Hi Nasr,
Does the Tank software not carry out appropriate calculations? As has been stated in a previous post the brothers Roark should not be used in isolation for the design of storage tanks. If you need to get hold of a copy then you should be able to order one on the internet. The full reference is 'Roark's Formulas for Stress and Strain' by Warren C. Young ISBN-10: 0071210598, ISBN-13: 978-0071210591
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Stan McKay