited and added to the ICE Panel for Historical Engineering Works database.

James Dredge died in 1863, so if the dates are correct, then it seems likely that William Dredge did indeed complete the bridge using his father's patent design. William died in 1886.

The bridge is a classic James Dredge design, with the exception of the cast-iron A-frame towers. It spans either 120 feet or 42m across the river (depending which source you credit), supporting the deck from chained eyebar rods.

As in all Dredge bridges, the main "cables" diminish in size from support to the centre of the bridge, this being achieved by the expedient of diverting component bars downwards to support the bridge deck at each link in the chain.

There are five bars in each chain at the towers, reducing to two at midspan. At Doldowlod, two smaller bars are diverted downwards at each connection point.

The effect, in theory, is to closely match the amount of metal used in the main "cable" to the level of stress present at each point, and this works as a cantilever bridge, not a suspension bridge (where instead the force in the main "cable" is relatively equal all along its length). This should give an efficient use of material, but it is not a very robust design.

The diagonal hanger introduce a longitudinal force into the deck; this is balanced by forces in pairs of flat steel bars which run along each edge of the deck. Deck joists are held in place between these flat bars, and these carry timber plank flooring.

The bridge's appearance is visibly distorted, and it is certainly quite "lively" even under just one person walking across. However, it is otherwise in fair condition considering its age and likely fairly limited maintenance.

Some sources indicate that there are no more than 7 surviving Dredge bridges in the UK, although that figure does not include the bridge at Doldowlod. I've previously visited and reported on the surviving spans in Bath and Inverness.

Doldowlod Bridge appears to hold no protected heritage status, which while not unusual for an obscure engineering work, is very difficult to justify given the age and rarity of the structural form.

Alan Crow's book indicates that the bridge was repaired in 1962 after being struck by a floating tree, and that in 1989 it was dismantled, refurbished and reassembled by R.J. Hope and Son of Newbridge-on-Wye. He also indicates that there were previously two other suspension bridges on the Doldowlod Estate, of which only foundations now remain.

Further information: