Design of Rectangular tanks
Rectangular tanks are provided when small capacity tanks are required. For small capacities circular tanks prove uneconomical as the formwork for circular tanks is very costly. The rectangular tanks should be preferably square in plan from point of view of economy. It is desirable that longer side should not be greater than twice the smaller side.
For rectangular tanks in which ratio of length to breadth is less than 2, tank walls are designed as continuous frame subjected to pressure varying from zero at to top to maximum at H/4 or 1 m., from base, whichever is more. The bottom portion H/4 (or) 1 m whichever is more is designed as cantilever. In addition to bending, walls are subjected to direct tension caused by the hydrostatic pressure on the walls. The section is to be designed for direct tension and bending. Bending moments in the walls are found by moment distribution. Direct tension in long walls
and direct tension in short walls
For rectangular tanks in which ratio of length to breadth is greater than 2, the long walls are designed as cantilevers and short walls as slabs supported on long walls. Bottom portion of short walls H/4 or 1 m whichever ismore, is designed as cantilever.
Maximum B.M. in long walls at base
In the short walls maximum B.M. occurs at support and is given by
B.M. at center of short walls is taken as
For bottom portion of short wall, which is designed as cantilever maximum B.M. is given by
whichever is greater. In addition to B.M. short walls and long walls are subjected to direct tension. Direct tension on long walls is given by
For short walls it is assumed that end one meter width of long wall contributes to direct tension on the short walls. Direct tension on short wall is w(H-h).
Design of section for tension. It is assumed that entire tension is taken by steel. Let T be tension. Net B.M. = M –T x x. Steel reinforcement is provided for B.M. of M –T x x and direct tension T