The Forth Road Bridge is one of the world’s most significant long span suspension bridges. With a main span of 1006 metres between the two towers, it was the fourth longest in the world and the longest outside the United States when it opened in 1964. In total, the structure is over 2.5 km long. A staggering 39,000 tonnes of steel and 125,000 cubic metres of concrete was used in its construction.
The bridge’s two main towers, with their distinctive “St Andrew’s Cross” cross-bracing, support the majority of the weight of the suspended span. The two main cables sit on saddles at the summit of the towers, which pass the load back down to the ground.
The towers are of welded cellular high tensile steel construction and rise some 156 metres above high water level. The maximum thickness of the steel in the towers is about 25 mm.
The pier for the north tower was sited on the Mackintosh Rock, a whinstone outcrop which 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 found the pier on sandstone some 32 metres below high water.
The towers were strengthened in the late 1990s to allow for the increased weights of heavy goods vehicles crossing the bridge.
The side towers sit at the boundary between the approach viaducts and the suspended span. These substantial reinforced concrete structures help to support the weight of the main cable as well as the approach viaducts on either shore.
The entire suspended span and all the traffic is suspended from the bridge’s two main cables. These sit on top of the main towers and side towers and are anchored into the rock on either shore.
Each main cable is made up of 11,618 individual high tensile steel wires, compacted into a bundle approximately 60 centimetres thick. The total length of wire in the main cables would reach around the world one and a quarter times.
The cables were formed by spinning a few wires at a time back and forth across the estuary, gradually building up a cable capable of taking the load of the bridge. When spinning was complete the cable was hexagonal in shape – it was then compacted into the round shape we see today and wrapped laterally with 9 gauge galvanised wire. The cable was further protected by painting using red lead paste between the cable and wrapping wire and the whole cable was then painted.
This technique for spinning parallel wire suspension cables was first developed by John Roebling on the Brooklyn Bridge in the 19th century, and has subsequently been used on the Golden Gate, the Severn and many other famous suspension bridges worldwide.
The main cables are anchored at each end to take the 13,800 tonnes of total load in each cable. These anchorages are made of concrete cast in tunnels of tapering section cut into the rock at an inclination of 30º to the horizontal. The concrete is strengthened by steel post-tensioning strands grouted into conduits. The tunnel lengths vary between 56 and 79 metres.
The deck of the bridge is suspended from the main cables by 768 steel hanger ropes. These measure 57 mm in diameter on the side spans and 48 mm in diameter on the main span. The shortest hanger is 2.4 metres long, the longest is 90 metres.
All the bridge’s hanger ropes were replaced between 1998 and 2000, following the discovery of fraying on one of the ropes on the West cable. Each new hanger comprises a pair of ropes, each with its own single socket. This updated socket design is an improvement on the original, as it allows for just one rope of the pair to be replaced as required.
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 span between the two main towers measures 1006 metres long. The two side spans between the main towers and side towers are 408 metres long.
On the main span the deck is an orthotropic stiffened steel plate. However, on the side spans the deck is of composite construction – a 225 mm thick reinforced concrete slab on steel beams. On all suspended spans the surfacing is limited to a thickness of 38 mm.
Main expansion joints
The main expansion joints are embedded in the roadway underneath the main towers. Articulated trains slide over curved girders, allowing the suspended deck to expand and contract as required by temperature, wind loading and weight of traffic. They are the oldest and the largest of their kind in Europe.
The joints had been due for replacement in 2009, but the project has been deferred until after the opening of the Replacement Forth Crossing, in order to minimise costs and disruption to traffic. Failsafe devices have been fitted to ensure the safety of bridge users in the meantime.
The approach viaducts are significant structures in their own right. Reinforced concrete piers support a continuous deck structure consisting of twin steel box beams with transverse beams and outriggers. The deck is comprised of a reinforced concrete composite slab and 38 mm surfacing.
The South Viaduct is 438 metres long, and is supported by ten concrete piers and the side tower. It is divided into two separate structures with an expansion joint at the third pier from the side tower. The piers between the expansion joint and the side tower are designed to flex whereas the remainder are on roller bearings which allow the structure to move through live load and temperature.
On the North Viaduct, at 253 metres long, is a continuous structure, and the intermediate piers are designed to flex with the structure.
At the time the bridge was built, the risk of ships colliding with the structure had not been addressed. To remedy this concern, significant improvement work began in 1996 to construct defences around the base of each of the main towers. Completing the work was not entirely straightforward, however – the breeding patterns of a colony of rare roseate terns nesting on the nearby Long Craig Rock restricted when work could be carried out.