01 November 2020

"Bridging: by Foot and Bicycle" by Ney & Partners

Here is an excellent book devoted to  the designers du jour in the world of bridges, Ney and Partners. Once past its ungrammatical title, Bridging. By Foot and Bicycle (Archibooks, 264pp, 2019) is an excellent and very well-presented survey of the firm's pedestrian bridge oeuvre, filled with photos, visualisations, drawings, diagrams and informative text (in both French and English). As with the firm's bridges, the book is not completely without its flaws, but I can recommend it to anyone with a serious interest in the very best bridge design.

An introduction titled "A bridge has to be designed!" by Laurent Ney sets the tone. Ney reports his experience working on a project with a large multi-disciplinary firm, who apparently responded with embarrassed silence when Ney asked "Who designed the bridge?" While this hardly rings true (such firms are obviously well populated with people who design bridges) his point is that the norm in design firms when presented with a bridge project is not to design from a tabula rasa, but to select and adapt from an existing structural typology. His argument is that there is no real "design" in this approach.

This is the very opposite of Ney's own modus operandi which is (to at least some extent), to see how context and constraints can point the way to creative opportunities, minimising preconceptions wherever possible. This book illustrates that effectively, explaining in detail the design decisions that were made on the various projects featured.

From the outset, this also highlights a lacuna that recurs throughout the book. It is the designer's perspective, and largely a history of various designed objects and why they are how they are. There is little in the way of self-criticism, and essentially no space for the voice of those who commission or use these bridges. This is not uncommon in an architectural monograph, and in this case, where much of the discussion is inevitably somewhat technical, the sense that there are people on the bridges and they may have something to tell us sometimes vanishes entirely.

The technical element in the book is inevitable given the superlative engineering at the core of many of Ney and Partners' bridges. Sometimes, their astonishing bridges seem to be the output of a designer who is operating as a naïf, ignorant of the conventions of bridge design and hence able to devise solutions that would never even enter the peripheral vision of an experienced engineer. At other times, they seem the work of an auteur, someone with an in-depth understanding of the bridge engineering craft but the desire to pursue excellence and never accept compromise.

Fellow engineers often admire Ney and Partners' bridges because this approach - creative, challenging, courageous - leads to structures which are remarkable in their geometric perfection and economy of detail, slimmed down well beyond what others ever attempt, let alone achieve. This is why the technical issues are key throughout the book and it could never be purely about the architectural aspects of design.

Although the bridge designs featured often take the idea of refinement to extremes, the bridges are only rarely completely rational in their conception. It's always clear that intuition has been applied, and subjective choices made. This is particularly the case in the first few bridges presented, which share a theme of history and context.

The as-yet unbuilt Poissy Footbridge is a proposal for a new bridge on the site of the remnants of a historic bridge across the River Seine. The historic Pont de Poissy was largely destroyed in 1944, and never rebuilt. The Ney design follows the same alignment but flies above the remaining bridge piers, supported from tetradactyl steel supports sitting in between the original masonry. The new bridge's longest span is 93m, yet the deck is formed from a single, ultra-slender folded steel plate. The impression is of a gently undulating ribbon, dancing across the river, dipping down over the existing piers but darting back away from them as if suspended on air. It is both a little incongruous, giving the initial impression of being structurally unreasonable, and also rather stunning.

Dejima Footbridge, completed in 2017 in Nagasaki is very different but illustrates some of the same aspects of the Ney philosophy. This is a cantilever bridge, arranged with one fixed end so that the "free" end imposes as little load as possible on the more archaeologically sensitive of the two river banks. The two edge girders feature multiple rows of stiffening plates, with the web perforated in a visually interesting manner. However, the stiffeners are to a great extent decorative, and the shape of the girders gives the visual impression that the bridge functions like an arch. It's a beautiful structure, but the engineering and architecture are not integrated in the way that many other Ney designs achieve.

The book's discussion of the Tintagel Footbridge serves to illustrate the point that Ney and Partners don't entirely ignore conventional typologies, but that their design process can allow them to take or adapt those standard forms in interesting and site-appropriate ways. At this site, Ney's analysis of the normal bridge forms led them towards an arch as a visually and contextually desirable proposal for the site.

A key issue at Tintagel was the difficult access for construction, leading to consideration of a bridge which was built by cantilevering from its supports (a well-trodden method for building metal arch bridges). The bridge as-built retains the cantilever form, as making it into a genuine arch would have been structurally far more challenging (a result of the sag curve of the pathway, and the consequences for thermal restraint).

