General Concepts and Construction Details
The basis of the Preload system of design and construction of pre-stressed concrete circular structures was the Preload stressing machine, or ‘wire winder’ which could apply large quantities of stressed high tensile steel wire to circular structures rapidly and efficiently. The wire winder was a self propelled machine capable of continuous motion around the tank wall. The wire was applied at a predetermined and uniform stress, by drawing the wire through a die, and laid directly on to the wall thus reducing frictional losses to a minimum. The stressing wire used was a cold drawn high tensile steel wire, usually galvanised. In most cases the initial wire diameter of 0.200” was drawn down during wire winding to 0.175”.
By the late 60’s some 2,000 liquid storage tanks had been built world wide using this process, the majority of which were in the United States. Up to the end of 1967, about 100 such tanks had been built in the UK.
The standard Preload roof was a spherical shell dome having a rise at the crown 1/10th of the span and a minimum shell thickness of 3”. The peripheral ring of the dome was stressed by winding the design number of laps around the springing line. In the 60’s, domes were either ‘fixed’ or ‘floating’ depending on their diameter. The fixed dome was cast monolithically with the tank wall whereas the floating dome sat on a rubber bearing pad at the top of the wall. Developments in the late 60’s included the concept of the floating dome supported off corbels cast on the internal face of the wall, thus removing the need for the dome ring beam.
Other developments in the 60’s included the ‘banded’ wall design. Previously, the core wall was wound with stressing wire over its full height, with the number of laps per metre rise varying to design loadings. The whole wall was then covered with sprayed concrete. With the banded design, the wires were concentrated into preformed grooves. The grooves or bands were subsequently covered over with sprayed mortar. As the sprayed mortar was in narrow bands it was possible to have a floated finish to produce an aesthetically improved appearance. These bands were quite often painted, with a cementitious paint.
All walls were vertically stressed. Vertical stressing was carried out, either using Macalloy bars or tendons. Up until the late 60’s the Macalloy bars were grouted after stressing, subsequently all vertical tendons were left un bonded.
The last ‘Preload’ tank built in the UK was constructed in 1981 to a modified design by Prestressed Structures Limited, the successors of Preload Limited, for the Bournemouth Water Company.