Showing posts with label Thomas Telford. Show all posts
Showing posts with label Thomas Telford. Show all posts

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.


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04 January 2020

Some recent books about bridges

I have a couple of posts on recent bridge visits in preparation, but meanwhile here's a quick round-up of a few books about bridges that have recently arrived at Pontist Towers ...

From Brycgstow to Bristol in 45 Bridges by Jeff Lucas and Thilo Gross (Bristol Books, 144pp, ISBN 978-1-90944-618-2, 2019) is a catalogue of all the bridges spanning Bristol's main waterways (at least, those that can be crossed by foot), presented in the order of a possible walking tour (albeit quite a long walk). The city is mentioned as Brycgstow in the Anglo-Saxon Chronicle, translated as "place by the bridge", sited at the confluence of the Rivers Frome and Avon. Over the years, it expanded as a port, especially after the creation in the early 19th century of the New Cut to divert the main river, turning the remainder into the "Floating Harbour".

Inevitably, a profusion of bridges resulted, and Jeff Lucas shares their stories with his own photographs in this book. It's a general interest book, so more of a travel guide or social history than a book on architecture or engineering, which is fine. The idea to write the book arose from an article by mathematician Thilo Gross applying the Königsberg Bridge Problem to Bristol, and a chapter by Gross explains this topological network puzzle in more detail.

Thomas Telford's remarkable bridge over the Menai Straits was opened in 1826, so to find an excuse for a bicentennial history, Menai Suspension Bridge: The First 200 Years (Menai Heritage, 206pp, ISBN 978-0-9932351-3-9, 2019), the author Bob Daimond has had to date events to the laying of the first stone, in August 1819. Spanning 176m, this was the longest bridge in the world when completed, a tremendous achievement given the state of engineering knowledge at the time.

Daimond's book is a definitive history of Telford's masterpiece, and very well illustrated with extracts from archive drawings, photographs etc. It discusses in detail the planning, testing, design and construction of the bridge, and its subsequent history including storm-induced failures, alterations and eventual reconstruction in the mid-20th century. As a history of engineering it is exemplary; my only complaint would be that it has little to say beyond that, on the bridge's cultural status, on the people who use it, and on its place in the wider history of suspension bridges.

Bridges by David Ross (Amber Books, 224pp, ISBN 978-1-78274-576-1, 2018) is essentially just a coffee-table photo book, a collection of photographs (with short descriptive text), arranged chronologically. The bridges are from all around the world and the photos are from a variety of photographers, so there's no special theme or style. Nonetheless, I found it a very enjoyable book. The photos are very well-chosen, and well presented, often across two pages. There are plenty of familiar bridges, plus quite a few that are less well-known, or were to me, anyway. A real effort has been made to span the globe, and the result is a fine reminder of the variety and ingenuity that bridge-builders have brought to their art over many centuries.

Ann-Mary Paterson is the great-grand-niece of William and Murdoch Paterson, two of the engineers responsible for construction of various railway lines radiating from Inverness in Scotland in the late 19th century. Her 2017 book, Spanning the Gaps: Highland Railway Bridges and Viaducts (Highland Railway Society, 96pp, ISBN 978-0-9927311-1-3; my copy was purchased from Old School Beauly) describes the history of the Highland Railways, with a focus on the structures that carried traffic through often quite difficult terrain.

The book is very well illustrated, with a mixture of historic and modern photographs, and several historic drawings. There are some informative photographs of construction, and some following various disasters, such as the 1989 collapse of the Ness Viaduct. There are some fascinating and impressive bridges along these railway routes: Culloden Viaduct, William Fairbairn's box girder bridges across the Rivers Findhorn and Spey; the timber Aultnaslanach Viaduct; Findhorn Viaduct; swing bridges over the Caledonian Canal; ornate castellated viaducts at Blair Atholl and elsewhere; and many more.
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15 August 2012

Scottish Bridges: 40. Craigellachie Bridge


I guess that of all the bridges we visited in Scotland, this was by far the best known.

As the "further information" section at the end of this post suggests, this is a very well-documented bridge, so I will say as little as possible about its history, and concentrate on how it looks today.

The facts, in brief: spanning 150 feet across the River Spey, Craigellachie Bridge was designed by Thomas Telford. It was his second cast iron arch road bridge of this type, the first being the now-gone Bonar Bridge. The bridge at Craigellachie was completed in 1814 at a cost of £8,200. It was extensively refurbished in 1964, and closed to all traffic in 1972. It's a Listed Building, Scheduled Monument, and an ASCE International Historic Civil Engineering Landmark.

