Britannia BridgeEdit profile
Britannia Bridge (Welsh: Pont Britannia) is a bridge across the Menai Strait between the island of Anglesey and the mainland of Wales. It was originally designed and built by Robert Stephenson as a tubular bridge of wrought iron rectangular box-section spans for carrying rail traffic. Following a fire in 1970 it was rebuilt as a two-tier steel truss arch bridge, carrying both road and rail traffic.The bridge design
The opening of the Menai Bridge in 1826, a mile (1.6 km) to the east of where Britannia Bridge was later built, provided the first fixed road link between Anglesey and the mainland. The increasing popularity of rail travel necessitated a second bridge to provide a direct rail link between London and the port of Holyhead, the Chester and Holyhead Railway.
Other railway schemes were proposed, including one in 1838 to cross Thomas Telford's existing Menai Bridge. Railway pioneer George Stephenson was invited to comment on this proposal but stated his concern about re-using the suspension bridge. By 1840, a Treasury committee decided broadly in favour of Stephenson's proposals, with final consent to the route including Britannia Bridge given in 1845. Stephenson's son Robert was appointed as chief engineer.
The design required the strait to remain accessible to shipping and the bridge to be sufficiently stiff to support the heavy loading associated with trains, so Stephenson constructed a bridge with two main spans of 460-feet (140-m) long rectangular iron tubes, each weighing 1,500 long tons (1,700 short tons) , supported by masonry piers, the centre one of which was built on the Britannia Rock. Two additional spans of 230-feet (70-m) length completed the bridge making a 1,511-feet (461-m) long continuous girder. The trains were to run inside the tubes. Up until then the longest wrought iron span had been 31 feet 6 inches (9.6 m).
Stephenson retained the services of two distinguished engineers as consultants. William Fairbairn was an old friend of his father. Eaton Hodgkinson was a leading theorist on strength of materials. Hodgkinson believed that it would be impractical to make the tubes stiff enough, and advised auxiliary suspension from chains. However, Fairbairn believed chains unnecessary declaring:
The consensus of received engineering opinion was with Hodgkinson, but Stephenson, rather nervously, backed Fairbairn's analysis. A 75 feet (23 m) span model was constructed and tested at Fairbairn's Millwall shipyard, and used as a basis for the final design. Although Stephenson had pressed for the tubes to be elliptical in section, Fairbairn's preferred rectangular section was adopted. Fairbairn was responsible both for the cellular construction of the top part of the tubes, and for developing the stiffening of the side panels.Lions
The bridge was decorated by four large lions sculpted in limestone by John Thomas, two at either end. These were immortalised in the following Welsh rhyme by the bard John Evans (1826 - 1888), who was born in nearby Porthaethwy :
The lions cannot be seen from the A55 although the idea of raising them to road level has been suggested from time to time.Construction and use
Begun in 1846, the bridge was opened on 5 March 1850. For its time, it was a bridge of "magnitude and singular novelty", far surpassing in length contemporary cast beam or plate girder iron bridges. One aspect of its method of construction was also novel; the box sections were assembled on-shore, then floated out into position before being gradually lifted into place using powerful jacks.
There was originally a railway station on the east side of the bridge at the entrance to the tunnel, run by the Chester and Holyhead Railway company, which served local rail traffic in both directions. This station closed after 8½ years in operation owing to low passenger volumes. Nothing now remains of the station other than the remnants of the lower-level station building.
Stephenson went on, in short order, to design the High Level Bridge in Newcastle Upon Tyne, which can be seen as a second and more elegant version of the Britannia Bridge; and the design of the bridge and the construction techniques employed also influenced Isambard Kingdom Brunel in the design and construction of the Royal Albert Bridge across the River Tamar at Saltash.Fire and reconstruction
During the evening of 23 May 1970 the bridge was greatly damaged when boys playing inside the bridge dropped a burning torch, setting alight the tar-coated wooden roof of the tubes (see Britannia Bridge Official Fire Report, BBC News video). Despite the best efforts of the Caernarfonshire and Anglesey fire brigades, the bridge's height, construction and the lack of an adequate water supply meant they were unable to control the fire which spread all the way across from the mainland to the Anglesey side. After the fire had burned itself out the bridge was still standing but the structural integrity of the iron tubes had been fatally compromised by the intense heat. As a consequence the bridge was completely rebuilt by Husband & Co.
The new design was for an arched bridge. Concrete supports were built under the approach spans and steel archways constructed under the long spans either side of the central Britannia Tower. The bridge reopened to rail traffic (albeit with only a single line of rails and with reduced speed) on 30 January 1972. Over subsequent months the original box tubes were removed and the stonework of the towers was restored. The original deck below the rail lines was cosmetically restored.
In 1980, almost 10 years after the fire, the upper road level opened which carried a single-carriageway section of the A55 road.Proposed bridge improvement
In November 2007, a Public Consultation exercise into the ‘A55 Britannia Bridge Improvement’ commenced. The perceived problems stated include:
- It is the only non dual carriageway section along the A55
- Congestion during morning and afternoon peak periods
- Congestion from seasonal and ferry traffic from Holyhead
- Queuing at the junctions at either end
- Traffic is expected to significantly increase over the next 10 years or so
In the document, four options are presented, each with their own pros and cons:
- Do Nothing. Congestion will increase as traffic levels increase.
- Widen Existing Bridge. To do this, the towers would have to be removed to make room for the extra lanes. This is an issue as the bridge is a Grade 2 Listed Structure and also as the bridge is owned by Network Rail. The extra lanes would have to be of reduced width as the existing structure is not capable of supporting 4 full width lanes.
- New multi span concrete box bridge alongside. Building a separate bridge would allow the existing bridge to be used as normal during construction. The bridge would require support pillar(s) in the Menai Strait which is an environmental issue as the Strait is a Special Area of Conservation. Visual impact would be low as the pillars and road surface would be aligned with the current bridge.
- New Single Span Cable Stayed Bridge. This would obviate the need for pillars in the Strait, but the bridge would have a large impact on the landscape due to the height of the cable support pillars. This is also the most costly option.
Respondents were overwhelmingly in favour of seeing some improvements, with 70% favouring the solution of building a second bridge.Bibliography
- Norrie, Charles Matthew (1956) Bridging the Years - a short history of British Civil Engineering, Edward Arnold (Publishers) Ltd
- Rolt, L.T.C. (1960) George and Robert Stephenson: The Railway Revolution, Penguin, Ch. 15, ISBN 0-14-007646-8
- Rapley, John (2003) The Britannia and other Tubular Bridges, Tempus, ISBN 0-7524-2753-9