Dear Andre,
In the Code ASME VIII Division 1 Appendix 13, there is an analysis procedure for vessels of non-circular cross section. The design rules cover many flavors and varieties of these types of vessels. PV Elite and CodeCalc do analyze these rectangular vessels. We will forward a sample printout to you as requested.
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Scott MayeuxCADWorx & Analysis Solutions
Intergraph Process, Power, & Marine