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Civil Engineering heritage: Scotland - Highlands and Islands
Date 2007
Event ID 962878
Category Descriptive Accounts
Type Publication Account
Permalink http://canmore.org.uk/event/962878
Bernera Bridge
(Institute Civil Engineers Historic Engineering Works no. HEW 1812)
This structure, linking Bernera with Lewis in 1953, is believed to be the first pre-stressed concrete bridge built in Britain. It has three spans of 108 ft, each of which is formed of three parallel beams of pre-cast concrete segments stressed together and capped by an in situ concrete deck slab. The width is 13 ft between parapets. Because of the limited carrying capacity of the roads and other bridges in the area, it was agreed that the bridge should be designed for two-thirds of the full Ministry of Transport loading.
Each span consists of three pre-cast ‘U’ girders posttensioned on shore each with two straight and two curved cables of 32 wires by the Magnel–Blaton system. The beams were cast on shore in eight ‘U’ segments 11 ft 8 in. long, 4 ft 8 in. deep and 3 ft wide and two solid end blocks 5 ft 6 in. long. The completed beams each weighed 52 tons and their erection over water was facilitated by a temporary steel-lattice girder. The abutments and piers are of conventional design and consist of cylindrical shafts 7 ft 3 in. diameter placed 14 ft apart. The piers are founded on a concrete base slab founded on stiff boulder clay, and are capped by reinforced-concrete beams on which the pre-stressed beams are carried.
The engineers were Blyth & Blyth, Edinburgh, in collaboration with the County Engineer J. A. Shaw. The main contractor was A. A. Stuart & Sons (Glasgow) Ltd and the subcontractor for the pre-stressing was Stressed Concrete Design Ltd. The cost was £70 111.
While underwater work was in progress, divers reported traces of what appeared to have been a stone-built causeway about 6 ft west of the centreline of the bridge and parallel to it. It was about 7 ft wide and built of hand placed boulders ‘some of which were of considerable size’. There is no record of a causeway here, but it may be significant that two standing stones in Bernera overlook the site.
R Paxton and J Shipway, 2007.
Reproduced from 'Civil Engineering heritage: Scotland - Highlands and Islands' with kind permission from Thomas Telford Publishers.
