14 January 2019

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:

08 January 2019

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.