While designing vessel as per ASME Sec VIII Div 2 Ed 2007 Add 2008 in PVElite 2009 version, the top and bottom hemispherical head fails in combined loading even if there is no wind and earthquake load only self weight is there on vessel. The minimum thickness required to withstand design pressure is not sufficient. So, thickness of heads needs to be increased. can anyone tell me why the heads fail in combined loading? Has anyone faced this sort of issue?