29 September 2011

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:

27 September 2011

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:

25 September 2011

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.

21 September 2011

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:

20 September 2011

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:

19 September 2011

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:

18 September 2011

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.

15 September 2011

Paolo Soleri's Bridges

Tallbridgeguy mentioned this before me, but I had planned to feature it here anyway.

ArchDaily has a very interesting post with text and images of the American architect Paolo Soleri's bridge designs. Only one of his bridges has ever been built, a horrible footbridge at Scottsdale, and judging from the many other examples showcased, that may be no bad thing.

Soleri is a classic example of an architect whose sympathy for structure is superficial at best, with even his best known bridge design, a proposal featured in Elizabeth Mock's seminal The Architecture of Bridges, showing a thorough contempt for the imperatives of construction. Pictured above, this design was criticised by the Italian engineer Pier Luigi Nervi for "start[ing] from a purely formal conception  ... with the sub-conscious thought that the 'calculator' will manage to make it stand and the constructor will be able to build it."

Sometimes, this lack of embedment within a discipline can result in a design which shatters hidebound preconception and reminds us quite how trapped we often are within the habits of tradition. Many of Soleri's designs look lovely on paper, science fiction fantasies of what a bridge might be unconstrained by gravity or cost.

I'm particularly struck by his Ponte Lussemburgo proposal shown above, described as a "double cantilever inter-related three-dimensional structural members in ferro-cemento" (click on any image for a full-size version). Its sinewy curves are artistic rather than rational, but there's a family resemblance to Sergio Musmeci's astonishing Basento Viaduct.

I'm reminded that there are many histories of structural engineering yet to be written. Those few that do exist are partial and often hamstrung by linguistic limitations: the likes of Musmeci are rarely featured in books on bridge engineering. Beyond the bridge world and considering wider structural engineering, the English-speaking world often seems largely ignorant and unappreciative of great designers like Heinz Hossdorf, Ulrich Müther or Eladio Dieste.

If a history of the unknown would be welcome, a history of the unbuilt and unbuildable would be equally so, and Soleri's bridges would certainly merit inclusion.

14 September 2011

Bridges shortlisted for IStructE awards

This year's Institution of Structural Engineers awards feature a number of bridge schemes.

The pedestrian bridges shortlist has:
I will reserve comment, due to lack of time, but feel free to click the comment button and add your own!

The reconstruction of the Mizen Head Footbridge is shortlisted in the Heritage category, while the Sheikh Zayed Bridge is shortlisted under Transportation Structures.

The winners will be announced on the 8th November.

13 September 2011

Bridges news roundup: Mega Edition

It's been some time since I've done a roundup of bridge-related news, so there is quite a bit to cover this time ...

Italy's bridges weighed down by locks of love
I recently wrote about the tradition of adorning Wrocław's Tumski Bridge with padlocks as a symbol of marital or romantic commitment. It appears the Venetian authorities are less than keen on seeing the same tradition applied to the Rialto Bridge. I didn't spot any locks when I visited it, but perhaps this was simply because the authorities were quick on the draw with their boltcutters. It's a shame they are less effective at removing graffiti.

Daredevil takes on Gateshead Millennium Bridge stunt
As this video shows, it's not quite as crazy as the original article makes it sound.

Gateshead Millennium Bridge celebrates 10th anniversary
Designer Jim Eyre interviewed.

Barnard Castle Bridge moves step closer
Plans are afoot for a £1.3m, 265m long pedestrian suspension bridge. That strikes me as too little money for that much bridge, unless it really is done on the cheap, forestry-style.

New Tempe Town Lake pedestrian bridge draws attention
As well it should. See this site for more construction images, or play spot the difference.

Diamond idea for bridge
The suggestion is that the proposed New River Wear Crossing in Sunderland (pictured, right) should be named the Diamond Jubilee Bridge as part of a wheeze to help persuade cash out of the central government in order to support what is undoubtedly a local council vanity project. They are not the first to have this idea - you heard it here first.

New Wear Crossing funding bid
Meanwhile, Sunderland have submitted their revised bid for central government cash for the wider highway scheme of which the bridge is only the most prominent part. The bid cuts roughly £15m off the previous estimated £133m scheme cost. Of the new £117.6m figure, roughly £72m is for the bridge itself, with the rest for highways, land and utilities.

An examination of the documents reveals that considerable "value engineering" has taken place (cost-cutting, in normal parlance). The twisty masts of the bridge have been reduced in height, bespoke parapets and road lighting have been replaced with their bog-standard equivalents, and structural finishes have been reduced in quality. An alternative scheme with a conventional bridge is estimated at £109m, although much of that is due to additional design and land purchase work only necessitated by the fact that Sunderland have already spent large sums on the landmark bridge option. The difference between the landmark and alternative schemes is therefore a mere £8.6m, which is frankly almost impossible to believe. Sunderland have also made a commendable effort to calculate the regeneration and employment benefits of the landmark option, which they put at £33m.

Peace Bridge defenders say Toronto crossing not the same
Calatrava's CAN$25m footbridge in Calgary (the subject of many previous posts on this blog) is compared to the CAN$8m Puente de Lux in Toronto (see here). Both are trusses with curved web members, but the similarity ends there. The Toronto span is essentially a Warren truss with pretensions, while the Calatravan helical truss bridge is a more complex beast. And one is yellow and the other one red. Quite, quite different.

Is it all over for £6 million bridge?
Answer: it was probably all over before it ever started. This footbridge in Gainsborough was an ambitious landmark structure (pictured right, and see my previous post for full details) which always seemed at risk of cost escalation, and perhaps over-ambitious for a town which started with no funding in place and remains in the same position. What's odd is that the council granted a local body (now disbanded) £12,000 to develop the initial design but has, apparently, never had any real intention of taking the scheme further. If so, why waste the development cash?

