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

Richard La Trobe-Bateman: Part 2. Public Bridges

This is the second part of a two-part post on the bridges of artist/designer Richard La Trobe-Bateman. This time I'm featuring bridges which are publicly accessible (to varying degrees), but there's no hard-and-fast separation so please don't take my categorisation too seriously.

Something I didn't mention last time is that La Trobe-Bateman is the great-great-great-great-grandson of one of the most significant 19th century British engineers, Sir William Fairbairn. Fairbairn's daughter Ann married John Frederick La Trobe-Bateman, who was himself a highly notable engineer, becoming president of the Institution of Civil Engineers, and developing an early proposal for a tunnel across the English Channel. Whether any of this heritage has impacted the artist's path in life, I don't know!

The 14.5m span bridge at the National Pinetum in Bedgebury (1996), pictured below under construction, is in a Forestry Commission woodland (parking charges apply). I believe it's the only one of La Trobe-Bateman's timber bridges in the UK that the public can walk across. It was engineered by Mark Lovell Design Engineers, who have collaborated with the artist on several occasions.

It's not entirely clear from the photograph, but the central part of the bridge is a diamond-shaped truss in timber with steel wires. The upper compression chords continue to form the upper part of triangular frames at each end, which effectively support the diamond section. It's an elegant design which exemplifies the designer's desire for structural clarity, with clear separation of tension and compression elements. It is noticeably less three-dimensional than several of his later designs, consisting of two identical frames, cross-braced in timber for lateral stability.

A timber bridge which can be seen but not crossed can be found at Buscot Park in Oxfordshire. This is a National Trust property, so there is a fee for admission. The bridge consists of a series of three 6m long king-post trusses, built in 2011. Two specialist timber firms claim credit for helping with this bridge on their websites, The Timber Frame Company and WoodenHouse, possibly because the artist's son Will has worked for both firms.

A cable joins the tops of the three trusses and is a feature of how the bridge was installed - it was launched from one side of the lake, and can be rolled back again at any time. The photograph below (© Des Blenkinsopp [cc-by-sa/2.0]) shows the bridge partially retracted.

La Trobe-Bateman's work is both art and engineering; the identification of structurally stable arrangements is central to it, as is the search for economy of materials. Many of his more recent bridges also consider the economy of construction - the way in which they can be built is an important part of the work.

This is partly a reaction to his few encounters with the world of commercial bridge-building. In 1993, he designed a 19m span tied A-frame bridge over the River Ehen near Cleator in Cumbria, which was built using steel structural members (pictured above). This was winched across the river, although that's not something you see often in bridge construction!

The form of this bridge is unusual and (for me) difficult to understand. The photograph on the right, taken before the walkway was installed, perhaps gives a better idea of its form. The bridge consists of a main steel tripod, from which a suspended pyramid supports the walkway. The tripod legs are tied together by diagonal bracing below the bridge deck.

I find this design less appealing than the timber-and-cable bridges. It is clunky and slightly incomprehensible, and the lack of material distinction between tension and compression elements is less visually appealing.

In 1994, La Trobe-Bateman entered a design competition for a new footbridge in Bristol, but was unsuccessful. His proposal was for another tripod type bridge, with deck sections that swung out to either side. The eventual winner was designed by Arup and Eilis O'Connell.

A later steel footbridge at Langport in Somerset (2006) was designed with Mark Lovell Design Engineers. The artist's sketch shows a simply-supported span, a form of underspanned suspension bridge where the upper chord is at handrail level, and the lower element is a tie-bar, with two steel stanchions on each side of the bridge strutting the two main elements apart. The entire bridge was lifted into place by crane.

This is the better-looking of the two completed steel designs, but it still lacks the charm of the wood-and-wire bridges. Again, I think the use of a single material is less visually self-explanatory, and the parapet posts and infill are too solid.

More recent designs deliberately seek methods of hand-assembly and erection, even the 27m long timber arch truss built in 2017 for the Timber Framers Guild conference in Wisconsin, USA (which is available for sale from Areté Structures, for anyone interested).

There are two very interesting accounts online which describe this bridge's construction. A number of volunteers were involved in helping to plan the bridge, including helping to develop the design over a period of time, including completing a 1:5 scale model as a trial. The bridge itself was then assembled by conference attendees, taking 20 hours to erect, and 4 hours to take back down.