It's a spectacular bridge and the engineering and architectural ideas are well-explained in this book, but very little is said about how its users find it (especially with such filigree parapets high above a chasm, and with a gap in the floor at midspan).

The same is true of a structure like the Park Footbridge in Antwerp. This is one of a number of designs where Ney and Partners apply a "subtractive process", defining a stable shape and form, generally in thin sheet metal, and then looking to see what metal is unnecessary and can be cut away (the carbon balance of reduced material versus increased fabrication process is never discussed). The structure here is a hybrid between a bowstring arch and a box girder, although, as with many architects, that's a term that's never used - after all, who would celebrate being able to walk through the interior of a girder (it's not 1850 any more)?

It's an amazing work of structural engineering, absent all the bolts and stiffeners that a normal box girder interior displays. The webs are perforated in a manner that takes account of levels of stress, but is not entirely determined by that, with far more material left in place than can be structurally necessary - compare a proper bowstring arch where a handful of cables suffice to connect the arch to the bridge deck. It looks like an amazing experience to walk through but ... why is it a covered bridge at all? Will the patterns of illumination inside be tolerated by pedestrians susceptible to flicker? And is a floor-rail, an obvious trip-hazard, really the best way to stop cyclists bashing their heads on the girder web? To me, it feels like the uncompromising desire to maintain the purity of the design object results in a design that is not completely comfortable.

This is not the only Ney bridge where this can be said. The Vluchthaven Footbridge in Amsterdam has the same sinuous deck plate as the Poissy design, and an ingenious parapet design. The client was keen to address the common Amsterdam problem where bridge parapets serve as a favoured place for bicycles to be parked and locked. Ney and Partners came up with an elegant parapet design with all verticals - no top rail or handrail. If someone were foolish enough to lock a bicycle here, it could just be lifted off. It's smart and very well detailed - but handrails are a good thing for many bridge users, especially the elderly or infirm who may want to take the opportunity to stop and briefly secure a handhold. This does not appear to a bridge for stopping on.

A very different result from the subtractive approach mentioned above is the Knokke Footbridge, which I discussed right back in 2009. This is, in my view, one of the firm's masterpieces, using one curved steel plate to satisfy the requirements both of transverse load distribution (a curved skin acting in tension) and as the primary longitudinal structure (sharing the characteristics of a suspension bridge and also of a Robert Maillart arch, inverted). The Y-shaped bridge supports, and the way they hold up the deck, are brilliant.

In writing this book review I've been drawn to writing about the flaws in these designs, because I feel that the monograph style of the book (uncritical, celebratory) and the presentation entirely from the designer's perspective (their subjective view is privileged over anything else) do beg for some degree of challenge.

However, Ney and Partners are easily one of the best bridge designers working today, bringing together a very rare blend of imagination with the superlative technical ability required to turn their audaciousness into reality. They operate way beyond the level of the vast majority of bridge designers.

The breadth and variety of their designs shows that they do take context seriously (contrast their designs, for example, with someone like Calatrava). The detailing of their bridges is frequently exquisite, and the book's photographs and technical drawings make that abundantly clear. I can't imagine a bridge designer who wouldn't enjoy and learn from this book, and non-specialists should also find their work well worth discovering in more detail.

30 September 2020

Welsh Bridges: 20. Llantysilio Chain Bridge

This bridges goes by a number of names - Berwyn Chain Bridge may be equally as appropriate. Signs at the site just call it "The Chain Bridge". It is neither a Listed Building nor a Scheduled Monument, which will only be surprising if you incorrectly imagine that our heritage bodies are competent.

The first bridge across the River Dee at this location was the work of local man Exuperius Pickering, variously described as an entrepreneur or a "coalmaster". Pickering was looking for a way to transport his coal and other materials between the Llangollen Canal (1808) and Telford's recently improved London to Holyhead Road, without paying tolls to cross Llangollen Bridge. Conceived in 1814, his bridge was completed in 1818.

