PLYMOUTH |
The Encyclopaedia of Plymouth History |
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1 - THE DESIGN The Royal Albert Bridge, carrying the railway from Plymouth into Cornwall across the river Tamar, was a product of the Cornwall Railway Company and its engineer, the famous Mr Isambard Kindom Brunel. When the Cornwall Railway Act received the Royal Assent on August 3rd 1846, it contained a condition that the steam ferry across the Tamar should be replaced by a high-level bridge. Saltash was chosen as the crossing point, where the river was 1,100 feet wide, and the Admiralty stipulated that the floor of the bridge should be 100 feet above the highest tide. Brunel's first plan involved building a single-span bridge of some 850 feet but this had never been attempted before and the idea was replaced by a main span of 255 feet and six smaller spans of 105 feet each at a height of 80 feet above high water. Clearly this did not comply with the Admiralty's orders so unsuprisingly it had to be dropped. Next came a plan for two main spans of 300 feet each and two side spans of 200 feet, this time at the correct height. However, this plan had to be dropped when it was found that there was no natural rock at the base of the river upon which to construct the three piers that would be required to support the bridge. Finally, Brunel came up with an acceptable proposal: one pier in mid-stream supporting two spans of 455 feet each with seven approach spans on the Devon side and ten on the Cornish. This did not entirely solve all the problems as there was still no natural foundation for the one pier and there was no-where to secure the tension chains of the suspension bridge. Taking inspiration from the bridge he had already constructed at Chepstow, Brunel came up with the idea of making his trusses self-supporting. Each truss consisted of an oval tube of wrought iron, sixteen feet by twelve, made in the form of an arch. This was designed to rest on top of the piers.
You will readily see, however, that the weight of this immense structure (each truss weighed over 1,000 tons) would tend to push against the piers and force them over, so this outward thrust was countered by hanging huge chains on the piers which would pull the piers inward.
This had then to be tied together to give it strength and stability, so eleven strong girders were used to connect the trusses to the chains.
Finally the roadway was suspended from the braced chains at twenty-two points, the eleven girder connections but intermediate points.
Work could now begin on the preliminary work.
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