The mode of sinking the Queensferry caissons
The mode of sinking the Queensferry caissons
Approach viaducts The approaches were constructed and designed by James Carswell under separate contract . The approach viaducts to the north and south had to be carried at 39.78m above the level of high water , and it was decided to build them at a lower level and then raise them in tandem with the construction of the masonry piers .
Two spans were attached together to make a continuous girder , with an expansion joint between each pair of spans . Due to the slope of the hill under the viaducts , the girders were assembled at different heights , and only joined when they had reached the same level . Lifting was done using large hydraulic rams , and took place in increments of around 1.0m every four days .
Building the superstructure The tubular members were constructed in a workshop further up the hill at Queensferry . To bend plates into the required shape , they were first heated in a gas furnace , and then pressed into the correct curve . The curved plates were then assembled on a mandrel , and holes drilled for rivets , before they were marked individually and moved to the correct location to be added to the structure . Lattice members and other parts
The tilted caisson
were also assembled at South Queensferry , using cranes and highly efficient hydraulic rivetters .
The main compression members are steel tubes ranging up to 3,6m in diameter , the tubular form being adopted for two reasons , firstly , because experiments have shown that inch for inch the tubular form is stronger than any other , and , secondly , because the amount of stiffening and secondary bracing is thereby reduced to the lowest percentage .
Assembly of the cantilevers took 4 years to complete . At the peak , approximately 4,600 workers were employed in the bridge ' s construction . Wilhelm Westhofen recorded in 1890 that 57 lives were lost .
Opening The bridge was completed in December 1889 , and load testing of the completed bridge was carried out on 21 January 1890 . Two trains , each consisting of three heavy locomotives and 50 wagons loaded with coal , totaling 1,880 tons in weight , were driven slowly from South Queensferry to the middle of the north cantilever , stopping frequently to measure the deflection of the bridge . This represented more than twice the design load of the bridge . A few days previously there had been
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