City river footbridge to be lifted into place after year of delays
Hull's £7.5m pedestrian swing bridge (note it was only £6.5m when I last reported, in June) was finally ready for installation after considerable delays due to the local council drip-feeding the finance. It's unclear whether the revolving restaurant will have a tenant yet, or whether lack of cross-river development will make this something of a white elephant.

11 September 2011

London Bridges: 16. Royal Victoria Dock Bridge

The last in my current set of bridges from London's Docklands is further east than the others, at Royal Victoria Dock. In 1995, the London Docklands Development Corporation held a contest to design a bridge spanning this huge, if now little used, dock. I featured some of entries here previously.

The winning design was a spectacular 127.5 span "inverted Fink truss" bridge, designed by Techniker with Lifschutz Davidson, completed in 1998 at a cost of £5m. It was originally intended as a transporter bridge, with a gondola active at low level. The bridge has, however, never seen enough use to justify installation of the gondola. The adjacent ExCeL exhibition centre is one of the venues for the London 2012 Olympic Games, which may see the bridge getting much greater use, at least for a short while.

There are very few bridges of the "inverted Fink truss" type around. The only other examples which come to mind are the Forthside Footbridge, at Stirling, and the Passerelle La Défense, in Paris, although there have been unbuilt proposals.

A key challenge in designing the bridge was the need to maintain a 15m clearance to the dock, allowing tall-masted ships to pass below. This was imposed by the client upon competitors, and seems to me to have been what the kidz would now call an Epic Fail. In the absence of the gondola, to cross the bridge you have to ascend and descend multiple flights of stairs, or endure the mangy lifts. It's not even as if the view from the bridge is sufficiently exciting to justify the exertion. As the dock is hardly ever used by tall ships, I would have thought that a low-level structure with an opening element would have been a better choice.

If the gondola is ever installed, I think that opinion would be reversed, and I think that seeing the bridge operate as a modern-version of a Ferdinand Arnodin-style transporter bridge would be quite splendid.

A short distance to the north, an elevated walkway structure connects ExCeL to a Docklands Light Railway station, but the possibility of connecting this directly to the bridge via a ramp has so far been missed.

Viewed in elevation (as in the first photo above), the stair and lift towers, which are heavily metal clad, have a monolithic, blocky character totally out of keeping with the rest of the bridge. However, the bridge is generally viewed from a much more acute angle, and the towers are less problematic from most perspectives.

The bridge itself is a very strange beast, something like an elephant as it might be sculpted by Alberto Giacometti. The deck is just that, a promenade-style walkway translated vertically, and set upon comically spindly stilts. This is not entirely a bad thing, it emphasises the sense of the bridge as a platform rather than as a pathway.

The cable and mast arrangement above the deck has a certain logic relating to the diminishing forces to be carried by successive cantilevers away from the supports, an approach repeated by the other inverted Fink truss bridges and which here appears justified by the visual scale of the bridge in relation to the dock sides. Single masts on the bridge centreline give a clarity to the composition which isn't harmed at all by the consequent need to have twin pairs of cables connecting the masts to both sides of the deck. The mast and cable arrays form a set of overlapping tetrahedra, with the variation in mast height eliminating any geometric clash of cables (except for the centre bay, where a link arrangement has to be used to solve this problem, see the photo).

I find the relationship of the above-deck masts to the below-deck support legs to be less admirable - it would have been great to have seen single legs relying only on cables for lateral stability. The sense of incongruity is enhanced by the adoption of a cigar-shape for both the legs and the masts, although this makes structural sense.

The bridge deck has a dual personality - Iroko timber decking and handrails give the feel of a promenade from above, while from below, it has the qualities of a metallic millipede, or perhaps the snout of a sawfish. Structurally, there is a steel box-section spine beam, with the deck and balustrades supported from cantilevered fins. The box incorporates a slotted element in its soffit which currently carries a maintenance gantry and which would carry the proposed gondola. The designers, Techniker, make great play of the fact that the main deck elements are not bolted or welded together, but pegged together and prestressed, which is a highly unusual choice for a structure of this type and scale.

The balustrades are in perforated plate, which gives a greater feeling of security at this height than would a lighter-weight wire, rail or mesh solution, and also enhances aeroydynamic stability. As with most elements of the bridge, they seem to be lasting well.

The bridge deck at first seems very slender, although at deck level, it is apparent that this is because most of the spine beam box girder has been allowed to protrude upwards through the decking.

The steel box is profiled to carry the bending moment between points of support, in a similar manner to Castleford Footbridge. However, at Royal Victoria Dock, the opportunity to create areas of seating was missed. The boxes resemble upturned boat hulls, part of a nautical theme that persists throughout the structure (with its spars, cables, and bowsprits), but I think seats would have been preferable.

The Royal Victoria Dock Bridge is described by its designer, Matthew Wells, as having "reacted strongly against the prevailing fashion for iconic bridge structures presented as catalysts for urban renewal" (in Engineers: A History of Engineering and Structural Design (2010)). While there has been very little urban renewal at the site (the ExCeL centre is pretty much it), it's hard to see a bridge of this scale, idiosynractic choice of form, and modernist starkness as anything other than iconic.

I'll finish by noting that  there were plans to supplement this bridge with another further along the dock. In early 2010, a competition was announced, subsequently won by Ian Ritchie Architects. Ritchie came up with a number of suggestions, including a floating bridge, but a unique swing bridge solution, the "water boatman" was the favourite until Olympics funding cutbacks killed the entire scheme. It only emphasises what a mistake it was to require the 1998 structure to be so high in the air - while it resulted in a very heroic, spectacular design, it provides far from the ideal pedestrian experience.

Further information: