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)

First Schlaich Bergermann bridge in Britain?

Manchester City Football Club (MCFC) have recently submitted a planning application for a new sports training complex, across the road from their existing City of Manchester stadium. The scheme involves a dedicated mini-stadium, indoor and outdoor training pitches, and supporting buildings. It's probably the most ambitious football construction project in the UK at present.

The new facilities will be linked across a main road to the football stadium by a new landmark pedestrian bridge. The structural designer for the project generally is Capita Symonds, and their name is on the draft technical approval document for the bridge, but judging from the names on the planning drawings, I think this is actually a Schlaich Bergermann design. It looks like it could be their first bridge to be built in Britain, with their closest previous attempt failing to get off the ground.

The present design is for a 90m span tubular arch bridge, with two inclined arches. The arches have a common springing at one end, where the bridge deck diverges in a Y-shape, and twin springings at the opposite end, where there is only the single straight deck. The tubes therefore intersect in one place above the deck, giving a plan arrangement very like the Christian ichthys symbol. I wonder whether the football club's owners, the Abu Dhabi United Group, have spotted this.

The deck is a minimum 5m wide trapezoidal steel box girder with steel rib cantilevers supporting a concrete deck slab. The parapets are in stainless steel, with horizontal tensioned rods as the infill. The deck is suspended on spiral strand rope hangers, which also run between and stabilise the main arch members.

The asymmetry of the arrangement will lead to some asymmetry of loading on the arch, but I would guess not enough to be of any significance. More interestingly, the angle of the hangers will be asymmetrical, which should lead to an interesting visual effect, with the hangers shallower and hence "opening out" as you walk away from the main football stadium, rather than towards it.

As ever, click on any image for a full-size version. All images have been taken from the planning submission documents. The images on the Manchester City website give a wider impression of the context for the new footbridge.

West Country Bridges: 3. Halgavor Footbridge

This little suspension footbridge at Halgavor, near Bodmin in Cornwall, was on my must-visit list.

It carries a footpath and bridleway 47m across the A30 highway, poking out shyly from between the trees on either side. Driving under it, you can see the main span but not the support towers, while from above, it is similarly difficult to find a viewpoint taking in the entire bridge. This sense of modesty is unusual for such an adventurous little structure.

Halgavor footbridge was designed by Wilkinson Eyre and Flint and Neill for the Highways Agency, opening in 2001, and even at first sight is an unusual structure for that body. At this span, some form of metal truss would have been a straightforward solution, and probably with lower initial cost than what was built.

It's a reasonably conventional suspension bridge, although the towers are tilted back from the span (see picture, click on any picture for the full size version), and the parapet posts arranged radially for visual effect. However, its dainty appearance hides its innovative nature.

The steel towers are the only part of the bridge likely to require substantial maintenance, in the form of repainting. The main suspension cables are in galvanised steel. The hanger cables, parapet posts, and parapet infill mesh are all in stainless steel, helping to minimise the need for future maintenance over the highway. The lower part of the parapets is in timber boarding, to meet bridleway requirements.

The deck is the feature that has got the bridge the most attention, being constructed largely out of glass-reinforced plastic (GRP). With the GRP resin formulated to protect against ultra-violet attack, it should therefore require little or no maintenance, minimising future disruption to the highway. It's paved in rubber blocks made from shredded, recycled rubber, which provide a happier surface for horses' hooves and also contribute to the structure's damping.

The upper-layer of the deck is a double-skinned sandwich plate, providing the necessary stiffness to resist out-of-plane pedestrian loading. Channel sections form the sides, and a series of diaphragms and a lower plate are assembled into a box-girder form. It was reportedly the first GRP bridge built in the UK by a publicly-funded body, and as such, considerable innovation had to go into its design and construction.

The difficulty with bridges like this is putting them in a context where the innovation becomes repeatable and more widely applicable. In the ten years since Halgavor Footbridge was opened, very few comparable structures have been built in the UK, and generally of much less aesthetic value. Most of the GRP and CFRP structural systems available are based around automated fabrication (such as pultruded sections). Elaborate lay-up and assembly appear less common, and such examples as I have seen are very ugly.

I'm reminded of the Lockmeadow Footbridge, by the same design team, another lightweight, aesthetically sensitive structure where the more innovative elements (an interlocking aluminium deck, and GRP parapet posts) have failed to see any wider adoption.

Visually, I think there is much to admire at Halgavor. The parapets are particularly good, especially with the combination of timber and stainless steel, but mainly for the attention paid to keeping it simple and transparent. The suspension arrangement is more attractive than the use of vertical hangers, at least in this setting.

There are peculiar arms which extend from the abutments to cradle the deck, and I struggled to work out what they were for. According to a technical paper about the bridge, they are there to soften the disjuncture in stiffness between the suspended deck and its connection with the abutments, which is fixed (thermal movement is addressed by vertical flexing of the lightly-arched deck). As in a stress-ribbon bridge, the bending moment at the support is otherwise too high for a slender deck such as this to accommodate.

