Royal Albert Bridge - Design

Design

The structure was the third in a series of three large wrought iron bridges built in the middle of the nineteenth century and was influenced by the preceding two, both of which had been designed by Robert Stephenson. The two central sections of Brunel's bridge are novel adaptations of the design Stephenson employed for the High Level Bridge across the River Tyne in Newcastle Upon Tyne in 1849. Brunel was present when Stephenson raised the girders of his Britannia Bridge across the Menai Strait in the same year. From 1849 to 1853 Brunel was erecting an iron bridge of his own; the Chepstow Bridge carried the South Wales Railway across the River Wye and featured a main truss of 300 feet (91 m) with a curving tubular main member and three conventional plate-girder approach spans of 100 feet (30 m), a similar solution to that adopted for crossing the River Tamar at Saltash.

The river is about 1,100 feet (340 m) wide at Saltash. Brunel's first thoughts had been to cross this on a timber viaduct with a central span of 255 feet (78 m) and six approach spans of 105 feet (32 m) with 80 feet (24 m) clearance above the water. This was rejected by the Admiralty, who had statutory responsibility for navigable waters, and Brunel thus produced a design to give 100 feet (30 m) clearance, with two spans of 300 feet (91 m) and two of 200 feet (61 m). The final design as built consists of two main iron spans of 455 feet (138.7 m) with 100 feet (30.5 m) clearance above mean high spring tide. These two spans are lenticular trusses with the top chord of each truss comprising a heavy tubular arch in compression, while the bottom chord comprises a pair of chains. Each of the trusses is simply supported and therefore no horizontal thrust is exerted on the piers, which is crucial in view of the curved track on either side. Between these two chords are supporting cross-bracing members and suspension standards which hang beneath the bottom chord to carry the railway deck which is a continuous plate beam. There are also seventeen much shorter and more conventional plate-girder approach spans on the shore. On the Cornish side there are ten which measure (from Saltash station towards the river): 67.5 feet (20.6 m), 69.5 feet (21.2 m), 69.5 feet (21.2 m), 69.5 feet (21.2 m), 69.5 feet (21.2 m), 69.5 feet (21.2 m), 72.5 feet (22.1 m), 78.0 feet (23.8 m), 83.5 feet (25.5 m), 93.0 feet (28.3 m), and seven on the Devon side of (from the river towards St Budeaux): 93.0 feet (28.3 m), 83.5 feet (25.5 m), 78.0 feet (23.8 m), 72.5 feet (22.1 m), 69.5 feet (21.2 m), 69.5 feet (21.2 m), 69.5 feet (21.2 m). This gives a total length for the nineteen spans of 2,187.5 feet (666.8 m).