The Lifting of the Bottom of a Circular Steel Tank

1. INTRODUCTION.
EXPERIENCE shows that, under two types of loading, the circular bottom of a large cylindrical steel tank may partially lift off the concrete base on which the tank rests, leaving a central area lying flat on the base. The first of these, indicated in Fig. 1 and examined in section 2, occurs when the tank is filled with fuel or water. The hydrostatic pressure on the shell causes a bending moment along the circumference of the bottom which tends to lift the bottom. The second is found when a tank is used for the storage of volatile spirit. To prevent excessive loss by evaporation the vents in the roof are loaded so that a small vapour pressure is maintained in the tank. Although this pressure is usually only a few pounds per square inch, yet it acts on such a large area of roof that the shell is put in vertical tension. The pressure persists when the tank is emptied, and then the edge of the bottom tends to lift as shown in Fig. 4, the disturbance caused by the shell being due partly to the bending moment as in the first case and partly to the upward force. The behaviour of the shell and the bottom with uniform pressure in the tank is therefore considered in section 3. The procedure in each case will be to calculate the relation between the bending moment
M., and the rotation è (Figs. 1 and 4) first for the bottom and then for the shell. Hence the absolute magnitudes of these quantities can be obtained. When this has been done, the usual methods suffice to determine the deflexions and stresses throughout both the bottom and the shell.

A.M. Binnie