Further information:

• Google maps
• Structurae
• Flint and Neill
• Wilkinson Eyre
• New Materials for New Bridges: Halgavor Bridge, UK (Firth & Cooper, Structural Engineering International, 2002)
• Application of Safety Factors in the Design of Halgavor Bridge (Cooper, 5th CoSACNet Workshop, 2002) (includes several images of fabrication and construction)
• A Tale of Two Bridges: The Lockmeadow and Halgavor bridges (Firth, The Structural Engineer, 2002)
• New Materials for Modern Footbridges (Firth, Footbridge 2002)
• Bridges (Arcila, 2003)
• Stress Ribbon and Cable-Supported Pedestrian Bridges (Strasky, 2005)
• Cornwall's Bridge and Viaduct Heritage (Kentley, 2005)
• Footbridges (Schlaich & Baus, 2008)
• An Encyclopaedia of Britain's Bridges (McFetrich, 2010)

West Country Bridges: 2. Newquay Suspension Bridge

I am an absolute sucker for these early-20th century suspension footbridges. A large number were designed and built by David Rowell & Co., with latticework towers and often unstiffened bridge decks suspended from wire ropes. Another prolific designer and builder was Louis Harper, who worked for James Abernethy as well as in his own right.

This particular example crosses from the Cornish mainland to Towan Island at Newquay, and is well known to locals and tourists, although seemingly not to the bridge community as it is not covered in any of the obvious books.

The excellent Harper Bridges website records that this bridge was reportedly commissioned by Victorian novelist Hal Caine to provide access to a summer house on the island. The bridge was originally similar in form to most of Louis Harper's suspension bridges of the periods, with lattice metalwork towers topped by pointed finials.

Another source reports that at one time there was a teashop on the island, with a toll of 2 old pennies charged to cross the bridge. The present house was built in the 1930s, and the bridge and island are both currently privately owned, although you could buy it for a mere £1.25m.

At some time, the latticework towers have been encased in concrete, presumably as a simple solution to a corrosion problem given the proximity to salts from the sea, which passes below the bridge.

It's an unusually spectacular location for such a bridge, by British standards, with most of the contemporary examples spanning more gently between riverbanks. As it's private, I wasn't able to gain access to the bridge itself, but hopefully the photos give a reasonably clear idea of the structure.

The bridge spans only about 30m, short by the standards of modern suspension bridges. I think it's a huge shame that the suspension form is rarely used for this sort of structure today (at least in the UK - there appear to be bucketloads of them in Japan), and would attribute some of that to the tyranny of codes of practice and methods of analysis. Bridges like the Newquay one are relics from an age where bridges were the products of craftsmen who learned what worked from experience as much as relying on their methods of calculation.

The equivalent firms to Rowell and Harper today, the specialist footbridge fabricators, produce in the main simple steel truss designs which are straightforward to analyse and efficient to erect. Little or no specialist knowledge is required. I wonder whether a place might ever arise again for a firm which would invest in R&D, develop its own system, use patent protection as a key source of income or monopoly, and help return us to structures which are lightweight and visually attractive.

Further information:

• Google maps / Bing maps
• Harper Bridges
• Cornwall's Bridge and Viaduct Heritage (Kentley, 2005)
• The Nineteenth Century Footbridges of Harpers of Aberdeen (Harper & Day, Industrial Archaeology Review, 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)

Concepts published for St Botolph's Footbridge, Boston

That's Boston, Lincolnshire, not Boston, Massachusetts.

Lincolnshire County Council is proposing to demolish and replace the existing St Botolph's Bridge, which spans the River Witham in Boston. It's a remarkably ugly metal girder bridge (pictured, click on any image for the full-size version), which isn't suitable for use by the mobility-impaired and was found to have structural defects earlier this year. They have a £600k budget and a yearning for something better.

Three design concepts have been released for public consultation, all from local architect Neil Dowlman Architecture (there's no mention of an engineer). All three options are supposed to improve accessibility, reduce maintenance, enhance the environment, and provide improved views of the river and the nearby St Botolph's Church.

The first option is for a so-called bowstring arch, with twin tubular steel arches inclined against each other. I say "so-called", because the deck and arch are only connected in the visualisation by a transverse member which would seem insufficient for a proper connection, and hence the bowstring behaviour will be compromised. This is straightforward to amend, however.

It spans about 35m, with 15m ramps either end, and varies in width from 3m to 4m. The available finance therefore works out at about £2.6k per square metre. The concept has structural glass balustrades with concealed handrail lighting, which together are likely to be a costly component, and don't seem to fit well with the aspiration for reduced maintenance problems.

I don't know the town so don't know how well this design fits within the landscape, but I would have thought a lower-profile solution might be more appropriate, if opening up the views is of primary interest.

The second option is described as the "traditional" design, with a deck-type bridge resting on gently curved lattice beams, with guardrail-style parapets. Lighting is from antique-style lamp-posts. Longer ramps are required because the bridge must sit higher above the river level.

The final option is derivative of the first, with "bowstring trusses" of lower height. These are in a Vierendeel style. The relatively low angle of the upper chord at the ends of the bridge will make structural design difficult, but I would see the main objection to this option as being the fact that the lower-height truss interrupts views off the bridge.

To me, there are two obvious options which seem to have been neglected. The first is the traditional half-through girder design, with twin edge girders cradling the bridge deck, and topped by handrails to provide transparency of views off the bridge. At a 35m span, this should just about be achievable without requiring the girder to extend all the way up to handrail height. It would offer better views than the "bowstring" options, and shorter ramps than the "traditional" alternative.

A second option would be a spine-girder solution, with a single central beam raised above deck level and doubling to provide seating. The views would be the most transparent of any solution, although the depth of girder required at 35m would mean higher ramps than some alternatives (albeit still much less than shown for the "traditional" design).

I always find it irritating when a public consultation presents a false or partial choice, but it remains an early stage for this project, and in my own work I have been amazed how different the final design sometimes turns out from the early concepts presented. In this instance, a quick look on Google Street View and at pictures of the nearby church suggests to me that prominence is best avoided.