Thanks Dave for your prompt response.

The method you suggest to check the model, is not suitable as both the code B31.3 and B31.1 uses an effective section modulus for the branch.

But i tried with a simple model,
First model- with all 3 elements with user deifned SIF and no branch type defination.
Second model with all three element with user defined SIF and one element with type of branch. Note that SIF values in both the model are same.

As a results, first model produces higher branch stresses compared to second model. I understand first model uses exact section modulues of branch and second model uses effective section modules for branch.