Facts and figures
The Forth Road Bridge is a long span suspension bridge which when opened in 1964 was the largest in the world outside the United States and, together with the approach viaducts is a little over 2.5 km in length.
The bridge has a main span of 1006 metres between the two main towers. The side spans which carry the deck to the side towers are each 408 metres long. The approach viaducts are 252 metres and 438 metres long on the north and south sides respectively.
The two main aerially spun cables from which the suspended deck is hung are 590 mm in diameter, and each is made up of 11,618 high tensile wires with a 4.98 mm diameter. The suspended deck is made up of a steel stiffening truss, with three longitudinal air gaps at roadway level to improve aerodynamic stability.
The main cables are anchored at each end to take the 13,800 tonnes of total load in each cable. These anchorages are concrete, cast in tunnels of tapering section, cut into rock at an inclination of 30º to the horizontal. The tunnel lengths vary between 56 and 79 metres.
The suspended span decks are hung from the main cables by 192 sets of four hanger ropes with diameters of 44.5 and 52.4 mm. These hanger sets take loads of 176 and 224 tonnes. Between 1998 and 2000 all these hanger sets were replaced without interruption to traffic.
The deck on the main and side spans carries traffic loadings to the main cables via the hangers and does not contribute significantly towards the resistance of the truss to global effects. On the main span the deck is an orthotropic stiffened steel plate. However, on the side spans the deck is of composite construction with a 200 mm thick reinforced concrete slab on steel beams. On all the suspended spans the surfacing is limited to a thickness of 38 mm.
The pier from the north tower was sited on the Mackintosh Rock, a whinstone outcrop that made an ideal foundation. The south tower was located about as far from the shore as the depth of bedrock would allow the construction of a foundation with compressed air working. Caissons were used to enable the pier to be founded on sandstone some 32 metres below high water. Pier defences were constructed around both piers in the late 1990's to withstand collision loads from shipping.
The main towers are of welded cellular high tensile steel construction and rise up over 150 metres above high water level. The maximum thickness of the steel in the towers is about 25 mm. These towers were strengthened in the late 1990's to take the ever increasing weights of heavy goods vehicles crossing the bridge.
The approach viaducts are significant structures in their own right. Reinforced concrete piers support a continuous deck structure that consists of twin steel box beams with transverse beams and outriggers having a reinforced concrete composite deck slab and 38 mm surfacing.
The traffic loading on the bridge is now significantly higher than expected by the engineers who designed the structure in the 1950's. This has been due to the increasing weight and number of heavy goods vehicles on UK roads. A large capital programme to strengthen the bridge to take these increased loads is continuing.