This was a period of rapid development in cable or chain-supported bridges within the United Kingdom. Granted, the Winch Bridge, an iron chain catenary structure, had been built over the River Tees in 1741. However, it was the early 19th century when cable and chain bridges took off, with stayed bridges in Galashiels (1816), King's Meadow Bridge (1817) and Dryburgh Abbey Bridge (1817, rebuilt as a suspension bridge in 1818), and the Union Chain Bridge (1820, suspension bridge). Things advanced rapidly enough for Robert Stevenson to present an article surveying these and other designs in 1821, as well as proposing his own bridge at Cramond, an underspanned suspension bridge, which was never built.

Pickering's bridge sits right in the middle of this chronology. Happily for posterity, drawings of the bridge were made by the French traveller Joseph-Michel Dutens (see below). These show the bridge to be an underspanned suspension bridge, with eyebar chains supporting the deck, and an additional tension rod below this, perhaps to enhance stability. The bridges I mentioned above were well-reported, and it's often stated that Stevenson was the first to propose an underspanned suspension bridge, and James Smith's Micklewood Bridge (1831) the first to be built. In reality, Pickering got there first, although how much of an improvement his structure was over a simple catenary bridge might be doubtful.

The first drawing by Dutens shows half of the bridge (it was a three-span structure), while the second drawing gives cross-sections and details of the chains. A dozen chains passed below the bridge deck to provide support.

In addition to the drawings, photographs of Pickering's bridge survive, although showing it enhanced on one side by a timber truss.

The bridge lasted remarkably well, until it became unsafe and was dismantled in 1870. In 1876, Henry Robertson, owner of Brymbo Ironworks, rebuilt the three spans and re-used the original chains, again adopting the underspanned system (photograph below). This one was destroyed in flooding in 1928.

Roberton's son rebuilt the bridge the following year, but this time with only a single pier in the river. The chains were re-used, but now to form a suspension bridge, with three suspension chains on each edge, and two stiffening chains connected along the deck underneath.

One tower sits on an outcrop of rock within the river, and the other on the river wall at the north edge. The river tower was protected by a large concrete pier, rendering the new bridge far less susceptible to flood damage.

The chains at the south end of the bridge were anchored into the ground, while at the north end they pass over the adjacent Chainbridge Hotel and were anchored into rock high above the canal. The deck chains were anchored into the ground using an adjustable tensioning system.

A pair of bars hang downwards from each chain link, and these are connected to a triangulated system of lower hangers. These in turn carry the lower deck chains and the timber deck.

The bridge was load-tested with 45 people when it opened, and lasted reasonably well, becoming gradually more dilapidated until being closed as unsafe in 1984. In 2014-15, it was completely refurbished, with all the metalwork carefully dismantled and then reinstated.

The works were completed by local firm Shemec Ltd to a design by consultants Ramboll. The engineers completed a careful structural assessment of the bridge, determining that even if corroded ironwork was replaced, it could not carry anywhere near modern loading requirements, being limited to 1.5 kPa of load. This equates to roughly 5 tonnes of load on the 24m main span, or around 60 people. Llangollen Town Council, who had taken over responsibility for the bridge, agreed that this was sufficient. Warning signs at the end of the bridge request that no more than ten people use it at once.

The reconstruction works are well documented in a paper by Ramboll and in photos on the Chain Bridge Project website. I'm not clear what proportion of the original metalwork was preserved and reused, but new pieces were fabricated in mild steel to match the existing details and dimensions wherever any piece could not be reused. All the chain pins had to be replaced. Nonetheless, in the rebuilt bridge it is claimed that these are the oldest bridge suspension chains in Britain to remain in use.

Prior to the refurbishment, there was no parapet remaining on the bridge. The reconstruction introduced a series of new parapet posts, a tensioned upper cable, and a mesh infill system. I'm not sure how well these match any parapet that had been there in the past, but I doubt the new system is compliant with normal modern standards.

Indeed it's interesting to compare the refurbishment work at Llantysilio with what was done at Brabyns Park Bridge in Marple, which I discussed in a recent post. The chain bridge project is an exemplary piece of conservation engineering, where even though the structure is not Listed, it has been treated with integrity and the original details preserved as closely as possible. The engineers sensibly recognised that compliance with modern standards would have been entirely inappropriate. By contrast, the Marple structure is Listed Grade II, but senseless attempts to impose modern standards on it have largely ruined its appearance (although thankfully not irreversibly).

The Llantysilio Chain Bridge is unique both in the complex history of its surviving structural fabric, and in its form and details. It is well worth visiting, in a setting within view of two other fine bridges, and with plenty more to see within walking distance.