This bridge belongs to a family of La Trobe-Bateman structures which spring from pyramids or tetrahedra at each end. Other examples on the artist's website are more of a cable-stayed form. With bridges of this type, horizontal thrust occurs at the foundations, and they must either be anchored into the ground or rely on friction. Built on a building roof, the Timber Framers Guild installation used temporary tie cables to hold the supports together.

At this scale, the necessities of structural engineering make the structure more like a conventional bridge and less like a work of art. The use of both timber and wire becomes more of a challenge, as the lack of stiffness in slender wires would have a more noticeable impact on the structure's performance.

The latest bridge project is the artist's largest, a 32m span cable-stayed bridge planned to be erected at the Burning Man Festival in 2019.

The intention again is for this bridge to be erected by hand avoiding typical construction plant as much as possible. A physical gap will be left at the middle of the bridge, reminiscent of the planned footbridge at Tintagel Castle in Cornwall. There's a nice little video at Tamara Stubbs' website where Richard explains the project.

The economy of materials in many of La Trobe-Bateman's works is key to their visual success. "Real" pedestrian bridge designers and builders would struggle to match it. For the spans involved, wood is a reasonably robust material, but thin steel wires lack stiffness and can only be pre-tensioned to a certain degree. Stiffness drives the use of thicker members on real bridges. Long-term durability is also a problem, as is the need to comply with standards and regulations, especially for parapets. The artist bridges certainly look better because of their minimal (and sometimes absent) parapets.

As a professional bridge designer, these designs feel aspirational to me, indicative of an ideal to strive towards rather than something that I could readily achieve. I also admire a sense of contradiction that several of the designs exhibit: they look at first to be simple, but on further examination seem more complex; or they look at first to be complex, but turn out to be based on simple forms and ideas. For me, they always demand attention, to try and understand how they are formed and how they work.

Further Information:

• Richard La Trobe-Bateman website
• British Library: National Life Stories Collection: Crafts' Lives
• A bridge too far-sighted to be built, The Independent (1994)
• Wood Awards 2003: Swing-Lift Footbridge
• Richard La Trobe-Bateman, Donald Fortescue (2009)
• Ruthin Craft Centre exhibition (2012)
• Getting Dressed: The La Trobe-Batemans, The Oldie (2018)

Richard La Trobe-Bateman: Part 1. Private Bridges

I've wanted to feature artist and bridge-maker Richard La Trobe-Bateman here for some time. Late last year, I finally got hold of a copy of his "Making Triangles" book, and Richard has also kindly shared several photographs for me to use.

La Trobe-Bateman has been creating art since the late 1960s, initially making wooden furniture. His early chairs were in an arts-and-crafts style, evolving over time into examples with far fewer individual elements. His works also moved away from a rectilinear style towards the increasing use of triangular frames.

As any truss designer knows, this geometry provide stiffness with minimal material and without bending of individual elements. In three dimensions, the triangles naturally became tetrahedra, and in 1987 La Trobe-Bateman made his first bridge.

Pictured above, this "hanging boom" bridge was a short span over a stream, a simple A-frame mast supporting a single crane-like cantilever spar, with the deck suspended from thin galvanised steel wires and the various timber pieces stabilised by a further array of triangulated wires.

Since then, roughly thirty of the artist's bridges have been built and exhibited throughout the UK, and in one or two cases abroad. The use of timber and wire is a common theme, and the structures are often experimental in arrangement, if often inevitably drawing on forms better known from larger bridges.

Many of the bridges are private commissions. Several are variations on a tripod form, with a couple of examples pictured below, a 15m long span passing through a tree (1995), and two 9m span twin bridges (1994), built with various oak, douglas-fir and stainless steel elements. These give the initial impression of something anyone could throw together with a bit of trial and error: beginning with a simple form and adding only one element at a time until the whole assembly is stable.

La Trobe-Bateman's methodology is to develop geometry using smaller scale models, to ensure the form exhibits structural clarity while minimising material, both typically the concerns of bridge engineers. The models also allow the method of construction to be rehearsed, and confirm that any given geometry is, in principle, stable.

A more recent example of the same form was built at Tassajara Monastery in California, a 15m span douglas-fir tripod structure which was first trial-assembled in the studio before being erected on site in 2006 using only a single chain-winch. The construction process was recorded online by Courtney Skott at Flickr, and also in a book by Donald Fortescue, who later documented its rebuilding in 2012.

This design illustrates one way in which these are sculptures as well as footbridges - the structural form is self-contained, simply supported, held together with tie cables like a triangular bowstring truss. So far as I can tell, all these designs avoid imparting any horizontal thrust onto their foundations, they are as stable on a smooth gallery floor as they are on site.

