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Caesar II and RISA 3D

In this blog I will compare results of RISA 3D stress analysis (structural software) and Caesar II stress analysis (piping software). At the first glance, I was expecting that displacements and forces calculated in structural software RISA 3D will match displacements and forces calculated in Caesar II. Of course, calculated stresses will not be matched, since both software uses relevant code in the stress calculation. At my surprise, calculated forces and displacements from RISA 3D and Caesar II shows great discrepancies. In model simulation 4″ pipe has been used, having thermal load of 250 degree F and ambient temperature of 65 degree F. Please see the figure below.
1-caesar-piping-model

Displacements from thermal load calculated in Caesar II are shown on figure bellow.

2-caesar-ii-displacements

Same model has been stress analyzed in RISA 3D (structural software) and thermal displacement s are shown on the figure below.

3-risa-d3d-displacemnts

From the figures above, it is evident that vertical and horizontal displacements are matched, but Mz rotation shows great divergence. Consequently, forces and moments must shows divergence too. Please see the figures below.

5-risa-3d-forces

Reason for mismatching results lies in a fact that Caesar II takes account elbow curvature as well as elbow flexibility in the FEA analysis, whereas RISA 3D will simply takes two beams connected at 90 degree angle, and rigidity of such joint is much greater than rigidity of the elbow, producing higher forces. Please see the figure below.
6-caesar-ii-vs-risa-3d
In conclusion, structural software should not be used for forces and moments analysis since it will always produce incorrect result. However, RISA 3D can be used for stress analysis of plate weldment structure (e.g boxed section of crane girders). Below is the snapshot of RISA 3D output, showing shear force for the transfer car structure.
7-rsa-3d-girder
After determining forces and moments, the stress was calculated manually, as well as plate buckling. Please see figure below.8-manual-calculation