Further information:

27 September 2020

Welsh Bridges: 19. Lôn Las Ogwen Footbridge

Not far from the Britannia and Menai Bridges, the dedicated pontist may happen upon this lesser-known footbridge.

It carries the Lôn Las Ogwen, a walking and cycling route, over the A4244 highway. The trail follows the line of the former Penrhyn Quarry Railway, which was closed in 1962.

The footbridge diverts from the original line of the railway, presumably to allow a small railway junkyard to be preserved on the south abutment of the original railway bridge.

I don't know who designed the bridge, possibly local consultancy YGC, but it was fabricated by D. Hughes Welding and Fabrications, and built by contractor Mulcair Ltd. At a guess, the main span probably doesn't exceed 20m.

At first glance, it's a steel arch bridge with a rather chunky looking parapet, decked out in the patriotic Welsh colours of green, white and red.

A second look makes clear that it is, as the fabricator says on their website, "a Vierendeel Construction with a Decorative Arch".

Opinions on this may vary. Some may note that it is just another in a long line of fake arch bridges, and hardly as egregious as some examples. Others may wonder if the emphasis on superficiality over substance combines with the colouring to act as a sly post-modern comment upon the inherent hollowness of nationalism.

I'm not sure I would go that far, but I can say that I don't like it.

Further information:

23 September 2020

Welsh Bridges: 18. Britannia Bridge

Thomas Telford built two significant suspension bridges on the north Wales coastline: the Menai Suspension Bridge (1819-1826), and Conwy Suspension Bridge (1822-1826). These formed part of a significant and much-needed improvement to the nation's highways. However, they were completed just four years before George Stephenson's Liverpool and Manchester Railway would kick start a very different transport revolution.

Roughly two decades after Telford did so for roads, it was George's son Robert Stephenson's turn to bring the railways to north Wales and Anglesey. He built two revolutionary bridges to span the exact same stretches of water as Telford: the Conwy Railway Bridge (1846-1849), and the Britannia Bridge (1846-1850). And just as had been the case for Telford, Stephenson could not do it alone.

The bridge across the Menai Strait was the most challenging element in the Chester and Holyhead Railway, and decisions on how to span the Strait remained unresolved while designs progressed for other parts of the line. Some thought was given as to whether the Menai Suspension Bridge could be modified to carry trains, but the loads required for a railway far exceeded those imposed by the horsepower that initially crossed Telford's bridge.

As in Telford's time, consideration turned to building a new arch bridge, but the Admiralty insisted on the provision of full clearance for high-masted ships across the full width of the Menai. Having settled on an alignment that made use of Britannia Rock in the middle of the channel, Stephenson proposed a flat span structure, with girders supported from above by suspension chains. The bridge towers were designed and then constructed tall enough to support such chains, although in the end they were never installed.

It seems that Stephenson conceived initially of a suspension bridge, and then sought a way in which it could be made sufficiently stiff to carry railway loads. He turned to William Fairbairn to investigate the feasibility of tubular stiffening girders, through which the railway tracks could run. Fairbairn rapidly came to the conclusion that the suspension chains would be too flexible, and should be dispensed with, but the less confident Stephenson kept provision for the chains until the bridge was complete.

Fairbairn undertook many experiments on tubular cross-sections, and in turn involved the mathematician Eaton Hodgkinson to analyse the experimental results. Stephenson's preferred girder design was for an elliptical cross-section, but Fairbairn soon determined that a rectangular section was more efficient. It rapidly became clear that buckling of the top flange of the girder was the key issue, a problem that was resolved by adopting a cellular upper flange to the girder, initially comprising two hollow circular tubes joined together, and later evolving into multiple cells side-by-side. 

Fairbairn constructed a 75ft span model tubular girder to resolve the final details of the rectangular tube design. The side walls required internal stiffening, and in the final design both the top and bottom flanges were made cellular. Although Fairbairn's experiments had been on single spans, the bridge was built as a continuous girder, giving it additional strength and stiffness.

Some of the other key participants in the project included Stephenson's assistant Edwin Clark, and Fairbairn's assistant Mr Blair, who was largely responsible for producing all the bridge's design drawings. After Fairbairn and Stephenson fell out in a dispute over recognition as being the true designer of the bridge, it was Clark who wrote the account setting out Stephenson's side of the story. Fairbairn published his own, and it seems generally to be regarded as the more honest version.

