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Bonar Bridge
Kyle of Sutherland, Bonar Bridge, Highland, Scotland, UK
Bonar Bridge
associated engineer
Thomas Telford
Crouch & Hogg
date  Sept 1811 - Nov 1812, 1892 - 6th July 1893, 1972 - 14th Dec 1973
era  Georgian  |  category  Bridge  |  reference  NH608915
ICE reference number  HEW 2550
photo  courtesy Graces Guide
The present bridge at Bonar is the third at the site. The first, by Thomas Telford, employed a prefabricated cast iron lattice arch at the time the longest ever made. It was destroyed in a flood and replaced by a steel bowspring bridge. This bridge in turn had to be replaced due to corrosion, and a modern steel arch bridge took its place in 1973.
Bonar Bridge is located at the southern (downstream) end of the Kyle of Sutherland, a tidal waterway that flows into the Dornoch Firth in the Scottish Highlands, fed by five rivers. Before the first bridge was constructed, it was crossable at a ford, just downstream of the bridge site.
The local village was called 'Bonar' until Thomas Telford (1757-1834) built the first bridge (pictured), when it became known as Bonar Bridge. Situated on the main route north from Inverness to Caithness, the village grew with the coming of the railways in the 1860s and developed into a destination for visitors.
Telford designed his bridge in 1810. It consisted of a cast iron lattice spandrel arch, 45.7m in span with a rise of 6.1m rise. The soffit followed an elliptical curve. The truss was composed of four open, cast iron ribs. He used a similar design for Craigellachie Bridge over the River Spey, which was completed in 1814 and survives.
At the time, it was thought that a single span would provide sufficient waterway, subject to the available ground conditions for a river pier. However, there were difficulties with the foundations and the site lacked suitable bedrock on the west bank. A single span wasn't feasible, nor were twin iron arches. Two additional arch spans in masonry, of 15.2m and 18.3m, with rises of 5.1m and 6.1m respectively, were constructed on the west side, the second of which acted as a flood relief arch.
The bridge was constructed by John Simpson (1755-1815) and John Cargill (1772-1848). The lattice truss was prefabricated by ironfounder William Hazledine (1763-1840) and pre-erected offsite in June 1812. The bridge cost 13,971 to construct, of which the ironwork was 3,947.
Unfortunately, on 29th January 1892, Telford's bridge was swept away by floodwaters. The second bridge, erected in less than 11 months, was designed by Crouch & Hogg and constructed by Sir William Arrol & Co. The contract sum was 13,584.
The new three-span (21.3m, 32m and 42.7m) bridge used steel bowstring girders supported on two granite river piers, each with two towers linked by an arch beneath the deck. Caissons were sunk for the pier foundations and cofferdams built for construction of the abutments. The piers and abutments were topped by octagonal castellated turrets, providing refuges at road level.
By the 1970s, the Couch & Hogg bridge had become corroded at its springings. Its replacement, a segmental steel tied arch of 103.6m span with a rise of 19.5m, was constructed alongside it in 21 months. The old bridge was demolished soon afterwards.
The third designed by A.A. Cullen Wallace of Crouch & Hogg and constructed by William Tawse, with structural steelwork by Redpath Dorman Long. It was given a Steel Design Award in 1974. The contract sum was 405,000.
Plaques commemorating all three bridges are mounted on a cairn near the south east corner of the present bridge. The road it carries was designated the A9 until 1991, when the new Dornoch Bridge opened 16km downstream. Bonar Bridge is now part of the A836.
Masonry and ironwork erection (1811-12): Simpson & Cargill
Iron casting and prefabrication (1811-12): William Hazledine
Contractor (1892-3): Sir William Arrol & Co
Contractor (1972-3): William Tawse (North Region) Ltd
Steelwork (1972-3): Redpath, Dorman Long (Contracting) Ltd
Research: ECPK
bibliography
https://canmore.org.uk
www.gracesguide.co.uk
www.ice.org
www.tainmuseum.org.uk
reference sources   CEH SHIBDCE1
Location

Bonar Bridge