As per ASME B31.3 Table D300, 1.6 SIF shall be applied with lap joint flange. however, in caesar analysis flange input as rigid elements, i.e no code stress is calculated. when we click "SIF" with 13. lap joint, how CAESAR will apply to analysis?? in my test run, there is no difference.

To comply with B31.3 SIF requirement for lap joint, what we have to do???

If the pipe is element 1000 to 1010 and the flange 1010 to 1020, specify the SIF for node no. 1010. Caesar will consider the SIF 1.6 at 1010. Check this in the output "Stresses" or "Stresses Extended" for element 1000 to 1010. Of course for element 1010 to 1020 which is a rigid, it will not list anything.

Regards,