Beneath the eastern towers of Conwy Castle in northern Wales, the town’s river is crossed by a tightly bundled trio of bridges, each one entirely unalike the others. The most modern one, a simple steel arch from 1958, carries the main road from the east bank and allows motorists a simultaneous view of the other two, Thomas Telford’s 1826 suspension bridge and Robert Stephenson’s 1849 tubular railway bridge. These two structures mirror in miniature their engineers’ respective bridges across the Menai Strait- Telford’s Menai Bridge and Stephenson’s Britannia Bridge. Today, with the latter pair far apart and the Britannia Bridge greatly modified, Conwy is the best place to see their designs- which are among the most influential in the history of civil engineering- up close and in clear contrast.
The bridges and castle, viewed from the east
Telford, whose life I have previously chronicled, monopolized English engineering in the early decades of the nineteenth century. Britain is today crisscrossed by his roads and canals, and his bridge at Conwy initially carried one of the former. Like the Menai Bridge, the Conwy bridge adeptly uses the three major construction materials of the time; iron chains and rods bear the weight of the span, itself originally a bed of wooden beams, with the whole structure ultimately resting on crenellated stone towers and, at one end, anchored to the ancient rock walls of the castle. Today the bridge is reserved for pedestrians, many of whom find it to be the most beautiful of the three bridges. As Telford intended, it could be mistaken for the castle’s drawbridge, and the intricate chains and hangers invite close-up examination.
Top- The intricate technical ironwork; Bottom- the chains anchored to the castle walls
The imposing rail bridge beside it, despite boasting larger castlesque towers, probably inspires less admiration than its delicate counterpart. No ornamentation graces the iron panelling of the twin tubes that traverse the Conwy River, which by virtue of their volume cast the water below into Stygian shadow. Nevertheless, it is a work of genius on the part of Robert Stephenson, the same man who built the Rocket, the most famous of the earliest working locomotives. Time was money for the multitalented Stephenson, who designed the structure to be built quickly without sacrificing strength, for he correctly foresaw that trains would grow larger and more powerful over the coming century.
Top- The twin tubes of the bridge; Bottom- a close up view of the cladding
Both of the Conwy bridges are not especially significant in and of themselves- they are more or less prototypes for each engineer’s civil magnum opus on the Menai Strait. Spanning this water was a particular challenge not only because of the span- which could have been easily crossed by a large but precedented masonry arch- but the height demanded by the royal navy, whose tall warships made use of the strait as a passage. Telford, above all a brilliant theorist ahead of his time (as seen in his early use of cast iron), designed a suspension bridge with a span without equal, having extensively tested iron chains to prove they could bear such an immense load. Telford realized something that engineers still hold true to this day- in most cases, a suspension system is the most material-efficient way to cross an unusually long gap. The Menai bridge was, however, expensive and the materials imperfect- it has undergone multiple strengthening renovations.
Top- Britannia Bridge; Bottom- Menai Suspension Bridge
Where Telford was theoretical, Stephenson was resourceful. Materials at the time were not strong enough to construct a suspension bridge for trains (these would not materialize until John Roebling made use of steel cables), so Stephenson decided that he had to span the strait with two beams either side of a small central island. Stephenson’s lightbulb moment came when he realized that the rigid, rugged iron girders then just starting to be used to support small bridges could be massively expanded so that a train could run through the center. The resulting tube was both incredibly strong and fast and cheap to produce. In both the Britannia and Conwy bridges they were manufactured offsite and floated into position. The bridges were definite successes, accommodating changes in rail travel for over a century until the unfortunate destruction of the Britannia Bridge in a 1970 fire that could have been prevented by restoration.
In their own individual ways both designs revolutionized bridge construction. It could be argued that all modern suspension bridges have their genesis in Telford’s Menai Bridge, elements of which, like multiple chains spaced across the width, established this form of bridge as the first choice for long spans and were used extensively by future engineers. Stephenson’s tubes, though within a few years made obsolete by trusses, proved that iron beam bridges, when designed through the processes of experimentation and calculation that Stephenson utilized, are strong, safe, and economical- which is why they are used today for mass-manufactured bridges ranging from short overpasses to miles-long multispan structures like China’s Hangzhou Bay Bridge. Today, it is hard to believe that this pastoral corner of Wales was for a few decades the Florence of civil engineering, where a renaissance spearheaded by two industrial masters transformed the field forever.
Sketches in public domain, all photos by author
Total Views: 1034 ,