Britannia Bridge - the first box girder bridge

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

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 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-ft (140-m) long rectangular iron tubes, each weighing 1,500 tons [2], supported by masonry piers, the centre one of which was built on the Britannia Rock. Two additional spans of 230-ft (70-m) length completed the bridge making a 1511-ft (461-m) long continuous girder. Up until then the longest wrought iron span had been 31 ft 6 in (9.6 m). The bridge was decorated by four large lions sculpted by John Thomas, two at either end.