A professional bridge designer might take a different approach, using thrust-bearing foundations to ensure that parts of the frame are stable temporarily before all elements are complete. There are other details which a professional designer might avoid - most notably the lap splices visible on two of the main struts. I think a professional would have made a butt-splice, or a machined lap-splice to minimise eccentricity of loading and reliance solely on the strength of fixings.

This is evidence of an artist's philosophy rather than an engineer's - one of La Trobe-Bateman's stated design principles is to "get closer to the earlier stages in converting trees to useable pieces", and I guess that means avoiding over-engineering.

Several of the bridges also owe a direct and presumably conscious debt to the patent truss bridges of the 19th century. This next example, completed in 2008, is described by La Trobe-Bateman as a 6m span Bollman truss, and does share Wendel Bollman's arrangement of multiple suspension cables each attached near one end of the upper chord (here doubling as the handrail).

Another small bridge borrows (and adapts) the form of Albert Fink's truss, which used overlapping triangular frames below bridge deck level. In La Trobe-Bateman's 10m long 1989 adaptation, the truss struts are continued above deck to support the bridge handrail.

A similar idea was used in 1992 for what the designer refers to as a "bowstring bridge". This was a mere 4m long, in the form of an underspanned suspension bridge (compare for example Maryhill House Footbridge and Roxburgh Viaduct Footbridge as 19th century antecedents). I like the sense in this bridge that the parapet assembly is perched precariously on the central point, as well as the slenderness of the deck.

La Trobe-Bateman describes his work as "art as a byproduct of design". All the bridges are clearly functional, but it's notable that they are generally photographed without people standing upon them, emphasising the sense that these are art objects as well as bridges.

They may be designed to be experienced close at hand, but as an engineer, much of the appeal for me is in seeing them as structural assemblies and considering the relationship between static behaviour, embodied craftsmanship, and aesthetics.

This post is getting very long, so I'll conclude in a second post, covering examples of more publicly accessible bridges.

"China's Unique Woven Timber Arch Bridges" by Zhou et al

I don't normally make a point of mentioning technical papers on this blog, but maybe it's something I should do from time to time. I have previously wondered about putting together a semi-regular roundup of papers that might be relevant or interesting to my readers. However, I don't want to make more work for myself, so this may only very rarely happen!

I did think that this paper was worth drawing to wider attention: "China's unique woven timber arch bridges" (Zhou, Leng, Zhou, Chun, Hassanein and Zhong, Proc. ICE - Civil Engineering, August 2018).

This gives an overview of timber bridges in China of a type that dates back over 1000 years. I first properly encountered them in Ronald Knapp and Chester Ong's excellent book Chinese Bridges, which presents several bridges from the Zheijang and Fujian regions. The design and construction of these bridges is considered important enough for them to be included on UNESCO's Intangible Cultural Heritage List since 2009. A historic example, the Rainbow Bridge, is illustrated on the Song Dynasty painting Along the River During the Qingming Festival (~1085-1145), pictured above.

Nearly 100 of these woven timber arch bridges survive. Several have fallen victim to disaster through fire or flooding, including at least one of the bridges featured in Knapp's book. However, the construction skills have undergone a revival, such that some of these bridges have since been rebuilt. Indeed, the paper lists some 19 woven arch bridges which have been rebuilt or newly built since 1999.

The essence of these bridges is the structural form of a woven polygonal arch, which is described in detail in the paper, including several construction photographs. It consists of two sets of arch members which alternate across the width of the bridge, so that there are two superimposed polygons. These are locked together by transverse timbers, creating a triangulated system which in one way behaves not like an arch, but like a beam. However, it must also behave as an arch, as the main timbers are carefully butted together to be able to transmit axial load.

The paper in the ICE Proceedings is a short (6 pages) but very clear and useful introduction to these amazing bridges, and definitely worth a read if you have access to it. In case anyone would like to learn more about the woven arch bridges, I've collected a set of links to more detailed technical papers at the bottom of this page.

It wasn't until several centuries later that a similar bridge design was developed in Europe by Leonardo da Vinci. His design is discussed in a 2004 paper by Ceraldi and Ermolli, which compares da Vinci's design to the earlier Chinese bridges. Da Vinci's solution does not use the butted timbers, and is an open frame rather than having many alternating arches all immediately adjacent to each other.

