The Forth Bridge
and how it was built
by Jasmine Macleod and Malissa-Ann Surtees
step ONE The Tay Bridge collapsed in 1878, lowering public confidence due to a poorly structured bridge. As well as this, rail services were compromised as there was no route to get across the river, meaning that a replacement ferry service had to transport the contents which was more timeconsuming and a large inconvenience to the rail services. The Forth Bridge’s construction commenced in 1882, with the rocky shore being levelled to 7 feet for plants, materials, huts and other facilities for workmen. Construction of the landing stage at the Inchgarvie centre (one of three centres, the other two being Queensferry and Fife) began in April 1883.
Did you know? The Forth Bridge was one of the first cantilever bridges in Britain, and Britain’s first all-steel bridge
Foundations of the three centres being levelled
Caisson
step TWO Three towers of cantilevers are each seated on four circular piers. The foundations were excavated with the assistance of caissons. Did you know? The Forth Bridge is a 53,000 tonne bridge!
Six caissons were excavated by pneumatic process, using positive air pressure inside the sealed caissons to allow dry working conditions. The first caisson was launched on 26th May 1884. The last caisson was launched on 29th May 1885.
step THREE On New Year’s Day 1885 there was a low-tide which caused one of the caissons to sink into the mud. The caisson then flooded and didn’t drain so wooden struts had to be put onto the caisson to reinforce it whilst the water was being pumped out. However, the caisson adopted a slight tilt and so plates had to be bolted onto the caisson to raise the edge above the water level and then reinforced with more wooden struts. The caisson was refloated on 19th October 1885.
Did you know? The bridge stretches to 110m above high water at the top of each cantilever
Caisson tilting (Left)
Wooden struts to reinforce caisson (Right)
Viaducts
step FOUR The viaducts were built at a lower level than the foundations. The viaducts were then built upwards too the same level as the masonry piers. There were fifteen spans built between two viaducts. This meant that two spancs were attached together to make a continuous girder, with an expansion joint between each pair of spans.
Did you know? The two main spans of the bridge, of 521m, were the longest and second longest bridge spans in the world for 28 years!
step FIVE The cantilevers were constructed at the workshop on the hill at Queensferry. To bend the plates nto the required shape, they were first heated in a gas furnace and then pressed into the correct curve. Did you know? There were 26,000 accidents and 73 deaths throughout the making of the bridge, the majority of the deaths being that the workers had fallen to their death
Cantilevers
Cantilevers from far back viewpoint
step SIX The cantilevers were structured in the shape that they were because triangular shapes are deemed as stable shapes, enhancing the confidence that the public have in the bridge to be strong. By having three centres and three massive triangular shaped structures this increases the public’s perception as to how strong and well-structured the bridge actually is.
Did you know? The original designers of the Forth Bridge were English engineers, Sir John Fowler and Sir Benjamin Baker
step SEVEN As a result of the measurements of the metal spans of the bridge not being accurate, there had to be a method to resolve the issue of not being able to connect the cantilevers together.
Did you know?
The solution that they came up with was to expand the metal by heating it, then joining it together and reinforcing it with wooden struts.
The Forth Bridge sees 200 train movements daily, showing how reliant the rail services are on this iconic bridge.
This scared people as the bridge was supposed to be this strong, unbreakable structure, yet it had faults before it was even opened.
Metal gap to be heated and joined, reinforced by wooden strut
by Jasmine Macleod and Malissa-Ann Surtees