Hydraulic power was used in the construction of the bridge in many places, but nowhere more critically than in the closure of the top chords of the arches.
The two-hinged arch
Bradfield's design was for a 'two-hinged arch' bridge, however to achieve this it was necessary to construct each half arch from each shore and for these to come to rest on a temporary pin to form a "three hinged arch". This allowed guide pins to be used to assist alignment of the arches and it allowed for the removal of the support cables.
The completion of the 'three-hinged arch' was not the end of the construction process: the upper and lower chords had to be fixed in place, thus in effect removing the central pin at the centre top of the arch. Movement of the Bridge due to live loads such as traffic, temperature change and wind would then be distributed throughout the arch and taken up at the main bearings on each shore.
This process required considerable skill and some very large hydraulic jacks.
To quote the Director of Construction, Lawrence Ennis, on the process: ‘… the arch was completed so as to leave a gap between the top chords of 24 inches [610 mm], into which was inserted four hydraulic jacks in each chord. These eight jacks were capable of exerting a combined force of nearly 8000 tons [8128 tonnes], at an operating pressure of 4 tons per square inch [61·8 MPa]. The two top chords were forced apart to a predetermined extent, and in the space so made carefully machined steel slabs were inserted as packing pieces. By forcing the two central top-chord members apart, the crown of the arch was converted into a rigid structure, and thus into its final 'two-hinged' condition, the main bearings alone acting as hinges.’ (Ennis, 1932, p 46)
The main arch of the Bridge has two top chords on the eastern side and two top chords on the western side. Each chord on the eastern side is adjacent to one another; similarly on the western side.
Four interconnected hydraulic jacks were placed between the two top chords on the eastern side. These were loaded to separate the chords enough to allow a forged steel block to be hung between the chords. When the blocks were in place, the hydraulic jacks were unloaded and removed. Mechanical screw jacks were also used as a precaution against hydraulic failure. When complete, the same process was carried out on the western chords.