The forth bridge - where did they start?

[mariner2]

When they built the railway bridge over the Firth of Forth, what's the FIRST thing thing they actually did apart from constructing something on the bank?
I was thinking about it and just couldn't work out where you would start . .

[New Judge]

[Part One]

The Forth railway bridge consists of three main cantilevered towers and some minor approach sections. One of the towers sits on a small island in the estuary, but to accommodate the other two artificial bases had to be built. This was done by sinking a cast iron cylinder (called a caisson) into the river bed, pumping out the water and filling it with rock and concrete. So the first job was to construct these on land and tow them out and sink them in the estuary.

The caissons weighed about 500 tons and one of them keeled over when being towed and it took ten months to right it.

As an aside, after the water had been pumped out the caissons were sealed and filled with compressed air to keep the water out. Men then worked at the bottom sealing it to the riverbed. Working in compressed air the men inside sometimes suffered �the bends�. This was before scuba diving had been developed and decompression sickness - the proper name for �the bends� - was firstly known as �caisson disease�.

Once the bases had been built construction of the main towers began. They grew outwards from the middle at the same rate each side so that they did not topple over. When two of the spans met they were bolted together using bolts each the size of a man. This relied upon the steel being at around 55 degrees Fahrenheit as the boltholes would not line up at other temperatures.

[New Judge]

[Part Two]

The bridge was opened by the Prince of Wales on March 4th 1890. People were initially reluctant to travel over it. Memories of the Tay Bridge, which collapsed carrying a train into the river in 1879, were still vivid. That bridge�s designer (Thomas Bouch) had produced a design for a Forth crossing but he committed suicide soon after the Tay Bridge collapse.

The structure cost �3.2m and took eight years to complete. 57 lives were lost during construction, the majority of these being due to either falls from the structure or being struck by falling objects.

At 1710 feet each span is slightly smaller than the world�s longest cantilever span in Quebec. However, with two main spans of that length and lengthy approaches the overall length is more than a mile and a half. The main towers are 330 feet high and the spans provide 150 feet of headroom at high water. The main steel tubes forming the cantilevers are big enough to accommodate a London tube train.

The legendary continuous painting takes four years to complete. However, this will soon be a thing of the past as the latest high-tech coating currently being applied (due for completion in 2009) will last 20 years.

In 1990 to celebrate its centenary the bridge was opened to pedestrians for just one day and I was privileged to take a stroll across the mighty structure.

[heathfield]

As with any such structure, the very first thing to do before construction can start is to drill for soil and rock samples to see what sort of foundations will have to be designed to support it.