Credit is also due to architect Francis Thompson, who designed the masonry elements in a vaguely Egyptian style, as well as several other works along the railway. Thompson later worked again with Stephenson on Victoria Bridge, Montreal, another tubular bridge, as part of the Grand Trunk Railway in Canada.

Four sculptural lions were installed, one at each corner of Britannia, Bridge,sculpted by John Thomas, who also worked on the Palace of Westminster.

Hodgkinson had also fallen out with Fairbairn, essentially over the latter's willingness to extrapolate the results of his experimental work in the absence of a justifying mathematical theory. Around this time, Fairbairn began building many girder bridges with tubular (box) girders, but suitable theory was only just becoming available to practicing engineers. The sheer scale of the Britannia structure went well beyond what had been attempted previously - just as Telford's Menai Bridge had done a quarter of a century before.

The project innovated in many ways. The extensive reliance on wrought iron was pioneering, and the span was exceptional for a flat-span bridge. The range of experimental work relied upon was impressive, as was the idea for the cellular construction. Even the erection of the bridge required major innovation, with the girders lifted into place by jacking upwards with massive hydraulic jacks. The slots for the jacking process remain visible on the towers, and part of one jack can still be seen near the bridge on its south-west side.

Two million rivets were reported to be used, with workers having to squirm through the box cells to install many of them. This, more than anything else, determined the size of the cells used.

On 24 May 1847, while construction of the Britannia Bridge progressed, one of Stephenson's other railway bridges collapsed, killing five people. The bridge over the River Dee near Chester was constructed of three cast iron girder sections connected with wrought-iron link bars. It was a popular design at that moment of time, with at least thirty-four built prior to the Dee failure. Fairbairn had proposed in 1846 that Stephenson should use a wrought-iron tubular girder bridge across the Dee, but had been turned down.

This incident exposed Stephenson's lack of expertise as a structural engineer, and Fairbairn's views prevailed both at Britannia and more widely - he was involved in over 100 more tubular girder bridges (albeit predominantly with the girders sitting beside the tracks, rather than containing the tracks) within a 5 year period.

While the tubular girder was successful in the short-term for short and medium span bridges, it was not the optimal solution for larger structures, and the Britannia Bridge design would prove a dead-end. Before long, various forms of lattice-girder and truss bridges took over, although early lattice-girder railway bridges experienced their own problems. For more detail I can wholeheartedly recommend John Rapley's and Richard Byrom's books (see list of references below), both of which are excellent.

Britannia Bridge was bold, if not entirely beautiful, but I think there is a great deal to admire in its simplicity of line. It lasted 120 years until, on 23rd May 1970, a fire broke out, irreparably damaging the bridge's two tubular girders.

The replacement bridge seen today was built between 1971 and 1974, with two main truss arch spans over the Menai Strait. Both Telford and Stephenson had considered arch bridges, and finally the navigational restrictions that had forced both into bolder and more innovative designs were no longer an issue.

The form of the present-day bridge, designed by Husband and Co. (merged into Mott, Hay and Anderson in 1989, now Mott MacDonald), owes a great deal to the challenges of safely dismantling the damaged tubular girders, as well as to the need to reinstate a railway line as quickly as possible. 10,500 tons of metalwork had to be removed, forming a load well in excess of the railway traffic that the replacement structure would carry, and the arches were therefore designed and sized primarily to act as support to the demolition operation. Once the tubes were safely and temporarily supported, railway services were reopened through one of the damaged tubes in January 1972. The tubes themselves were cut into short sections, and then hauled off the end of the bridge using small locomotives.

The bases of the towers were extended with small concrete skewbacks to carry stainless steel pins, which carry the entire load of the new bridge. The steelwork for the new arches was assembled by Cleveland Bridge four miles from the bridge, at Port Dinorwic, and floated into place on barges.

The spans were cantilevered outwards from the central tower, with adjustable tie bars passing through the tower to provide temporary support. Lifting gantries moved along the upper chord of the arch truss to lift each new truss unit into place, as can be seen in the construction photograph below (taken from a souvenir booklet about the bridge).

During construction, the arches each briefly formed a three-pinned arch before pre-load was jacked into the upper member to transform the whole system into a two-pinned arch.

Because the arches had capacity well in excess of railway loading, this created the opportunity to add a second deck to carry highway loading, and openings in the towers were enlarged to facilitate this. The railway bridge was finished in 1974 (albeit with only one deck carrying services, as railway traffic was much diminished), and the road deck eventually completed by Fairclough Civil Engineering and Fairfield Mabey in 1980.

