|Date||2 October 2010|
|Location||Britannia Bridge, Menai Strait||SH 54254 70899||53.21509°N, 4.18414°W|
“Opened in 1850 to carry the Chester & Holyhead Railway, this tubular bridge, and that at Conwy, were the forerunners of modern box girder bridges. Their engineer, Robert Stephenson, surmised that if the top of the trough girders were enclosed, the girders might be self-supporting, a concept confirmed by strength of materal studies and large-scale model testing.
“As at Conwy, the tracks were carried within two riveted tubes formed of wrought iron plates. The four tubes for the two 460 feet mainstream spans, each weighing 1800 tonnes, were built on the Caernarfon shore then floated out and jacked up 100 feet onto the towers. The 230 feet side spans and the main spans for each track were connected end to end through the towers to form 1511 feet long girders, providing material economy through continuity. Hitherto, the longest wrought iron span was 31 feet and 6 inches.
“In 1970 a fire destroyed the protective timber roof above the tube, the heat causing the roof to tear apart, losing continuity. The bridge was replaced with new main spans of steel arches, the side spans being divided into three and rebuilt in reinforced concrete. The new bridge also carries the A5 road above the rail tracks to relieve the Menai Suspension Bridge of heavy traffic.”
— Gwynedd Council information board.
The original construction used limestone from the nearby quarries at Penmon. Following the fire in 1970, the bridge was reopened to trains in 1972 and the new upper road deck opened in 1980.