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.

23 December 2018

Design published for new Polcevera viaduct

Details have been made public for the new Polcevera viaduct in Genoa, Italy, the proposed replacement for the bridge which collapsed in August this year killing 43 people.

The bridge design is credited to architect Renzo Piano, who offered his services free-of-charge in the aftermath of the August disaster. I imagine a structural engineer is involved, but I haven't found out who they are yet.

The €202 million construction contract has been awarded to Pergenova, a joint venture of Italian contractors Salini Impregilo and Fincantieri Infrastructure. They state that the new bridge will be complete in 12 months from the site becoming available, which will be when demolition of the existing structure finishes in March 2019.

The new structure is to be an elegant and, I think, consciously unspectacular steel viaduct, some 1.1km long, comprising twenty spans mostly 50m long, with some 100m long where required to span the railway and river. The reinforced concrete bridge piers are elliptical in section.

The design is sober and straightforward. The only nod towards the site's tragic history comes with the lighting columns positioned above each bridge pier. These extend in height and support a number of solar-powered lanterns, reportedly one for each victim of the bridge collapse.

I do wonder about the width of the piers - in some of the renderings, the deck is shown as much wider than its supports, which for me looks visually unsatisfactory, potentially unbalanced if only one side of the roadway is heavily loaded.

Of course, there will be no issue if the bridge deck is heavy enough, and Genoa is not in one of the more seismically active areas of Italy, classed as having a peak ground acceleration between 0.05g and 0.15g, for a 50-year return period, and from what I can see, at the lower end of this scale even for a longer 475-year period.

The Piano design was chosen against one other competing contractor, Cimolai, who submitted four proposals, three of them designed by Santiago Calatrava. The proposals featured a girder viaduct with a series of 125m spans; a cable-stayed viaduct with 140m spans; and a 550m-span tied arch bridge, all pictured below. You can find more detail in Cimolai's video and in a gallery on the Domus website.

It's no surprise that Cimolai were unsuccessful: which public authority would wish to run the risk of an over-budget fiasco at this particular site, as so often seems to accompany anything Calatrava designs? It's also apparent that any of these solutions would take longer to build than the Pergenova proposal.

Visually, two of the three lack a sense of lateral stability, and all of them are visually over-complicated and inappropriately dramatic. The arch design, with its awkward stiffening truss at deck level, is particularly poor.

Cimolai's non-Calatrava option (no designer appears to be credited), is worse still:

Meanwhile, some 20 people are reportedly under investigation on suspicion of involuntary manslaughter in connection with the original bridge failure.

25 August 2018

London bridges series: 52. London Wall Place Highwalks

Look, up in the sky! Is it a bridge? Is it a walkway? Is it a skywalk? No, apparently it's the return of London's pedways.

The original pedway scheme was the idea of London's town planners in the 1950s and 1960s, an attempt to elevate pedestrians above increasingly car-dominated city streets. Several new buildings were required to include provision for pedway access at first-floor level, but few of the pedways  planned were actually built.

They were perhaps most fully realised in the vicinity of the Barbican development, spanning across roads, and connecting the new Barbican cultural centre to the residential blocks which surrounded it.

However, with most foot journeys originating and ending at ground level, pedways were often as much of an inconvenience as a boon, and the idea soon died away. Despite their lack of success, pedways continue to exert a strong fascination for design and architecture critics and bloggers, and I've included various relevant links below providing more information.

With the construction of a massive new office development at London Wall Place, the idea has been revived. This was an original pedway location, so the new development is essentially reinstating what had once been there, albeit with a 21st century sense of style.

The new walkways span one busy road (and others that are less busy), and at least do serve a useful function in connecting into the Barbican's pedways, which do still form a useful connection. However, the walk below the pedways at ground level also an attractive route with extensive public realm enhancements in the St Alphage Garden section.

The walkways were designed by the team also responsible for the office building, Make Architects and WSP. Spacehub contributed to the landscape architecture.

The walkways are accessible from several staircases, and there's a new lift, although that wasn't in operation when I visited. The detailing on the main staircase is particularly nice.

