Бриттания көпірі: тарихы мен қайта құрылысы

Road rail bridge over the Menai Strait

A fire in May 1970 caused extensive damage to the Britannia Bridge. Subsequent investigation determined that the damage to the tubes was so extensive that they were not realistically repairable. The bridge was rebuilt in a quite different configuration, reusing the piers while employing new arches to support not one but two decks, as the new Britannia Bridge was to function as a combined road and rail bridge. The bridge was rebuilt in phases, initially reopening in 1972 as a single tier steel truss arch bridge, carrying only rail traffic. Over the next eight years more of the structure was replaced, allowing for more trains to run and a second tier to be completed. The second tier was opened to accommodate road traffic in 1980. The bridge was subject to a £4 million four month in depth maintenance programme during 2011. Since the 1990s, there has been talk of increasing road capacity over the Menai Strait, either by extending the road deck of the existing bridge or via the construction of a third bridge.

The opening of the Menai Bridge in 1826, 1 mile (2 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 shortly 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 a single carriageway of the suspension bridge, as bridges of this type were unsuited to locomotive use. By 1840, a Treasury committee decided broadly in favour of Stephenson's proposals, however, final consent to the route, including Britannia Bridge, would not be granted until 30 June 1845, the date on which the corresponding Parliamentary Bill received royal assent. supported by masonry piers, the centre one of which was to be built on the Britannia Rock. Two additional spans of 70.10 m length would complete the bridge, making a 1,511 feet (461 m) continuous girder. The trains were to run inside the tubes (inside the box girders). Up until then, the longest wrought iron span had been 31 feet (9 m), barely one fifteenth of the bridge's spans of 140.21 m. As originally envisaged by Stephenson, the tubular construction would give a structure sufficiently stiff to support the heavy loading associated with trains, but the tubes would not be fully self supporting, some of their weight having to be taken by suspension chains. It became apparent from Fairbairn's experiments that without special precautions the failure mode for the tube under load would be buckling of the top plate in compression, the theoretical analysis of which gave Hodgkinson some difficulty. When Stephenson reported to the directors of the railway in February 1846, he attached reports by both Hodgkinson and Fairbairn. From his analysis of the resistance to buckling of tubes with single top plates, Hodgkinson believed that it would require an impracticably thick (and therefore heavy) top plate to make the tubes stiff enough to support their own weight, and advised auxiliary suspension from link chains.

On 10 April 1846, the foundation stone for the Britannia Bridge was laid, marking the official commencement of construction work at the site. However, this station was closed after only 8 1/2 years in operation owing to low passenger volumes. In the present day, little remains of this station, other than the remnants of the lower level station building. A new station named Menai Bridge was opened shortly afterwards.

The bridge was decorated by four large lions sculpted in limestone by John Thomas, two at either end.

During the evening of 23 May 1970, the bridge was heavily damaged when boys playing inside the structure dropped a burning torch, setting alight the tar coated wooden roof of the tubes. 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. carrying a single carriageway section of the A5 road (now the A55). During 2011, national railway infrastructure owner Network Rail, the Welsh Assembly Government and the English Highways Agency undertook a £4 million joint programme to strengthen the 160 year old structure and improve its reliability.

Very few other tubular iron bridges were ever built since more economical bridge designs were soon developed. The most notable of the other tubular bridges were Stephenson's Conwy railway bridge between Llandudno Junction and Conwy, the first Sainte Anne de Bellevue (Québec) Grand Trunk Railway bridge, which was the prototype of the Victoria Bridge across the Saint Lawrence River at Montreal. The Conwy railway bridge remains in use, and is the only remaining tubular bridge; however, intermediate piers have been added to strengthen it. The bridge can be seen at close quarters from Thomas Telford's adjacent 1826 Conwy Suspension Bridge. The Victoria Bridge was the first bridge to cross the St. Lawrence River, and was the longest bridge in the world when it was completed in 1859. It was rebuilt as a truss bridge in 1898.