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Forth Road Bridge
Firth of Forth, Queensferry, Edinburgh, Scotland
associated engineer
Mott Hay & Anderson
Freeman Fox & Partners
date  September 1958 - 4th September 1964
era  Modern  |  category  Bridge  |  reference  NT124795
ICE reference number  HEW 203
The steel and concrete Forth Road Bridge crosses the Firth of Forth at a point just west of the City of Edinburgh. It's a suspension bridge with a main span just over 1km in length, and the cables used for it were constructed in a way new to Britain. The bridge has suffered from serious corrosion problems in recent years and a second crossing is expected to open in 2016.
The cable technique used is called cable spinning, which involves binding large numbers of thin wires together. This was already an accepted technique in America at the time the Forth Road Bridge was constructed.
Before the building of the nearby Forth Rail Bridge (completed 1890), ferries were the only means of crossing the Forth near Edinburgh. The possibility of a bridge had been under discussion from the mid 18th century onwards, and a road bridge was proposed first in 1923 by Edinburgh journalist James Inglis Ker. It gained the support of civil engineer Sir Henry Maybury (1864-1943) the following year. Maybury was Director General of the Roads Department, Ministry of Transport (1919-28) and in 1926 the ministry invited Mott Hay & Anderson to prepare a design. However, no action was taken.
In 1934-5 proposals were made for different sites, including one upstream of the rail bridge at Mackintosh Rock, the eventual chosen location. World War II (1939-45) intervened and it was not until 1947 that the government gave permission for the Forth Road Bridge Joint Board to obtain proposals and prepare for construction. At this time, the estimated cost was 6.2m.
Construction was sanctioned on 10th February 1958, and preparations began on site in September that year. Freeman Fox & Partners worked alongside Mott Hay & Anderson, overseeing construction. Cable spinning took place in 1961-2 and the main span was completed in 1963. The bridge contains 39,624 tonnes of steel and 115,000 cu m of concrete.
The depth of water in the northern half of the site made foundations in mid-channel impracticable, and navigational headroom of 45.7m was required for shipping using Rosyth Dockyard. These considerations made a long-span suspension bridge logical and it's the first such bridge built in Britain in modern times.
The main span is 1,006m long, flanked by side spans of 408.5m. The approach viaduct lengths are 438m on the south side and 256.5m on the north. The two steel girder pylon towers rise to a height of 156m above mean tide level. The approaches are carried on shorter, reinforced concrete arched pylons.
For the suspension cables, 11,618 galvanised high tensile steel wires each of 5mm diameter were laid side by side and then bound together into a single cable, 610mm in diameter. The total length of wire in both cables is 49,557km, and the sag-to-span ratio of the cables is 1 in 11. The ends of the cables are secured by massive concrete anchorages excavated into bedrock on either bank. The 44mm diameter hangers between the cables and the bridge deck are each made from 113 wires and range from 1m to 106m in length.
The steel lattice truss deck was constructed to carry dual two-lane carriageways, 7.3m wide. Cycle tracks 2.75m wide and footways of 1.8m are supported on cross girders extending from the trusses. Wind tunnel tests on a model had shown that to restrict undesirable oscillations, the deck required longitudinal vents along its full length of 2,517m to assist in the dispersal of wind loads.
The superstructure was constructed by a consortium called A.C.D. Bridge Co Ltd, which was made up of three well-known names in engineering Sir William Arrol & Co Ltd, Cleveland Bridge Engineering Co and Dorman Long & Co. The bridge was opened by the Queen on 4th September 1964 and traffic was crossing the bridge by 6pm. It was briefly (78 days) the first bridge in the world with a span longer than 1km not designed and built by American engineers, and had the longest span in Europe. The final construction cost was 19.5m, including the approach roads and associated infrastructure.
It was originally a toll bridge, with a toll plaza at the southern end. The initial government construction loan of 14.35m was paid off by the end of 1993 and toll revenues were diverted to fund maintenance, operation and improvements. One-way tolling was introduced in 1997 and tolls were abolished altogether from 11th February 2008. The toll plaza was removed in 2008. Funding for the bridge now comes directly from the Scottish Parliament.
Since the bridge opened, the maximum weight of heavy goods vehicles on British roads has doubled to 44 tonnes, and the originally forecast traffic flows of 60,000 vehicles per day are exceeded regularly. To cope with the increased weight and volume of traffic, the towers were strengthened in the 1990s by the insertion of new steel columns inside the pylons. In 1996, structures were erected around the piers of the two towers to protect them from ship collision something that was not tackled in the original design. This work cost 7.5m.
Replacement of all 192 steel hangers was carried out in 1999-2000 after frayed and corroded wires were found in nine of the hanger cables. The old hangers were replaced by a total of 26km of new steel ropes, at a cost of 7.8m.
The bridge was designated a Category A listed structure in March 2001. The Forth Estuary Transport Authority superseded the Forth Road Bridge Joint Board in 2002. The new body is responsible for operating and maintaining the bridge, as well as other projects.
In March 2004, the Forth Road Bridge became the first suspension bridge in Europe to have its suspension cables opened up for corrosion checks. Some 10% of their strength was found to have been lost through corrosion resulting from water entering through cracks in their paint finish. To prevent further damage, the 4km long cables needed to be dried out. To do this, they were sealed inside neoprene membranes and dry air pumped in continuously to remove moisture and reduce the relative humidity to less than 40%. This system was installed in October 2009 at a cost of more than 12m. Further cable inspections will follow.
In December 2007, the Scottish Government declared its intention to construct a second road crossing over the Firth of Forth. In 2008, an inspection revealed that nine of the nuts on the metal brackets that connect the hangers to the main cables were cracked. All 1,888 nuts and their corresponding bolts were renewed during the 1999-2000 hanger replacement project, and another full set may yet be required.
A 1.75m project to install four under-deck remote access gantries, replace ladders with stairways and upgrade or install access walkways was carried out in 2010. Recent carriageway resurfacing cost 3.5m. The bearings on the north and south approach viaducts are being replaced in a 13.6m contract that began in May 2010 and should be completed by the end of 2012.
Architect: Sir Giles Gilbert Scott & Partners
Contractor (foundations): John Howard & Co. Ltd
Contractor (superstructure): A.C.D. Bridge Co. Ltd consortium
Design engineers (1998-2009): W.A. Fairhurst & Partners (remedial works)
Design engineers (2010): Atkins (under-deck access)
Suspension hanger works (1999-2000): Monberg & Thorsen
Bearing works (2010-12): Balfour Beatty Regional Civil Engineering
Contractor (2010): Raynesway Construction (under-deck access)
Research: ECPK
bibliography
http://canmore.rcahms.gov.uk
www.forthbridges.org.uk
www.forthroadbridge.org
www.nce.co.uk
reference sources   CEH SLB
Location

Forth Road Bridge