CAESAR II uses the "3D Beam Element" in its formulation and solution. All of the assumptions inherent with this element apply. Additionally, the piping Codes state that the (Code computed) SIFs are only applicable for D/t ratios less than 100. If you exceed this defined limit, no one knows if the results are accurate or not. Note, the is not a CAESAR II limitation, but rather a limit on the amount of testing that has been performed on actual fittings.