What could be best method to approximate Run pipe dynamic behavior in doing Branch pipe dynamic analysis seperately from Run pipe.

If anyone can sugegst how do we input Run pipe dynamic displacement/Forces-moments in Branch pipe dynamic run. As I would have to seperate stress runs of Branch and run pipes inorder to manage them. I was told that its not that straight
Please help me on this

I don't think you can do this, because the mass & stiffness of the run pipe will affect the dynamic response of the branch pipe.