Forth Rail Bridge facts
constructed ... 1882 - 1890
overall length ... 2,465m
maximum span ... 521m
height of towers ... 104m
height above river (high tide) ... 46m
steel in superstructure ... 50,000 tons
number of rivets ... 6,500,000
maximum workforce ... 4,600
cost ... £2.5m
The Forth Rail Bridge
BUILDING THE BRIDGE
The construction challenges posed by the Forth Bridge
were immense. The spans necessary were almost four times as large as any railway bridge previously built in the United Kingdom, which as yet had no cantilever bridges.
Also, steel, the proposed material, was considered relatively untried for bridge applications. From the mid 1800s most railway bridges were of cast iron, although steel could offer a 50 per cent increase in maximum working stresses, clearly a great attraction with long spans.
Safety was a primary concern after the Tay Bridge disaster. A new wind loading of 56 lb per sq ft (previously 10 lb per sq ft) had been imposed and the Board of Trade stated that the bridge: 'should gain the confidence of the public, and enjoy a reputation of not only being the biggest and strongest, but also the stiffest bridge in the world'.
The construction of the colossal spans was tremendous task that was undertaken by William Arrol of Glasgow, who had also worked on the massive new Tay Bridge. Initial site work involved building the support piers. No special difficulty arose with the work executed either in tidal conditions, or in half-tide or full-tide cofferdams.
Each of the three main towers is supported on four separate granite foundations, constructed within iron caissons 21m (70ft) in diameter. The caissons were founded at depths varying from 4.25m (14ft) to 27m (89ft). A delay was caused when one of the caissons accidentally tilted and, through a combination of very high and low tides, sank unevenly into the mud. It took ten months before it was refloated and sunk again in the correct position.
By 1887, the year of Queen Victoria's Golden Jubilee, the core of the towers had reached their full height and it remained to extend their arms towards each other and close the gap. It was the task of constructing the cantilever sections that fully demonstrated the the practicality and buildability of cantilever design. Because construction takes place by building out from one support to the next, no temporary support structure is needed. This dramatically reduces the amount of material required and the time taken. All the steel superstructure, which used large riveted tubes for the compression members, was fabricated on site.
In September 1889, a bridge worker clamboured from the Queensferry to the central Inchgarvie cantilever across a ladder placed between crane jacks, working some 60m (200ft) above the water. On 15th October a more secure and formal crossing was made, and by 6th November the central girder was ready to be connected. This was delayed for over a week until the temperature changed sufficiently to cause the necessary expansion that would allow the key plates to be driven in and the girder fixed between its supporting cantilevers.
The bridge was opened by the Prince of Wales on 5th March 1890 and is still in use today. It held the world record for a cantilever bridge until 1917.
Not only is the Firth of Forth Rail Bridge still in use today, it is also a structure that will continue to inspire generations of engineers. And how long might such a monument last? 'Forever if you look after it' according to the Firth of Forth Bridge Engineer in 1890.