"River Axe Crossings: from Mouth to Source"

I strongly suspect 2012 is going to be a fairly quiet year here at The Happy Pontist. The myriad obstructions which clutter "real life" have obliged a fever of activity, both necessary and unnecessary, which will leave limited time for my more idle whims such as blogging.

With this in mind, it's perhaps appropriate that my first post of the year is to feature a book, Colin Sackett's "River Axe Crossings: from Mouth to Source" (2008, www.colinsackett.co.uk), in which bridges are essentially conspicuous by their absence.

Opening the book from what is normally seen as the "front", it is titled "Upstream: River Axe Crossings from Mouth to Source". Each right-hand page then offers a picture of the River Axe looking upstream, photographed from each of forty-one consecutive river crossings (weirs, fords and rail bridges are excepted, not unreasonably). This takes the reader on a journey through Devon, Somerset and Dorset, all predominantly rural counties in England's south west.

Opening from the "back", the book's title is "Downstream: River Axe Crossings from Source to Mouth", and every left-hand page depicts ... oh, you probably guessed already. The photographs taken at the river crossings are bookended by images from the river's source and mouth.

Every photograph is in black-and-white, and accompanied by a brief paragraph with details on the crossing in question, or on what can be seen in the distance. The change in appearance of the river and its banks from a tightly constrained woodland watercourse, through open fields, back through woodlands, and eventually into a wide, open floodplain is quietly interesting, and a remarkably informative way to consider the details of a highly particular landscape. You can find some example images from the book at the author's website.

Although the book is clearly a one-off, it's easy to imagine a series of similar volumes depicting other rivers, or imposing order on entirely different features of the landscape. I'm reminded of the artist Richard Long, and his landscape journeys planned according to imposition of an artificial geometry onto a survey map (one example). There is a similar sense of using an entirely arbitrary system to order a journey and hence disrupt the way we normally encounter the landscape.

Many of the photographs make clear how this particular river, flowing as it does through a floodplain of varying width, both determines how people have altered the landscape, and is affected by human actions. The nature of the river banks, often in deep cut, reflects the use of the river as a boundary, as well as its diversion past other boundaries. It also makes clear the predominantly agricultural nature of the area, cleared of vegetation which might restrict the river's ability to erode and meander.

Bridges do appear occasionally, in the distance, where they can be seen from another crossing, or by implication, whenever an image has clearly been taken from the perspective of a taller span. Although they define the entire structure of the book, they are invisible platforms, present only because they allow the photographer to stand on the centreline of the channel without getting his tripod wet. Nonetheless, you can tell a little about their nature by considering the gaps between the photographs - stretches of the river which don't merit a crossing, or where one is uneconomic.

As a pontist who normally photographs bridges as an object in their own right (the structural engineer's focus) or as an object within a landscape (a nod to architectural friends), the absence of bridges from this book of river crossings comes as something of a shock, but a welcome one. It's too easy for an engineer to forget that bridges are not only there to cross, but to stand upon, that they establish a relationship not just between the two river banks but also allow their user a very different perspective upon their surroundings.

Post-structuralists in literature have long been au fait with the possibility for the author to disappear from the text. Perhaps it is time for post-structural pontists to likewise celebrate the disappearance of bridges from the landscape.

West Country Bridges: 5. Royal Albert Bridge

What is there that I can sensibly say about Isambard Kingdom Brunel's greatest bridge (pictured, above left)?

First, the facts. The bridge was opened in May 1859, four months before Brunel's death. The two main spans of 138.7m are achieved using giant lenticular trusses in wrought iron. The upper chord of the truss is a single tube, oval in cross-section, 5.1m wide and 3.7m high. The lower chord consists of wrought iron chains, with vertical members carrying the loads from the deck into both chords. It's a design with split personalities - as well as the idea of the lenticular truss, you can think of it as a bowstring arch bridge, or as a self-anchored suspension bridge in which the main cable forces are anchored into an overhead strut rather than into the deck.

In this respect, the design was a development of Brunel's 1852 bridge at Chepstow, where again a simplified suspension arrangement relies on tubular strut for its anchorage (although at Chepstow, the strut was circular). Another predecessor was Brunel's 1849 bowstring arch at Windsor.

The Royal Albert Bridge also owed more than a little debt to Brunel's great friend and rival Robert Stephenson. His High Level Bridge in Newcastle was also completed in 1849, and has a series of bowstring arches carrying the roadway, with the railway on a separate deck above the arches. The idea of superimposed systems may have influenced the Royal Albert Bridge, but another Stephenson design was even more relevant, the Britannia Bridge.

Opened in 1850, the two main spans at Britannia were very similar to Royal Albert, at 140m. The same method of erection, by floating out and lifting vertically, was also used on both bridges. The Britannia Bridge (along with a related structure at Conwy) had pioneered the use of riveted wrought iron box girders, although used in a beam arrangement rather than as struts as in the more complex Brunel design. Stephenson had even suggested the use of an oval box girder at Britannia, although William Fairbairn's preference for a rectangular section proved more appropriate.

Brunel's Royal Albert Bridge was a more sophisticated design all round, but not of a type which would see much further use. The first major Warren truss bridge had been built in 1852, and lattice trusses such as the Runcorn Railway Bridge were soon to come into favour. Truss bridges were simpler to assemble than box girder designs, and used significantly less material. Nonetheless, Brunel's lenticular truss certainly didn't stand alone, with several other examples built by von Pauli and Lohse in Germany, and by Lindenthal and the Berlin Iron Bridge Company in America.

