The opening of the MenaiBridge in 1826, a mile(1.6 km) to the east of where Britannia Bridge waslater 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. That rains were to run inside the tubes. Up until then the longest wrought iron span had been 31 feet 6 inch (9.6 m).
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.