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 Railway Bridge carried the South Wales Railway across the River Wye and featured a main truss of with a curving tubular main member and three conventional plate-girder approach spans of , a similar solution to that adopted for crossing the River Tamar at Saltash. The river is about wide at Saltash. Brunel's first thoughts had been to cross this on a double track timber viaduct with a central span of and six approach spans of with clearance above the water. This was rejected by the Admiralty, who had statutory responsibility for navigable waters, so Brunel produced a revised design to give clearance, with two spans of and two of . The Admiralty again rejected this plan, stipulating that there should not be more than one pier in the navigable part of the river. Brunel now abandoned plans for a double track timber structure and instead proposed a single track wrought iron design consisting of a single span. As the cost of this structure would have been around £500,000 at 1846 prices (equivalent to £ in ), he amended the design to one of two main spans of with clearance above mean high spring tide; this was approved by the Admiralty and the directors of the Cornwall Railway. The two spans are lenticular trusses with the top chord of each truss comprising a heavy tubular arch in compression, which tend to expand in length under load, while the bottom chord comprises a pair of chains, which tend to contract in length under load. By design, these two effects cancel so that there is no net change in length under load. This in turn enables each of the trusses to be supported with no horizontal thrust 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 17 shorter and more