Britannia Bridge is a bridge across the Menai Strait between the island of Anglesey and the mainland of Wales, originally made of wrought iron rectangular box section spans, and now a two-tier steel box girder and arch bridge.

The opening of the Menai Suspension Bridge in 1826, a mile (1.6 km) to the east of where Britannia Bridge was later built, provided the first road link between Anglesey and the mainland. However, the increasing popularity of rail travel necessitated a second bridge to provide a direct rail link between London and the port of Holyhead.

The task of building such a bridge fell to Robert Stephenson, son of the locomotive pioneer George Stephenson. Constrained by the fact that the strait must remain accessible to shipping, and that it must be sufficiently stiff to support the heavy loading associated with trains, he constructed a bridge with two main spans of 460 feet (140 m) long rectangular iron tubes, each weighing 1800 tonnes supported by masonry piers, the centre of which was built on the existing Britannia Rock. Two additional spans of 230 feet (70 m) length completed the bridge making a 1511 foot (461 m) long continuous girder. Up until then the longest wrought iron span had been 31 ft 6 in (9.6 m). The bridge which was decorated by four large lions sculpted by John Thomas , two at either end.

Begun in 1846 following preliminary work by William Fairbairn and with the mathematical assistance of Professor Eaton Hodgkinson, the bridge was opened on March 5 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 lifted into place. In spite of the heavier loadings placed on it in its later life, the bridge was "one of the most easily maintained and successful railway bridges" in the UK, and "as the first really large wrought iron bridge of the girder type it has unique significance in civil engineering history". 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.

In 1970 the bridge was greatly damaged when boys playing in the bridge dropped a burning torch, starting a fire. As a consequence the bridge was completely rebuilt, with the spans supported by archways. The deck has two levels: the lower still carries the rail line, and the upper supports a single-carriageway section of the A55 road.

Stephenson's only other tubular iron bridge, the Conwy railway bridge between Llandudno Junction and Conwy, remains in use, and can be seen at close quarters from another of Telford's elegant suspension bridges crossing the River Conwy.

Reference

• Charles Matthew Norrie (1956). Bridging the Years - a short history of British Civil Engineering. Edward Arnold (Publishers) Ltd.