Britannia Bridge (1850)

 

Robert Stephenson's design for the Britannia railway bridge revolutionised the use of iron in bridge building. His experimental approach was a milestone in structural engineering, and led to the development of box girders. His Britannia Bridge over the Menai Strait was the largest tubular bridge constructed in Britain, but today only the stone elements of Stephenson's original remain. The bridge’s modern superstructure carries road and railway traffic between mainland Wales and the Isle of Anglesey.
In 1838, railway engineer George Stephenson (1781-1848) proposed extending the London to Chester line to Holyhead. To reach Holyhead, the railway had to cross the Menai Strait in north Wales. In 1840, George's son Robert Stephenson (1803-59) surveyed possible routes and suggested the most suitable solution would be to use Britannia Rock (SH541710) as the foundation for a mid-channel pier.
As engineer for the Chester & Holyhead Railway, Robert Stephenson was confronted with the difficulty of producing a bridge that could safely sustain locomotives travelling at high speed over an unusually wide span. The area’s prevailing weather conditions were poor and the bridge had to maintain a level height, 31.6m above high water, so as not interfere with the navigation of the largest sailing ships coming in from the Irish Sea.
Several early designs were rejected and Stephenson began to reconsider a roadbridge he had made in 1841, using trough-shaped iron girders. If the trough were closed across the top, would the girder then be self-supporting, even across significant spans? Rather than acting as a platform for the roadway, the tube might then carry the railway tracks through its middle.
He discussed the idea with the ironmaster, William Fairbairn (1789-1874), who experimented with the strength of this arrangement and the best shape for the iron tube. After selecting a rectangular tube section over circular or elliptical, because it would be easier to build and performed well in tests, a one-sixteenth scale model of Britannia Bridge (23.8m long) was tested. Professor Eaton Hodgkinson (1789-1861), a mathematician and writer on the strength of materials, was called upon to verify Fairburn's findings and produce formulae to describe and replicate them. The railway company funded this preparatory work to the tune of more than £6,000.
With Fairbairn's assurance, Stephenson eventually designed a bridge consisting of twin rectangular iron tubes running through three masonry support towers, to abutments at either end, without any further bracing or suspension. He was helped in the complicated structural stress calculations by Edwin Clark (1814-94), who went on to become resident engineer for the project.
For the same railway line, Stephenson produced a similar design for a smaller bridge over the River Conwy (SH785774). The construction method for the iron tubes on both bridges was technology borrowed from shipbuilding — they were made from riveted wrought iron plates 16mm thick, with cellular roofs and bases and sheeted sides.
The Britannia Bridge tubes crossed the strait in four spans, on three masonry support towers between masonry abutments. One tower sits on each bank with the tallest, 67.5m high, founded on Britannia Rock. The towers and abutments are rectangular in section, fashioned in 'Egyptian Pylon' style from Anglesey limestone and Runcorn sandstone. Each partly hollow tower has two square holes per side near the top, in all eight on each tower. Stone lintels 6.1m long span the rectangular voids for the twin tubes.
Each of the tubes consisted of four spans connected end to end within the towers. The two central bays had clear spans of 140m each, with a 70m clear span at either side. To allow for expansion and contraction, Stephenson had the tubes fixed in the central tower but mounted them on rollerbeds in the side towers and abutments.
The side tubes were built first, in situ, on wooden platforms. Meanwhile, the two great central tube sections, weighing 1,830 tonnes apiece, were built on the Caernarfon shore and floated, one at a time, into position. They were then jacked into place using hydraulic pumps, with supports being built underneath at every few inches. The Conwy bridge tubes had been successfully manoeuvred into position by the time the first Menai tube was ready. Nonetheless, Stephenson lost many nights' sleep over his method of construction.
Isambard Kingdom Brunel(1806-59), Stephenson’s friend and professional rival, is said to have remarked, "If your bridge succeeds, then mine have all been magnificent failures". The two men stood together to watch the first huge tube float out on its pontoons on 20th June 1849. Brunel was later to use the same technique for the Royal Albert Bridge at Saltash (SX434588) in Cornwall.
Once in place, the separate lengths of tube were joined to form parallel prestressed continuous structures, each 460.6m long and weighing 5,350 tonnes. The prestressing increased the loadbearing capacity and reduced deflection. The tubes were 4.5m wide and between 7m and 9.1m in overall depth, with a clear 3m gap between them. They were supported on a series of 4.6m long cast iron beams embedded in the stonework of the towers.
To protect the iron from the weather, an arched timber roof was constructed to cover both tubes. It was about 12m wide, continuous over their whole length, and covered with tarred hessian. A 3.7m wide central walkway above the roof provided maintenance access.
The Parliamentary Bill brought to construct Britannia Bridge received royal assent on 30th June 1845. The foundation stone was laid on 10th April 1846, and the first rivet driven on 10th August 1847. The tubes were manoeuvred into place between June 1849 and February 1850. Altogether, the bridge took over three years to complete and Stephenson fitted the last rivet on 5th March 1850. A single tube opened to rail traffic on 18th March 1850. Both tubes were open by 21st October the same year.
The success of Stephenson's bridges at Conwy and Menai changed the construction of iron bridges forever, and their tubular form can be considered the forerunner of the modern box girder.
On 23rd May 1970, an accidental fire destroyed the tarred timber roof, weakening the iron tubes beneath. The tubes split open at the three piers and the spans began to sag. The long spans deflected 490mm and 710mm. Rail crossings ceased. More details about the fire can be read in the article Britannia Bridge (1974), which describes its reconstruction.
Stephenson’s wrought iron tubes have been replaced by steel arches, supporting a reinforced concrete deck. The original masonry towers, abutments and the four plump stone lions that guarded the ends of the bridge have been retained, although the lions are now obscured. They date from 1848 and were sculpted by architect John Thomas (1813-62).
Rail services resumed in January 1972 and the rebuilt bridge was completed in 1974. In 1977-80, an upper level two-lane roadway was added to the bridge, carrying the A55 across the waters.
A small section of Stephenson’s wrought iron tube is preserved as a monument (SH542708) to the original Britannia Bridge, and is located on the south bank of the Menai Strait.
Britannia Bridge was Grade II listed in March 1966, and retains this designation despite alterations to its superstructure as a result of the fire. The citation notes that here remains "significant evidence for the original engineering", and that it was "one of the most audacious and exciting monuments of the great age of engineering".
Also Grade II listed is the Britannia Bridge Memorial (SH537711) in the churchyard of St Mary at Llanfair Pwllgwyngyll, Anglesey. Erected in 1850, it commemorates those who died during the bridge’s construction — 16 men and two children