Even today, more than a century after it was built, the Forth Bridge is regarded as an engineering marvel.
The steel-built cantilever bridge is 2.5 kilometres long and carries a double railway track 46 metres (151 feet) above the Forth at high tide.
The three cantilever structures are each just over 100 metres in height, with each tower resting on a massive granite pier.
The superstructure weighs in at over 50,000 tonnes and something like 6.5 million rivets were used in its construction.
It’s a feat of engineering that has been described as years ahead of its time, yet it was also exactly of its time.
The Forth Bridge was the first major structure in Britain to be constructed of steel (rather than iron) and this was only possible because of an advance in steel manufacture.
Large amounts of steel had become available after the invention of the Bessemer process in 1855 but its strength was unpredictable.
It wasn’t until the development of the Siemens-Martin process in 1875 that steel of consistent quality could be obtained. The steel used was produced by two steelworks in Scotland and one in Wales.
The cantilever design of the bridge was not new – it had been in use for centuries in the east – but the scale of the Forth Bridge was completely unprecedented, as were many of the technical challenges.
With the Tay Bridge disaster fresh in everyone’s mind, the designers and engineers had to consider calculations for wind pressures and the effects of temperature changes, yet they still created what, at the time, was the longest cantilever bridge span in the world and even yet is the second-longest single span cantilever.