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Bernera Bridge
Lewis to Great Bernera, Outer Hebrides, Scotland, UK
Bernera Bridge
associated engineer
Blyth & Blyth
date  opened 22nd July 1953
era  Modern  |  category  Bridge  |  reference  NB164341
ICE reference number  HEW 1812
photo  © and licensed for reuse under this
Bernera Bridge links the islands of Lewis and Great Bernera in the Outer Hebrides. It was one of the first pre-stressed post-tensioned concrete bridges in Britain, and remains in service as the only crossing between the islands.
Great Bernera is a small islet some 9.5km long and 4.8km wide, lying on the north west side of the Isle of Lewis, divided from it by a narrow waterway at the closest point. Until the bridge was built everything Bernera’s residents needed had to be transported by boat — not always possible given the strong currents and frequent gales.
Pre-stressed concrete is likely to have been chosen for Bernera Bridge because of the shortage of steel immediately after World War II (1939-45). Using it can reduce the amount of steel required by over 50% against an equivalent reinforced concrete span.
From about 1943, precast pre-stressed beams were used for emergency bridge repairs but the first post-war pre-stressed bridge in the country is probably Adam Railway Viaduct near Wigan in Lancashire (built 1946) — so it was still a relatively new technique when Bernera Bridge was built. Bernera is believed to be the first pre-stressed post-tensioned concrete road bridge in the country. A model of it was exhibited the Festival of Britain in 1951.
In line with the limited carrying capacity of the roads and other bridges in the area, it was agreed that the bridge could be designed for two-thirds the usual Ministry of Transport specified traffic loading at the time. Preparatory rock blasting was required, as the route on the Great Bernera side had to turn sharply in line with the terrain.
The abutments and piers are of conventional design. The piers consist of cylindrical concrete shafts 2.2m diameter, placed 4.3m apart and infilled with concrete. They rest on a concrete base slabs founded on stiff boulder clay, and are capped by transverse concrete beams that support the main spans.
Bernera Bridge has three spans of 33m, each constructed using three parallel beams made of precast concrete segments stressed together and capped with an in situ concrete deck slab. The roadway was constructed as 4m wide between parapets.
The beams were cast on shore. Each consists of eight U-shaped segments 3.6m long, 1.4m deep and 910mm wide, with two solid end blocks 1.7m long. The completed beams each weighed 53 tonnes. Each group of three beams was post-tensioned with two straight and two curved cables of 32 wires, using the Magnel–Blaton system developed by Belgian engineer Professor Gustave Magnel (1889-1955). The beams were erected over the water via a temporary steel lattice girder and then lowered onto the pier caps.
The bridge cost some £70,000 and opened on 22nd July 1953, when 4,000 people walked across it. The Stornoway Gazette reported, "On that day Bernera ceased to be an island and became part of Lewis. Or perhaps it would be more accurate to say Lewis ceased to be an island and became part of Bernera."
While underwater work was in progress to position the bridge piers, divers reported finding traces of a stone causeway, about 2m west of the centreline of the bridge and parallel to it. The causeway of hand-placed boulders was about 2m wide and the remains may be seen at very low tide. A causeway was mentioned in a late 17th century book, though its age was unknown.
The bridge was widened in 1998 to accommodate bus and truck traffic.
County engineer: J.A. Shaw
Main contractor: A.A., Stuart & Sons (Glasgow) Ltd
Pre-stressing: Stressed Concrete Design Ltd
Research: ECPK
bibliography
"Early Applications of Prestressed Concrete in the United Kingdom"
by Chris Burgoyne, Ned H. Burns Symposium, University of Cambridge, Cambridge, 2004
Information board at Bernera Bridge, images available at www.flickr.com
http://canmore.rcahms.gov.uk
http://theconstructor.org
www.concrete.org.uk
www.scottish-places.info
www.undiscoveredscotland.co.uk
reference sources   CEHSHI
Location

Bernera Bridge