Mariog is correct, piping flexibility programs use a 3D Beam Element to solve the FEA solution to [K] {x} = {f}. A 3D Beam Element is an infinitely thin stick - there is no sense of volume, or a shell surface.
A rigid element is a 3D Beam Element that is much stiffer than the connecting pipe elements. You don't know the cross-section of the rigid, it is simply a numeric construct used to transfer forces and moments (unaltered) from its start node to its end node.
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Regards,
Richard Ay - Consultant