## Overhead tanks

**Overhead Tanks**

These tanks may be rectangular or circular. The tanks are supported on staging which consists of masonry tower or a number of columns braced together. The tank walls are designed in the same way as the walls of tanks resting on the ground. The base slab of circular tanks is designed as circular slab supported on masonry or circular beam at the end. The slab of rectangular tanks is designed as two-way slab if length is less than twice the breadth the slab is designed as one-way slab. The base slab is subjected to bending moment at the end to direct tension, caused by the water pressure acting on vertical walls.

For large tanks base slab is supported on series of beams supported on columns.

The staging consists of a number of columns braced together at intervals. The columns are assumed to be fixed at the braces as well as to elevated tank, therefore, effective length of column is taken as distance between bracings.

The wind force acting on the tank and staging produces tension on the windward side columns and compression on the leeward side columns. The force in any column is proportional to its distance from C.G. of the column group.

Let 'P' be the total wind force acting at height 'h' from the base and r_{1}, r_{2}…….. be the distances of the columns from the C.G. of the column group, measured parallel to the direction of the wind. Force F_{1} in column 1 at distance r_{1} from C.G. of the column group is given by

In the design it is assumed that horizontal shear taken by inner columns is twice that taken by outer columns. The bracings are designed for B.M. and shear. Same reinforcement is put at top and bottom as the may blow from one side or the other.

Circular tanks are sometimes provided with inclined columns. In such cases the vertical component of the force to each column is found as given above. The horizontal shear in each column is given by deducting the sum of horizontal components of the forces in the columns from the wind force and dividing by number of columns.

The moments in the inclined braces meeting at a column can be found as follows-

The axes of moments in column above and below the brace will be at right angles to the direction of the wind. The axes of moments in the two braces will be at right angles to their axes. By completing the triangle of moments, the moments, the moments in the braces can be found.

Let O be the column and OA and OB be the braces meeting at column O. Oa is drawn perpendicular to OA and OB is drawn perpendicular to OB and ab is drawn at right angles to the direction of wind, ab gives moment in the column Oa and Ob will give the moments in braces OA and OB respectively. For moment to be maximum in OA, the wind should blow at right angles to the other brace OB. In such a case triangle of moments will be right angled triangle and side Oa will be hypotenuse.

**Foundation for elevated tanks.** The foundation for elevated tank columns may be combined foundation in the form of raft or independent footing may be provided for each column.