In this article we will discuss about how to design cylindrical tanks for storing domestic and industrial water.
Cylindrical tanks may have flat bottom or spherical bottom or conical bottom. A flat bottomed cylindrical tank having a height greater than the diameter is called a stand pipe. Such a tank rests on the foundation directly. The tank may also be provided with a conical roof.
Bending Stress:
Overhead Cylindrical Tanks:
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The walls of the tank will be made of plates of different diameters so that the consecutive courses may lap on each other. Lap joints are provided for horizontal joints as well as radial joints at the bottom. Vertical joints are usually provided as double riveted or triple riveted joints depending on the strength requirement.
Stress in the Bottom Plates of Tanks:
Case (i) Tanks with Cylindrical Bottom:
Case (ii) Tanks with Spherical Bottom:
If a spherical vessel is subjected to an internal pressure p, then the hoop stress in the wall of the vessel-
The stress given above is the radial stress as well as the circumferential stress, since the radius of curvature is the same everywhere.
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Hence in a tank with a spherical bottom, the hoop stress (radial as well as circumferential)
Case (iii) Tanks with Conical Bottom:
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Circumferential Stress:
Let θ be semi-vertex angle Consider a point in the conical part at a depth h. below the free water surface.
Radial Tension:
Consider any horizontal section XX at depth h below the free surface.
Connection between the Vertical and the Bottom Portion of the Tank:
It is necessary to provide a ring beam at the junction of the cylindrical and the conical portion in order to resist the horizontal component of the meridional force in the conical part.
Circular Girder or Ring Girder:
This is a girder provided to support the load of the tank and its contents. The girder is supported on a number of columns. Such a girder is subjected to maximum sagging bending moment at the middle of each span, maximum hogging bending moment over each support and maximum shear force near each support. The girder will also be subjected to torsion. The table below shows the values of the maximum bending moment and torsion, for a circular girder carrying a total uniformly distributed load W.
A column will be subjected to the following loads:
(i) Dead load and contents of the tank
(ii) Wind loads
If W be the total load due to dead load and contents of the tank, force on each column due to the above loading = W/n
where, n = number of columns.
The direct load transmitted to a column due to wind action is proportional to the distance of the column from the centroidal axis G.G. (See Fig. 15.9).
Let M = Moment of wind load about the base.
Let x1, x2, x3… be the distance of the column from the centroidal axis G.G. The direct loads on the columns due to wind load are given by-
These forces are compressive on the lee ward columns and tensile on the windward columns. Hence, the normal force for any column section-
General Arrangement of Plates:
The sides of the tank will consist of seven plates 1.25 m × 2.20 m and two plates of 0.30 m × 2.20 m. The end plates consist of three plates of 2 m × 2.20 m and two plates of 0.3 m × 2.20 m. The bottom plates will be 1.25 m wide running transversely and bent up to form a butt joint with the side plates.
The corner plates will preferably be a casting. The joints in the bottom of the tank are covered by T bars inside and turned upto form vertical stiffeners at the sides. All outer joints will be covered up by cover plates. The tank will be supported on four longitudinal girders at 2 metres centres which in turn will rest on beams carried by 6 metres high columns.
Consider a 1 metre wide strip of plate spanning between two transverse tee covers.
Design of the Bottom Tee Covers:
Fig. 15.11 shows the arrangement proposed for the Tee cover.
Section adopted – ISNT 150 inside
and 150 mm × 10 mm cover plate outside.
Load per Metre Run on the Span:
The computation pertaining to the determination of the neutral axis and moment of inertia a about the natural axis are given below:
Connection between the T and the Bottom Plate:
Transverse and longitudinal stays are provided across the tank at 1.25 m and 2 m intervals respectively at mid height.
For purposes of design, it will be assumed that the vertical plates are hinged at the junction of the curved part and the bottom plates.
The section can resist this bending moment safely. It may be noted that the same arrangement has been provided for the bottom plate joints. The tee covers for the bottom plate joints are subjected to a still greater bending moment and the T-cover section has already been designed for the greater bending moment.
Design of Columns:
Design of Foundation:
Design of Stays:
