Stephenson's Tubular Bridge footnotes
Stephenson’s Tubular Bridge
The railway tracks past Conwy Castle disappear into two iron “tunnels” which are also a bridge over the estuary. This was a pioneering design by renowned engineer Robert Stephenson (1803-59), although he depended on the help of others including Isambard Kingdom Brunel and William Fairbairn. The tubes were constructed on the shore, floated into position on pontoons and – with numerous setbacks – jacked up to the correct height. Stephenson himself rode the first locomotive to cross, on 18 April 1848. The first tube opened to traffic on 1 May that year. The second tube was placed in position in January 1849.
The trains enter the tubes through tall masonry portals, designed to complement the adjacent castle. The full span of the tubes is best seen from the south, near the bowling green off Llanrwst Road. The bridge is now owned and maintained by Network Rail.
Stephenson built three other bridges with the same principles, including a very long one in Canada, but only Conwy survives as a tubular bridge.
Also in this vicinity is a goods crane constructed by the London & North Western Railway at Crewe. The railway occupied a wide ledge built out over the north bank of the Gyffin river. Between the main lines and the town walls was a yard and goods shed, where freight was loaded or unloaded.
FOOTNOTES: More about Stephenson’s tubular bridges
Although Stephenson’s name is associated with Conwy’s tubular bridge, the science behind its construction came from William Fairbairn (1789-1874). Fairbairn conducted stress tests on the different shapes of potential tubes, and ruled out the circular or elliptical tubes Stephenson originally proposed. The tubes at Conwy are upright rectangles, in cross section, with smaller “cells” made of iron above and below to add strength. Each tube is 129 metres long and 7.8 metres high at its highest point (in the centre), and weighs about 1,300 tons.
Conwy’s tubular bridge was built concurrently with the Britannia Bridge, between the mainland and Anglesey. The Britannia Bridge, on a much bigger scale, opened in 1850. Its tubes were damaged beyond repair by fire in 1970.
Stephenson designed a tubular railway bridge which opened in 1851 over the river Aire near Knottingley. It remained in use until 1900.
In 1854 construction began on Stephenson’s last and most ambitious bridge, across the St Lawrence River at Montreal. This railway bridge was c.3,000 metres long, with 25 tubes placed end to end. The bridge opened in 1859, nine weeks after Stephenson’s death. The tubes were replaced in 1898.
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