The steelwork for the new railway bridge weighed less than half of Stephenson and Fairbairn's original wrought iron bridge, only 4,961 tons, although the road bridge (which is nearly twice as long as the rail bridge) incorporates another 4,338 tons of steel.

Although it is often noted that the ordinary observer prefers an arch bridge over any alternative, the modern bridge is, to my eyes, less loveable than the original. Partly this is because of the sheer quantity of truss bracing, and partly that the visual relationship between road and rail decks is uncomfortable. I think this is partly due to the sheer depth of the edge beams at railway level.

On the plus side, the history of the bridge is there to be seen. The excess tower height originally intended to carry suspension chains contributes to the support of the road deck and punctuates the span in a pleasing way (compare Sydney Harbour Bridge). The form of the arches betrays their origin as falsework for a demolition process. The preserved cross-section of  tubular girder (accessible via a path leading to the south-west corner of the bridge) is well worth visiting. The masonry still looks excellent today, and the bridge's best-kept secret, the "cathedral" vaults at each end, are still intact albeit normally inaccessible.

Further information:

20 September 2020

Welsh Bridges: 17. Menai Suspension Bridge

Where do you start when trying to write a simple blog post about a bridge like this? So much has already been written (see links at the end, which are selective and do ignore some of the more detailed publications)!

The first serious proposal for a bridge over the Menai Strait came in 1802, when John Rennie proposed a multi-span viaduct of masonry and cast iron. A few years later, in 1811, it was Thomas Telford's turn, presenting designs for either a multi-span cast iron viaduct similar to Rennie's or for a single cast-iron span. Neither of these ideas were adopted.

Telford revisited the site in 1818, and prepared plans for a suspension bridge instead. Construction work began on 10th August 1819, three years ahead of Telford's suspension bridge at Conwy. Both bridges were completed in the same year, 1826.

The bridge at Menai became the longest bridge span in the world, its 577 feet length exceeding the 449 feet of Samuel Brown's Union Chain Bridge, completed six years earlier in 1820. Brown's bridge had commenced construction only a few days before the Menai bridge, on 2nd August 1819, but was built much more quickly than Telford's bridge.

Menai Suspension Bridge held the span record for 8 years before being overtaken by the 889ft Fribourg Suspension Bridge, in  October 1834, a month after Telford's death. It's maybe worth noting as a historical aside that the Union Chain Bridge's earlier record is attached to some degree of doubt: the 1430 Chushul Chakzam footbridge in Tibet may have been a very similar span, although records are poor.

The Menai Bridge's span was a remarkable achievement, and if it isn't Telford's finest bridge, I think it's the most substantial engineering challenge that he ever took on.

Telford had been looking at suspension bridge ideas since 1814, when he was commissioned to develop a proposal for a road bridge at Runcorn. That design was for what would have been an astonishing 1000ft span, something that would not be achieved on any bridge until 1849. Telford proposed to form the Runcorn bridge's catenaries out of half-inch square iron bars, welded and bound together into sixteen "cables" each comprising 36 such bars. He arranged for extensive testing of the strength of iron to inform the design, and built a model suspension bridge 50ft long.

The promoters of the Runcorn crossing invited others to submit designs for review by Telford. The only submission to meet his approval was a suspension bridge proposal from Samuel Brown, who proposed catenaries comprising iron chains. Telford visited Brown's factory in February 1817, where he was driven across Brown's own model bridge, albeit quite a substantial model some 100ft in span. At the time, Brown was working with chains made from iron rods, as he was to use for the Union Chain Bridge, although he also developed chains made from flat iron plate.

The Runcorn bridge was never built, but when invited to develop the Menai crossing, Telford at first continued with his idea of square iron bars welded and bundled to form cables. Perhaps he was influenced by Brown's patenting his own chain bridge ideas in mid-1817. It was only later, as work proceeded on the masonry parts of the Menai bridge that Telford switched to flat-bar chains, supplied by William Hazledine. There were to be sixteen chains in total, with four groups of four chains arranged vertically above each other; one group at each edge of the bridge, and two on the centreline of the roadway. There is a good image showing the original suspension arrangement at Wikimedia Commons.