On the day that we visited, which as with most of our trip was particularly wet, it was hard to appreciate the bridge at its best. Even in these conditions, it is a remarkable bridge. Perhaps the swollen river below just helps to emphasise why such a bold structural feat was attempted.

Compare Carron Bridge, which we had visited just over an hour beforehand. Both bridges are 150 foot span cast iron arches across the same river, with the ribs surmounted by X-braced spandrels. Craigellachie has four ribs and Carron has three. Carron was completed in 1863, almost half-a-century after its forebear, but visually it is a far poorer bridge.

The arch ribs on Carron are flat-faced girders, whereas Telford's bridge used pierced-web girders. They make the bridge look lighter and add visual texture. They are also less deep than those at Carron, although to be fair it has to be remembered that Craigellachie was designed only for highway traffic, while Carron Bridge had to carry steam railway locomotives.

This factor also accounts for the noticeably heavier spandrel members at Carron. Those at Craigellachie are amazingly slender, especially when viewed from close at hand. It's almost impossible to believe a modern highway bridge could be designed with such slender struts (and indeed, Craigellachie Bridge could not carry modern highway loads).

The pattern of the X-bracing is also different on the two bridges. At Craigellachie the spandrel members change in angle across the span such that the "diamonds" vary in angle to be roughly perpendicular to the arch rib. At Carron, they are oriented so that the "diamonds" are always vertical. That seems more rational, as their main function is to carry vertical loads from the deck down to the arch ribs, but it seems to me to be less attractive visually.

Another feature forced upon Carron by the nature of what it carries is that the deck is level, giving the bridge an appearance which rather stiff. Craigellachie Bridge has a vertical curvature which is much nicer.

While many of these features are consequences of either the loads supported or changes in engineering understanding, the last key feature which differentiates the bridges seems primarily a matter of choice. The abutment faces at Craigellachie are inclined perpendicular to the arch ribs, creating the sense that the bridge springs across the river, and also that the abutments are working properly to resist the thrust of the arch. The vertical abutments at Carron give a more precarious impression - it appears far from impossible that both the arch and the deck could just slide vertically downwards into the river.

The abutments at Craigellachie were never uncontroversial, however, because of the masonry turrets which punctuate the end of the span. These looked archaic even in Telford's day. I think they're rather pleasant, they frame the bridge visually, and look less obtrusive because they are dwarfed by the cliff face on the north-west end of the bridge.

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09 September 2010

Worcestershire Bridges: 5. Holt Fleet Bridge


Continuing north from the Sabrina Bridge, the last structure I visited on this particular trip was Thomas Telford's Holt Fleet Bridge, which is the next span across the River Severn (there's an 1844 iron arch bridge at Bevere Island, but it only spans part of the river).

Telford's cast iron arch at Holt Fleet was completed in 1828, and spans 46m across the Severn, carrying a roadway. It was built by William Hazledine, and is now Listed Grade II. It replaced a ferry, and is now the only highway crossing in the 12 mile stretch of the Severn between Worcester and Stourport.

As originally built, the bridge had five cast iron ribs supporting the deck via X-shaped lattices. The ribs were cast in 6.8m long segments. The bridge was strengthened and widened in 1928, by encasing both the upper and lower members of each 1m deep rib in a slab of reinforced concrete. The more vertical spandrel struts were also encased in concrete, and the original 32mm thick iron deck plates were replaced with a concrete beam-and-slab deck. I imagine that access for inspection and maintenance between the two arch slabs must be difficult if not impossible.

The concrete works have deteriorated considerably since then, with a 7.5 tonne vehicular weight limit recently placed on the bridge following inspection and assessment work. At the time of writing, considerable work is planned throughout 2011 and early 2012 to refurbish the bridge, with design by Halcrow. This will involve essentially reconstructing the reinforced concrete struts from scratch. The photos here show that the condition is poor, with extensive spalling, and rust staining in places.

It will be good to see the bridge improved from its current somewhat dilapidate state. Nonetheless, it's a fine structure, and a good example of Telford's work. There's a similar Telford bridge at Eaton Hall in Cheshire, which preserves the cast iron spandrel tracery and is much better looking, plus of course his classic span at Craigellachie. Galton Bridge, at Smethwick, is also cited as similar, but seems to me to have a different spandrel bracing pattern.

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