1) Yes you could define different cross-sections, and build your model this way. However, it may be easier to just increase the density in a short element every so often to consider the weight of the ribs.
2) CAESAR II has no clue about stiffening, external pressure requirements, or buckling.
3) The flexibility calculations of BS-7159 are unaware of stiffening ring size/spacing.
Remember, pipe stress programs, like CAESAR II, use a 3D beam element formulation. This is essentially an infinitely thin stick, with properties such as length, diameter, thickness, modulus, and thermal expansion coefficient. The overall behavior of the system is obtained based on the displacements and rotations of the node points defining these elements. The flexibility stresses obtained are accurate as long as the overall system wide behavior is dominated by bending. Buckling and local element stiffening are beyond the scope of the 3D beam element.
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Regards,
Richard Ay - Consultant