In order to illustrate the use of tension and compression in the bridge , a demonstration in 1887 had the Japanese engineer Kaichi Watanabe supported between Fowler and Baker sitting in chairs . Fowler and Baker represent the cantilevers , with their arms in tension and the sticks under compression , and the bricks the cantilever end piers which are weighted with cast iron .
Construction The preparations at South Queensferry required the steep hillside to be terraced . Drill roads and workshops were built , as well as a drawing loft ( 61 by 18 m ) to allow full size drawings and templates to be laid out . A cable was laid across the Forth to allow telephone communication between the centres at Queensferry , Inchgarvie , and Fife , and girders from the collapsed Tay Bridge were laid across the railway to the west in order to allow access to the ground there . Many materials , including granite from Aberdeen ( 18,122 m 3 in total ), Arbroath rubble , sand , timber , and sometimes coke and coal , could be taken straight to the centre where they were required . Steel was delivered by train and prepared at the yard at South Queensferry before being painted with boiled linseed oil before being taken to where it was needed by barge . Near the shore a sawmill and cement store were erected . The cement used was Portland cement manufactured on the Medway .
After preparation works were done the main construction began in 1882 .
Caissons The three towers of the cantilever are each seated on four circular piers . Since the foundations were required to be constructed at or below sea level , they were excavated with the assistance of caissons and cofferdams . Six caissons were excavated by the pneumatic process , which allowed dry working conditions even at depths of up to 27 m . These caissons were constructed and assembled in Glasgow and transported in dismantled state to Queensferry . Then they were reassembled and floated to their final resting-places .
The first caisson , for the south-west pier at Queensferry was launched on 26 May 1884 , and the last caisson was launched on 29 May 1885 . When the caissons had been launched and moored , they were extended upwards with a temporary portion in order to keep water out and allow the granite pier to be built when in place . Excavation beneath the caissons was generally only carried out at high tide when the caisson was supported by buoyancy , and then when the tide fell the air pressure was reduced in order to allow the caisson to sink down , and digging would begin anew .
However not everything went smoothly . The north-west caisson was towed into place in December 1884 , but an exceptionally low tide on New Year ' s Day 1885 caused the caisson to sink into the mud of the river bed and adopt a slight tilt . When the tide rose , it flooded over the lower edge , filling the caisson with water . Even after the tide fell the situation did not get better since the water did not drain from the caisson but instead , its topheaviness caused to tilt the caisson further . Plates were bolted on by divers to raise the edge of the caisson above water level , and the caisson was reinforced with wooden struts as water was pumped out , but pumping took place too quickly and the water pressure tore a hole between 7.6 and 9.1 m long .
It was decided to construct a " barrel " of large timbers inside the caisson to reinforce it , and it was ten months before the caisson could be pumped out and dug free . The caisson was refloated on 19 October 1885 , and then moved into position and sunk with suitable modifications .
1 / 2017