None of this is to detract from the Royal Albert Bridge. It may have been a technological dead end, but it remains a remarkable design and a spectacular structure.

I particularly like the texture of the main arch girders, with the riveted plate form very visible. The evidence of handicraft is a welcome contrast to the more impersonal surfaces that modern welded construction produces.

Perhaps less appealing are features which I presume to have resulted from strengthening works over the years. Chief amongst these are the diagonal bracing members immediately below the suspension chain, which aren't visible on older photos, and detract from the clarity of the structural system.

Further information:

• Google maps / Bing maps
• Wikipedia
• Structurae
• Engineering Timelines
• English Heritage
• royalalbertbridge.co.uk
• Royal Albert Bridge (Buxton-Smith, 2007, student paper)
• British Railway Bridges (Walters, 1963)
• British Railway Bridges and Viaducts (Smith, 1994)
• Civil Engineering Heritage: Southern England (Otter, 1994)
• Cornwall's Bridge and Viaduct Heritage (Kentley, 2005)
• An Encyclopaedia of Britain's Bridges (McFetrich, 2010)

West Country Bridges: 4. Tamar Bridge

It's not often that I feature long-span bridges on this blog. The scale and complexity of major bridge projects makes them less easy to encapsulate in a short text. Smaller structures are easier to do justice to. However, in this current series of posts, there are two more bridges that it would be remiss not to include. This post will cover the one on the right, in the picture above.

Opening in 1961, and spanning 335m, Tamar Bridge was the first of three major British suspension bridges to be completed within a few years of each other, its siblings being the Forth Road Bridge (1964, span 1006m), and the Severn Bridge (1966, span 988m). Its nearest significant predecessor was the 101m span Chelsea Bridge, completed in 1937, although the longest spanning suspension bridge in Britain at the time dated back nearly a century, the Clifton Suspension Bridge (1864, span 214m) (I'm not counting the 300m Widnes-Runcorn Transporter Bridge, which is a special case).

By the standards of what had been built in American and elsewhere, the Tamar Bridge was not a major structure. The design, by Mott, Hay and Anderson, was of the old-fashioned stiffened deck-truss type, still the legacy of the Tacoma Narrows failure from two decades before.

However, this conservatism was forced on the designers by the proximity of Brunel's Royal Albert Bridge. Tamar Bridge shared its design and year of opening with the Runcorn-Widnes Bridge, which also had a very similar main span of 330m. At Runcorn, a 314m span suspension bridge was rejected in favour of a steel truss arch design as a result of wind eddies generated by the nearby Runcorn Railway Bridge. At Tamar, the wind tunnel tests showed that a small change in level of the new road bridge would be enough to solve a similar problem.

The Tamar trusses are some 4.9m deep, with a span-to-depth ratio of 68, almost absurdly deep for a suspension bridge even at the time. As built, it had only vertical hangers, but when it was widened at the end of the 1990s, stay cables were added, creating a hybrid form. Unusually, the main suspension cables are locked-coil steel ropes rather than aerially spun cables, with 31 ropes bundled together in each cable.

The design of the widening, by Hyder, involved extending the bridge deck on new cantilevers, which carried traffic during the construction period. The original concrete deck was replaced by a lighter orthotropic steel plate deck, and traffic then returned to its original lanes. The cantilevers now carry local traffic on one side, and a footway / cycleway on the other. The overall result is not just that the bridge is stronger, but also more attractive, as the main trusses are to some extent shadowed by the deck cantilevers.
Further information:

• Google maps / Bing maps
• Wikipedia
• Structurae
• Engineering Timelines
• Pathe film showing construction
• Modern British Bridges (Henry & Jerome, 1965)
• Civil Engineering Heritage: Southern England (Otter, 1994)
• In the Wake of Tacoma: Suspension Bridges and the Quest for Aerodynamic Stability (Scott, 2001)
• The Tamar Bridge (Brown, 2007, student report)
• An Encyclopaedia of Britain's Bridges (McFetrich, 2010)

West Country Bridges: 1. Postbridge

Ok, time for a few bridges from the south-west of England, all in the counties of Devon or Cornwall, or in some cases both.

The first, Postbridge, is a clapper bridge spanning the East Dart near Princetown. It's believed to date to the 13th century, and is a Grade II* Listed Building and Scheduled Ancient Monument.

The granite  slabs each span about 3m, and are reported to weigh up to 8 tons each.

The bridge's proximity to the roadway means this is one of the most visited of Britain's clapper bridges, as can be seen in the photographs. Tourists or not, it retains a timeless quality which belies the fact that it has been repaired and rebuilt on more than one occasion, most recently circa 1970, when the slabs were bedded on a resin mortar to eliminate movement.

I'd have preferred to have seen the bridge with fewer visitors, it seems almost uncouth for something so primitive to be so well-used.

As a designer, I wonder why it is acceptable to have no handrail on an ancient monument but one would be demanded on an equivalent bridge built today. Surely the risk of falling off is the same in either case? Does protection of heritage have more importance than the risk of injury, or should we simply recognise that the risk of injury is so low as to be an irrelevance, whether many centuries ago or today?

Further information:

• Google maps / Bing maps
• Structurae
• English Heritage
• Legendary Dartmoor
• Virtually Dartmoor
• British Bridges (Johnson & Scott-Giles, 1933)
• Bridges of Britain (Wright, 1974)
• Civil Engineering Heritage: Southern England (Otter, 1994)
• An Encyclopaedia of Britain's Bridges (McFetrich, 2010)