The Firth of Forth is a Scottish estuary that divides Fife on the north from Lothian on the south. Firth is probably derived from the Norse word Fjord, meaning narrow inlet. And it’s fortunate that it narrows sufficiently at the town of Queensferry to allow not one or two bridges but three.
Bridge number one is the oldest, and is a railway bridge, because cars didn’t exist when it was constructed between 1883 and 1890. Remarkably it still holds the record for being the world’s longest cantilever bridge. A Cantilever works something like this:
The Forth Bridge was also the world’s first major steel structure and is 2467 meters long, used 53000 tonnes of steel and 615 million rivets. During its construction 57 workers lost their lives. The first design for the bridge was by Thomas Bouch, but it was abandoned. The reason why was due to the collapse of another railway bridge in 1879 – an iron bridge that failed during a storm. Bouch’s reputation was destroyed with the bridge. The new designers John Fowler and Benjamin Baker submitted the cantilever design. Both men were involved in the construction of the London Underground, and Baker designed a vessel to carry Cleopatra’s Needle (an obelisk) from Egypt to London.
4600 men were employed, many left with crippling injuries. Caissons were sunk to create the piers and building them requires using compressed air to keep the water out. It was dangerous and uncomfortable work. The towers on top of the piers were completed, then the cantilevers stretched out to fill the gaps in a magical anti-gravity way. The completed bridge allows an uninterrupted train service from London to Aberdeen to this day.
The second bridge, the Forth Road Bridge, opened in 1964 and is a long span suspension bridge. At 2.5km it was the largest outside USA at the time. The first proposals for a road bridge were discussed in 1923. As well as bridge options, there were tunnel ideas too. The Great Depression and WW2 delayed the start. Whereas the Forth Bridge was designed by individual engineers, this newer bridge, like most large scale constructions these days, is the product of a consortium of engineers and designers.
One tower was constructed on a whinstone outcrop – Mackintosh Rock, while the southern tower was situated as far as way from the shore as possible. The cables, nearly 12,000 of them, are steel wires bundled together and the total length of them could go around the world one and half times.
The most recent bridge is the Queensferry Crossing and was opened in 2017. It is a cable stayed bridge, which looks similar to a suspension, but according to this neat little diagram, the forces work differently, so the deck isn’t suspended.
At 2.7km long, it is the longest, 3 tower, cable stayed bridge in the world. The record broken during its construction is the largest continuous underwater cement pouring. For 15 days, 16869 cubic metres of concrete were poured into the south tower caisson non-stop.
So they we have it. Three bridges, yet it has to be said, the Forth Bridge is the most iconic and recognisable. For who hasn’t heard of the saying it’s like ‘Painting the Forth Bridge’. (The idiom is used to indicate a job or task that is never completed.) As soon as the painting job is finished, it needs to be started again. The paint is required to protect the steel from the harsh elements. First a primer was applied, then an epoxy paint by hand to each of the rivets, then the famous red topcoat. But the truth is, the painting was completed in 2011, and should last 20-25 years. Regardless, we’ll stick to the idiom, it does have a great history.