Further information:

• Chinese Rainbow Bridges (Shen and Liu, World Conference on Timber Engineering, 2004)
• Timber Arch Bridges: a design by Leonardo (Ceraldi and Ermolli, 4th International Conference on Arch Bridges, 2004)
• Timber Arch Bridges in China (Yang, Chen and Gao, 5th International Conference on Arch Bridges, 2007)
• Tectonic Studies on Wooden Arched Bridge. As the Case of Span in Chinese Wooden Construction Tradition (Zhao, Feng and Bi, Third International Congress on Construction History, 2009)
• Traditional construction technology of China timber arch bridges (Yang, Nakamura, Chen and Nishikawa, Journal of JSCE, 2012)
• Structure, Construction and Craftsmanship of the Timber Woven-Arch Bridge in China (Liu, Second National Covered Bridge Conference, 2013)
• Chinese Ancient Arch Covered Bridges (Liu, Second National Covered Bridge Conference, 2013)
• Historical Culture and Traditional Craftsmanship of Taishun County (Ji, Second National Covered Bridge Conference, 2013)
• Discovering China's 'Forgotten' Bridges (Knapp, Second National Covered Bridge Conference, 2013)
• The Origin of Timber Arch Bridges in China (Yang, Nakamura, Chen and Nishikawa, Journal of JSCE, 2014)

Yorkshire Bridges: 26. Victoria Quays Footbridge, Leeds

This is a nice little bridge just a short distance away from the Centenary Bridge in Leeds.

It spans a basin between Flyboat House and Dock House, just off Navigation Walk, part of the Victoria Quays development, a series of waterside warehouses converted into residential property.

The basin doesn't especially need to be spanned, it's not huge, but no doubt the bridge provides some convenience for residents. It is a private structure, and can be seen but not used by the public.

Timber truss footbridges are reasonably unusual in the UK, and this one had the look of being historic, although it is not. It's form and details are interesting, and explained by the fact that it comprises two roof trussed removed from the adjacent Flyboat House, and used to create a bridge.

Judging from photos of restoration works undertaken by RDF Building Limited in late 2014, much of the timber was in poor condition and was presumably replaced. However, I think the bridge may date to 1986, when the warehouse buildings were converted.

Further information:

• Google maps

Australian Bridges: 6. Parsley Bay Bridge, Sydney

This charming suspension footbridge is tucked away in a quiet eastern Sydney suburb, little known to most of the city's residents let alone anyone else.

According to a history published online by the local municipal council, plans for a bridge across the beautiful Parsley Bay were first discussed in 1906, with the structure in place by 1910.

The bridge was designed by local town clerk and engineer Edwin Sautelle, and cost the tidy sum of £500 to build. Reportedly, it was built to improve access to ferries via the nearby Point Seymour.

The bridge is predominantly a timber structure, with the towers, deck and parapet rails all timber. The other parapet elements are in metalwork.

The towers are A-frame in form, which provides good stability in the longitudinal direction, but the bridge is evidently less stable laterally. Tie-back cables have been installed on all four corners, presumably to reduce lateral sway.

The bridge appears to have changed very little over its lifetime, judging from old photographs, although an image in Pictorial History Eastern Suburbs indicates that lamps or ornaments once sat above the bridge pylons. The same image doesn't appear to show the tie-back cables, although these can be seen in a postcard from circa 1930.

The other main change over time appears to have been the steady growth of surrounding vegetation.

The bridge was repainted and repaired in September 2003, and a further refurbishment was completed by GPM Constructions within the last couple of years.

The bridge's main asset is its beautiful setting. The Parsley Bay reserve features a fine sandy beach with water protected by shark netting, as well as a small patch of rainforest towards its rear. Views from the beach and from the bridge are both very attractive.

It seems to be a popular spot for bridge-jumping, although signs on the bridge make clear this is prohibited.

Further information:

• Google maps
• Structurae
• Bridgemeister
• Weekend Notes
• History of Parsley Bay Reserve
• Australia for Everyone

Australian Bridges: 4. Pyrmont Bridge, Sydney

My next few posts will cover a handful of bridges in Australia.

The best known bridge in Sydney is clearly the Harbour Bridge, but it's not the only historic engineering landmark in the city.

The Pyrmont Bridge is designated a National Engineering Landmark by the Institution of Engineers of Australia. It is a bridge rich in history and technical interest, and I can only scratch the surface here. I'd particularly recommend the original paper on the bridge in the ICE Proceedings (1907) and a series of articles in The Engineer (1917), the latter of which include excruciating detail in the form of description, drawings and photographs. There is an excellent history of the bridge in the document proposing the bridge as an engineering landmark, all links can be found at the end of this blog post.