In two places where the walkways span across roads, they are cantilevered from the office buildings, with the aid of stainless steel masts and stay bars. The walkway is suspended in an attractively-shaped weathering steel trough.

I found the mast-and-stay system visually awkward - it gives something of the impression that these are independent structures, although they obviously depend on the buildings for support. There's something about the arrangement that is out of place with the rest of the scheme.

The "centrepiece" of the walkway is another trough structure which snakes across the public space, skirting the ruins of an old church. This structure is a continuous beam cantilevering from each end, and it gives the impression that it's floating in space, thanks to the lack of intermediate supports.

It would have been so easy simply to have a multi-span walkway across this stretch, in the same style as other spans, but it's definitely this curved span that puts the whole scheme at a higher level of quality.

The various spans have spaces for seating, ensuring the walkways feel part of the public realm as a place to stop and breathe rather than just another way of traversing the city as rapidly as possible.

The whole scheme has been very well designed, and is a great addition to this one small corner of central London. I certainly can't see it inspiring yet more elevated pedways - the city streets are already too constrained and difficult an environment, but it's good to see it resurrected here.

Further information:

15 August 2018

Collapse of the Polcevera Viaduct

I'm sure most of my readers will have seen yesterday's tragic news that the Polcevera Viaduct in Genoa, Italy, collapsed, with at least 39 fatalities reported. At the time of writing, rescue and recovery efforts are ongoing. A state of emergency has been declared in the local region.

The 1.1km long viaduct carried a major toll road through Genoa, and was a key connection in the route from central and southern Italy to the south coast of France. It was completed in 1967 to a design by the famous Italian engineer Riccardo Morandi, one of a series of innovative concrete stayed bridges that he created starting with the Lake Maracaibo bridge in 1962.

These were distinguished by the use of very simple stay arrangements, using prestressed concrete stays rather than the steel cables already in wide use at the time (e.g. Strömsund Bridge in Sweden, 1955, and the Nordbrücke in Düsseldorf, 1957).

Eduardo Torroja's Tempul Aqueduct, built in 1926, is one of the few predecessors, although Torroja used concrete only as a protective material, it was not prestressed. Concrete-encased stays were also later used on the Prins Willem-Alexanderbrug in the Netherlands (1972) and the Metten Danube Bridge (1981). Morandi's tower arrangement was also used (without the concrete stays) on the Chaco Corrientes Bridge in Argentina (1973).

For more on Morandi's bridges and their relatives, see Walter Podolny's 1973 paper Cable-stayed Bridges of Prestressed Concrete.

Morandi's designs, although highly innovative, were a dead end. They required extensive temporary works to support the bridge deck until the stays were completed.

At Polcevera, temporary prestressing was used in the cantilevering deck sections, only to be removed once the stays had been installed. For his bridge at Wadi el Kuf, in Libya (1972), an array of temporary stay cables was used to support the longer spans, with all these cables then removed, rather than left in place as the permanent support system, which was the more logical and much more widespread solution. The Libyan bridge, incidentally, was recently closed for safety reasons.

Possibly relevant to the Genoa disaster, these designs also lack structural redundancy. The failure of any one key structural member of the bridge can lead to disproportionate collapse.

The Polcevera Viaduct, and its cousins, have been much admired by engineers and architects.

Michel Virlogeux, in his paper Bridges with Multiple Cable-stayed Spans, notes that the Lake Maracaibo Bridge was "much admired by architects who understand the evident flow of forces and who are sensitive to the impression of strength that emanates from the mass and shapes of the structure". Leonardo Fernandez Troyano described the same structure as "one of the great works in the recent history of bridges" in his book Bridge Engineering - A Global Perspective. In a 2010 paper summarising Morandi's work, Luca Sampo claimed that the Polcevera bridge's "technical features may still today be considered unsurpassed".

There is plenty of speculation on the internet regarding the cause of the collapse, which I won't repeat here.

The bridge's brand-new Wikipedia article is a pretty good source of information. There's a paper from 1995 which discusses previous remedial works to the bridge's main stays. Probably the best read is a contemporary article from 1968 with lots of construction drawings and photographs. All the images I've used here are taken from that article.

30 July 2018

"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: