Hello Pooria,
You can try this manually by generating the stiffness matrix of the pipe and the elbow and see the results. Normally the flexibility matrix of elbow is given and need to invert it to get the stiffness matrix.
You can simplify the model, by including only the vertical rigid pipe and elbow only. This would reduce the stiffness matrix to 12 x 12 matrix.
Any other opinion is highly appreciated.
Cheers!!
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Borzki