Incidentally, it is sometimes claimed that Telford sought permission from Sarah Guppy to use her 1811 patent for suspension bridges. Guppy's patent appears to have been for a catenary bridge, with the decking laid directly onto the suspension bridges, not for the type of bridge that Brown and Telford pursued. There seems to be little substance to this claim, but Telford certainly did rely very much on the assistance of others. Examples include learning from Brown's success in pioneering the use of iron chains; Hazledine's manufacturing capabilities; Telford's right-hand engineer William Provis; Peter Barlow's advice on the strength of iron; and Davies Gilbert's understanding of the mathematics of the catenary.

Telford's bridge encountered problems almost as soon as it was complete. Strong winds caused damage to the timber deck and to the hanger bars just one week after it opened. Remedial works were completed, but a storm in 1836 caused huge oscillations and significant damage, and then in 1839 another storm left the deck in ruins and the bridge impassable. Provis was employed to design a stronger, heavier deck.

Issues with wind on suspension bridges were by no means unique to Menai. Similar issues occurred around the same time on Samuel Brown's South Esk Bridge in Montrose, and wind-induced oscillation was also observed at Gattonside Bridge. Telford had not been unaware of the issue, and before the bridge was complete he was reported to have considered stiffening the deck with trusses, deciding eventually that if ever required, they could be retrofitted. The Menai Bridge was a giant engineering prototype, and as with any experiment, its performance was never entirely foreseeable.

The strengthened bridge lasted until 1893, when a new steel deck designed by Sir Benjamin Baker was introduced, largely to resolve problems with the deteriorated state of the timber deck. Further investigation and remedial work took place on several occasions before a decision was made that the bridge could no longer safely carry the loads required.

Between 1938 and 1940, the metal parts of the bridge were completely reconstructed, to a design prepared by Sir Alexander Gibb and Partners, and consultant Guy Maunsell. If the work had not already been underway, it's impossible to imagine it would have started once the Second World War began, given the quantity of steelwork involved and other demands for skilled labour. In any event the bridge was completed, but Maunsell was rapidly immersed in the war effort, turning his engineering skills towards sea forts and the concept behind the floating Mulberry Harbours. Due to the needs of wartime secrecy, his account of the Menai Bridge reconstruction was only published after the war had ended.

The masonry approach spans, which are themselves impressive structures, were left unaltered. Works were undertaken on the upper towers to slightly widen the portals through which vehicles pass. The masonry Bridge Master's House at the southern end of the bridge had its upper parts rebuilt to accommodate replacement of the suspension chains.

The suspension chain alterations included reconstruction of the anchorage elements hidden within tunnels at each end of the bridge. Temporary suspension cables were installed at the edges of the structure to relieve the load on the outer chains. The original sets of four chains directly above each other were replaced with sets of two chains directly above each other, with larger links in much stronger steel.

A new deck was constructed below the existing deck, to allow traffic to continue to use the bridge during the works. The existing deck was then removed (one lane at a time), allowing traffic to drive up and down ramps onto the lower deck. Once this stage was complete, the new deck was gradually raised into its final position. The original centre chains were removed entirely, with the only real evidence today of their existence being the empty slots in the face of the former Bridgemaster's House. The new edge trusses were then completed, considerably enhancing the load carrying capacity of the bridge.

I doubt that casual visitors to the bridge see it as anything other than Telford's structure. The profile remains the same, including the strange back-span arrangements where the chains are anchored directly down into the approach viaducts with hanger bars. Given the over-riding need to enhance the traffic capacity of the highway, the reconstruction was a relatively sensitive project. Even retaining chain catenaries was a technologically unusual choice in the mid-20th century: nobody was still building chain bridges at that point in time.

The trusses were foreseen by Telford, and don't mar the overall appearance of the structure, although the tacked-on cantilever footways are narrow and the new parapets feel over-tall. The detailing of the footway widening on the approach viaducts gives the impression that it was always there.

The bridge now provides one of the best viewpoints in the vicinity, and is one of the UK's most significant engineering landmarks. As with many such large bridges, it has come to define the character of the Menai Strait, visually structuring the way that visitors experience the area as well as remaining a key transport link.

Another bridge was built in 1850 to carry the railway across the Strait (later converted to become the main highway in the 1970s), and plans are under consideration for a third crossing. As with the Forth in Scotland, the prospect of a "family" of bridges is enticing, although it is too early to tell whether the new plans will be as visually successful.

Further information: