Articles
The Kessock Bridge:
Celebrating 40 Years
A9
The Kessock Bridge is a major crossing of the Beauly Firth, linking Inverness and the Black Isle. Designed and constructed by the Cleveland RDL Kessock Consortium, the harp type cable-stayed bridge was officially opened on 6th August 1982.
Discussions on the need for a bridge began in the 1960s as part of a study into improvements to the A9 north of Perth. As proposals developed it became clear that the construction of a new bridge, and the rerouting of the A9 through the Black Isle, offered significant benefits.
The ambitious and technically challenging project led to a significant reduction in journey times and combined with the construction of the Cromarty Bridge further north, shaved almost fifteen miles from a typical trip between Invergordon and Inverness.
For a time, Kessock was the only multi-cable stayed bridge in the UK and the longest in western Europe. The project, including the main approach roads, cost almost £30 million, equivalent to more than £100 million today.
Key Data
Location
A9, Inverness, Highland
Consulting Engineer
Crouch & Hogg
Ove Arup
Construction
June 1977
to August 1982
Project Cost
£30 million
(£100 million today)
Planning
In the late 1960s the Scottish Development Department announced that the A9 between Perth and Inverness would be comprehensively redeveloped, but the road north of Inverness would only be upgraded. In 1968, the Highlands & Islands development board had commissioned a report by Jack Holms Development Company to look at the Inverness and Black Isle area and suggest proposals for redevelopment. The report proved controversial, and the main road proposals included widening of the existing A9 to dual carriageway along its original line.
Frustrated by the existing route’s lengthy and circuitous path round the heads of the Beauly, Cromarty and Dornoch Firths, locals Raey Clark and John Smith developed the ‘Three Firths Concept’, a proposal to build a more direct route for the A9. This, they argued, would improve the economic prospects of towns in the far north of Scotland. A new bridge linking North and South Kessock was a key part of their idea.
In 1969 local industrialist and Chairman of AI Welders Pat Hunter Gordon spearheaded a campaign to push the plans forward. Intense lobbying of politicians followed and in 1970, the Scottish Development Department (SDD) appointed consulting engineer Crouch & Hogg to investigate alternative routes. A significantly shorter route across the Black Isle was selected.
Completion of the Kessock Bridge marked the end of a ferry service that had operated between North and South Kessock for several hundred years. Here, cars and passengers are seen disembarking from the Rosehaugh at Inverness in 1971. © Newsquest (Herald & Times)
In 1971 the Secretary of State for Scotland confirmed his approval of the proposal and a cable stayed steel box girder bridge with a main span of 240m was approved and put out to tender in 1975.
The returned tenders were each more than £30 million, almost double the original project estimate. Following a further aborted tender exercise later that year the SDD ordered a rethink. Crouch & Hogg joined forces with Ove Arup & Partners to restart the project, and after some consideration, it was decided to proceed with a design and build contract.
The Tender Process
To be sure a reasonably priced structure could be built at Kessock, the SDD invited contractors to tender on a ‘design and build’ basis—the first time this procurement method was used for a major bridge project in the UK. Sixteen applications were received, with six groups submitting final tenders in March 1977. Three steel and three concrete solutions were ultimately submitted.
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Several UK based consultants and contractors joined forces to submit tenders for the project. The image above shows the design proposed by Babtie and Professor F. Leonhardt as part of their joint venture with Wimpey, Arrol and Highland Fabricators.
Approved Design
Following assessment, and with the input of the Royal Fine Arts Commission for Scotland, the contract was awarded to the consortium of Cleveland Bridge and Redpath Dorman Long (RDL) in association with Dr Helmut Homberg and Trafalgar House Engineering Services on 21st June 1977. Their steel multi-cable stayed design was the lowest priced at £17.25 million.
By 1979 construction on the bridge's concrete supports was well underway. These were cast in pairs, working progressively from each shore of the Beauly Firth. The main span's supports are linked by an 8m deep cross beam.
Design and Construction
Construction of the bridge took a little over four years. Work on the foundations, reinforced concrete abutments and piers was undertaken by RDL with Cleveland Bridge responsible for the steel deck and towers.
The 22m wide deck comprising steel plate, trough stiffeners and 1.5m deep cross girders is supported on 3.25m deep plate girders visible along both edges of the bridge. The deck accommodates two 7.3m wide carriageways and 1.8m footways. These are surfaced with 40mm of mastic asphalt. Public services including water and electricity are carried under the deck. The 240m main span provides a minimum navigation clearance of 29m. Groups of eight spiral strand cables in harp formation support the main spans. The cables are held at their lower ends by anchorage boxes on the outside of the main girders. The main pylons, each 40m tall, rest on rubber pot bearings.
The bridge's superstructure was delivered in small sections which were assembled on site. These were installed using the cantilever erection method with work progressing outward from each shore. This photo shows work on the north side span 2.
The presence of the Great Glen fault line below the Beauly Firth meant an allowance had to be made for earth tremors. Two 400 tonne seismic buffers capable of withstanding accelerations of one tenth gravity were installed in a Scottish first. The main expansion joints can absorb movement of up to 500mm.
The 1052m continuous steel superstructure, incorporating almost 9000 tonnes of steel, was fabricated at the Cleveland Bridge works in Darlington and transported to the site in 16m long sections. These were installed using the ‘cantilever erection method’ with work proceeding outward from each shore of the Firth.
The bridge’s main approach roads, designed by Crouch & Hogg and constructed by Watson & McGregor Ltd, were completed in August 1975.
Contract Summary
Contract
Main Contractor
Tender Value
Bridge Foundations & Superstructure
Cleveland Bridge RDL Kessock Consortium
£17.25 million
Kessock Bridge Approaches
Watson & McGregor Ltd
£2.1 million
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By 1981 work to complete the 240m main span was well advanced. By this stage the 40m tall main pylons were also in position, these assisting with the erection of steelwork. On closing the gap, focus switched to completing painting and deck surfacing.
The Bridge Today
The Kessock Bridge is managed by Transport Scotland on behalf of the Scottish Ministers. It is estimated that more than 300 million vehicles have used the bridge since it was completed. Historic Environment Scotland listed the bridge as a Category B structure in October 2019, recognising its unique architectural and technical features.
It is a key link in the Scottish trunk road system and a series of multi-million pound projects are planned to ensure it remains operational for decades to come. Since 2010, works have been undertaken to refurbish the bridge deck and replace its surfacing in addition to the installation of a new inspection gantry and street lighting.
The bridge was waterproofed and resurfaced in 2013/14 with 50mm thick Gussasphalt, and H4a very high containment barriers installed at a total cost of £13.65m. Kessock bridge is the only bridge in Scotland to have Gussasphalt and has a design life of 30 years. Transport Scotland will invest more than £30 million in the years ahead to repaint the bridge and install a state of the art structural monitoring system. Works to replace its unique seismic buffers is already underway.
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Celebrating 40 Years
6th August 2022 marked Kessock's 40th anniversary. The anniversary was covered widely on TV and in the press. To mark the occasion the Scottish Roads Archive produced a special podcast and, with Transport Scotland's help and support, a booklet featuring new and unseen images from the archive.
Printed copies of the booklet are available at our Online Shop.
From the Archive
Documents
This article was first published in August 2022. With special thanks to Geoff Booth, Don Fraser, Ken Wilson, Norman Smart and Duncan Macknight.
Related Content
A special episode of the Scottish Roadscast celebrating the 40th anniversary of the bridge was released in August 2022. It can be heard below.