Forth Road Bridge

Forth Road Bridge
©2022 Gazetteer for Scotland

Forth Road Bridge

Located 9 miles (15 km) west of Edinburgh, the Forth Road Bridge is the second crossing to span the Firth of Forth between South Queensferry and North Queensferry, the first being the Forth Rail Bridge which opened in 1890. Construction began on this suspension bridge in 1958 with the work undertaken by a consortium of the three largest construction firms in Britain at the time; namely Sir William Arrol & Company, the Cleveland Bridge & Engineering Company and Dorman Long. The consultant architect was Sir Giles Gilbert Scott & Partners, who also designed the administration building by the south end of the bridge. When it was opened by HM Queen Elizabeth II on the 4th September, 1964, the bridge was the longest in the world outside the USA. In 2001, the bridge was 'Category-A listed' reflecting its importance as an engineering structure.

The bridge is nearly 1828m (2000 yards) long while its main span, of 1006m (3300 feet), is the tenth longest in the world; its high tensile steel towers are 156m (512 feet) in height. The deck, which comprises a pair of 7.3-m (24-foot) carriageways, carries four lanes of traffic alongside two walkways. The central span comprises an orthotropic stiffened steel plate with a road surface only 38mm (1½ inches) in thickness, while the side spans comprise 200-mm (8-inch) thick reinforced concrete slabs mounted on steel beams. The deck is suspended from cables which are 5.9cm (2.3 inches) in diameter and composed of 11,618 high-tensile steel wires, giving a total length of wire of 49,280 km (30,621 miles). The bridge is highly flexible; strong winds may displace the central span by several metres while a temperature rise of 20 °C will cause the centre of the bridge to drop by a metre.

While a road-bridge at Queensferry had been proposed since the 18th century, the government finally committed to the project in 1947. Costing £11.5 million, with a further £8 million needed for the approach roads, the bridge was established with tolls for motorised traffic but was free for pedestrians. The initial costs and accrued interest were paid off through these tolls by 1993, although the tolls were retained thereafter to cover the costs of maintenance and strengthening the bridge to cope with increasing traffic volumes. Northbound-only tolling was introduced in 1997 but, having become a political issue, the tolls were dropped in 2008. Traffic is heavy over the bridge, frequently exceeding 70,000 vehicles per day and long queues are now a feature of peak-time travel. The main towers were strengthened in the 1990s to cope with the increased weight of goods vehicles. With special permission, the maximum weight of a vehicle permitted on the bridge is 150 tonnes, although the heaviest load ever to have crossed it was 215 tons. Concerns over the safety of the ageing bridge came when corroded wires were found in nine of the steel hanger cables, which were all replaced 1999-2000. Then an inspection in 2004 revealed that up to 10% of the steel wires in the suspension cables were broken. The construction of a new Queensferry Crossing was approved in 2011, in part to deal with increasing traffic but also to provide an alternative route while the old bridge was completely refurbished. The Forth Road Bridge was closed for almost three weeks in 2015 when a 2-cm crack was found in a structural support. Following the opening of the Queensferry Crossing in 2017, use of the Forth Road Bridge was restricted to buses, taxis, cyclists and pedestrians.

The bridge was originally run by the Forth Road Bridge Joint Board on behalf of the government, but in 2001 the Scottish Executive replaced this body with the Forth Estuary Transport Authority. In 2015 management of the bridge passed to the private sector under a long-term contract.

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