A timber toll bridge crossed the Darling Harbour, dating from 1858, operating at a lower level than the present structure and incorporating a swing span. The replacement bridge was completed in 1902 to a design by Percy Allan, and was built at a higher level. Allan designed the bridge for the local public works department after a design competition had been held and all 41 entries had been rejected.

It was built as a highway bridge, being closed to highway traffic in 1981 following the construction of new highway viaducts nearby. The bridge was partially refurbished in the 1980s, reopening as a pedestrian boulevard in 1988. The bridge carried the Sydney Monorail from 1988 to 2013. The paper by Trueman linked below describes the refurbishment work.

The bridge is generous in proportion, originally accommodating a 40-foot wide carriageway and two 7-foot wide footways. The two central spans comprise cantilevering steel truss girders supported via an array of roller bearings on a central caisson pier. The steel was supplied from Belgium, with Australia lacking any significant steel industry at the time. However, all the approach spans are built as a variant on Howe trusses using Australian ironbark timber, primarily to save on construction cost, with the total price being roughly half what an all-steel bridge would have cost.

When built, the bridge was immediately recognised for its engineering significance. The opening spans were larger in area than the vast majority of moveable bridges built previously, and it was thought to be a pioneer in the use of electric power for its operation. The bridge is operated from a single control cabin at the centre of the opening section, with power supplied by cables running below the harbour bed. The cabin appears largely unaltered, although it was relocated to the edge of the bridge when the monorail was added.

The main perception at deck level is of generosity of space. It's unlikely a pedestrian promenade this wide would ever be built as new, it's purely a legacy of the bridge's historic use as a road bridge. Now there is space for banners, for people to admire the views of the harbour to either side, and for walkers, cyclists and roller-skaters to coexist reasonably happily.

Seen from below, the bridge appears in good condition, although I understand it is subject to an ongoing maintenance programme (BIM-empowered). When it was converted for pedestrian use, an extensive study into its defects and repair requirements was made. The engineers proposed the use of preservative treatment for the timber, but this was not immediately undertaken due to cost. The bridge was reassessed and only those repairs strictly necessary for safety reasons were completed: other damaged timber was left to continue to deteriorate. During a subsequent ten-year maintenance programme, timber preservatives were installed, both diffused into the timber core and applied as a barrier to the timber surface.

The timber trusses appear to have been overpainted at some stage, a largely cosmetic treatment for such a structure, I would think. The various connections are clearly visible. I believe some of these were altered during the 1980s restoration in order to locate connecting bolts in less damaged parts of the timber.

The west end of the bridge appears largely unaltered, with impressive stone approach structure. The east end has been more significantly altered, presumably because the bridge was cut short when the Western Distributor Highway was built. There are now escalators down to quayside level, and a cable-stayed footbridge provides a higher-level extension for those wishing to continue over the highway.

Overall, it's great to see that a bridge which was originally built as a cut-price alternative to other designs has survived so well, and I don't think engineers at the time of construction would have expected it to last so well. It has survived a variety of major changes in use, and its continued value as a key link in the Darling Harbour surrounds should hopefully ensure it survives for a long time to come.

Further information:

• Google maps
• Wikipedia
• Structurae
• NSW State Heritage Inventory
• Pyrmont Bridge, Darling Harbour
• Sydney Architecture (includes several historic and construction photos)
• The New Pyrmont Bridge (Sydney Morning Herald, 1902)
• The Pyrmont Bridge, Sydney, New South Wales, Proceedings of the Institution of Civil Engineers (Allan, 1907) (and Discussion)
• Pyrmont Electric Swing Bridge (The Engineer, 27 Jan 1917, 2 Feb 1917, 9 Feb 1917, 16 Feb 1917)
• Spanning Two Centuries: Historic Bridges of Australia (O'Connor, 1985)
• Pyrmont Bridge - Construction and Restoration (Trueman, National Conference on Engineering Heritage, 1988)
• Proposal to Landmark the Pyrmont Bridge, Darling Harbour, as a National Engineering Landmark (Institution of Engineers of Australia, 1991, this includes copies of several other key papers regarding the bridge)
• Management of an engineering landmark bridge: Pyrmont bridge, Darling Harbour, Sydney (Higgins, Bridge Management 2, 1993)
• The cabin man on Pyrmont Bridge (Sydney Morning Herald, 2014)
• Pyrmont Bridge (Kishan, ICE Australia, 2014, essay as part of the Bridge Ahoy! tour)