Hello Shabeer
A rectangular tank can be designed from Appendix 13 of ASME Section VIII, Division 1. That analysis is available in our CodeCalc (Component Anaysis) software. As you will probably have ribs around the tank, the load of 15 kN can be supported by one or two of the ribs. CodeCalc will not to that analysis, but it is a simple beam problem using basic beam theory.
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Sincerely,
Ray Delaforce
CADWorx & Analysis Solutions
Hexagon PPM