Elevated tanks with conic shell walls

Abstract

In Morrocco, recently, four elevated water tanks have been constructed, which are about 30 m above ground level. Parts of them are in prestressed concrete. Their aspect is pleasant, showing a harmonious outline. The four water tanks are fairly similar. One of them has a slender trunk, supporting the upper container. The other three differ from this one mainly in that the number of supporting columns which converge towards the top varies in each one. The water tank itself is made up of eight inverted conically shaped shell surfaces. They are paired symmetrically in four couples. These eight conical surfaces which constitute the bottom of the tank are separated from each other by vertical diaphragms or partitions. These are prestressed, but the compartments resulting from these partitions are connected at the bottom to ensure equal pressure on each side of the diaphragms. The structure, as a whole, constitutes a geometrical pattern whose simplicity and effectiveness can be easily appreciated from its subtle external beauty. Because of the complexity of analysing the strength properties of this structure with precision, an approximate method has been adopted, which is simple but ingenius. It is based on assuming a compression funicular of forces which is the mean of the external surface of the conical shells and provides a prudent margin of safety. The designer has also succeeded in inducing such a state of stress on the shell care to the concrete joints, as is the case in conventional concrete water tanks. These tanks, which are at present in service, have performed very closely as anticipated from the theoretical estimates. This has justified the author in his approximate calculation methods, which appear to be fully acceptable, and avoid the need of undertaking ardous mathematical operations.