Civil Engineering heritage: Scotland - Lowlands and Borders
Date 2007
Event ID 602846
Category Descriptive Accounts
Type Publication Account
Permalink http://canmore.org.uk/event/602846
A road bridge at Queensferry was first proposed in the 1920s and in 1926 the Ministry of Transport invited Messrs Mott, Hay & Anderson to prepare a design but no action was taken. In 1934–35 proposals were made for a bridge at different sites, including one half a mile upstream from the railway bridge, at Mackintosh Rock. But the war intervened and it was not until 1947 that the government gave permission for the Forth Bridge Joint Board to obtain proposals and prepare for construction. At this time the estimated cost of the bridge was £6.2 million.
Because of the depth of water at the bridge site at Mackintosh Rock, foundations in mid-channel were impracticable. A navigational headroom of 150 ft was required for access to Rosyth Dockyard. These considerations gave rise to a design for a long-span suspension bridge. The Forth Road Bridge was thus to be the first long-span suspension bridge built in Great Britain in modern times.
The construction of the bridge was to be the first use in Britain of ‘cable spinning’, by which small-diameter wires are laid side by side in large numbers and then bound together to form a single cable. Much of the technical expertise for this work was derived from USA practice. Wind tunnel tests on a model of the bridge showed that to restrict undesirable oscillations the bridge deck would require longitudinal vents along its full length to assist the dispersal of wind effects.
The construction of the bridge took place from 1959–64 and the bridge was opened on 4 September 1964. It was then the first in the world with a span of over 1 km not designed and built by American engineers, and the longest span bridge in Europe. The consulting engineers were Mott,Hay & Anderson and Freeman, Fox & Partners. The main contractorswere John Howard & Co. Ltd for the foundations, and the A. C. D. Bridge Co. for the superstructure. This latter firmwas a combination of SirWmArrol&Co. Ltd, Cleveland
Bridge Engineering Co., and Dorman Long & Co.
The principal dimensions are:
Main span 3300 ft
Side spans 1340 ft
South approach viaduct 1437 ft
North approach viaduct 842 ft
Total length of bridge between abutments 8259 ft
Height of main towers above mean tide level 512 ft
Diameter of cables 2 ft
No. of wires in each cable 11 618 (dia. 0.196 in.)
Total length of wire in both cables 30 800 miles
Sag/span ratio 1 :11
Navigational clearance under main span 150 ft
Carriageway width 24 ft
Cycle track width 9 ft
Footway width 6 ft
Total weight of steel in the bridge 39 000 tons.
R Paxton and J Shipway 2007
Reproduced from 'Civil Engineering heritage: Scotland - Lowlands and Borders' with kind permission of Thomas Telford Publishers.
