10. Bad Modeling practice and error messaging RBE3– warnings using 123456 coupling at independent grid points. *** WARNING # 340 RBE3 element 6300346 has nonzero weights at rotational (456) dofs of independent grids. This practice may result in undesirable load distribution - use with caution. Doubtful connections *** WARNING # 341 RBE3 element 10159308 has only two nodes. This practice may result in undesirable load distribution - use with caution.
11. Bad Modeling practice and error messaging RBE3– errors Independent spider that is not connected to any structure *** ERROR # 772 *** It is invalid to use a free grid as an independent grid in RBE3 data. Free spiders are not allowed to be independent grids in RBE3 data. RBE3 id = 10 grid id = 11 free component = 123 In NASTRAN, these free components are AUTOSPC’ed resulting in a grounded RBE3 that is now artificially stiff
12. Bad Modeling practice and error messaging RBE3– errors Independent spider that is connected with DOF 123456 to a solid element: *** ERROR # 773 *** When a grid has no rotational stiffness, then this rotational d.o.f. cannot be an independent d.o.f in an RBE3. RBE3 id = 10 grid id = 8 free component = 123456 In NASTRAN, these free components are AUTOSPC’ed resulting in a grounded RBE3 that is now artificially stiff.
13. Bad Modeling practice and error messaging Shell Elements– warning Use of very thin plate elements (.001 mm thick) on surfaces of solid elements. Will produce massless mechanisms unless the MID2 and MID3 are removed. *** WARNING # 1265 PSHELL 3000020 has thickness 0.001 or less and bending properties defined. This can lead to matrix singularities, causing message 153. If this element is intended to be only a membrane element, please leave MID2 and MID3 blank on the PSHELL data. Detected while reading line 8065770 from file ERROR_TEST.dat: "PSHELL 3000020 30000200.001 30000201.0 30000200.83333 4.60E-10"
14. Good Modeling Practices Always make sure that the model has 6 good rigid body modes Always use the GROUNDCHECK capability to verify rigid body modes quality Definition: Free-Free Eigenvalue Subcase, GROUNDCHECK=YES, ESE=YES Possible reasons for failing groundcheck Misaligned CELAS Elements with non-coincident nodes Inappropriate MPC equation definition (Tip: use rigid-elements for modeling welds) Passing groundcheck ensures model quality Otherwise model may be too stiff (constrained rigid body motion) Create problems especially when using inertia relief
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16. Accuracy can be increased buy using more modes and residual vectors (RESVEC’s)
17. Use 1.5 times the frequency of interest for Frequency Response Analysis
18. Use 1.5 time the frequencies vibration to be captured during transient Analysis
19. RESVEC’s are static “mode shapes” that capture the shape of static deformation due to dynamic loads and dynamic forces from viscous dampers
25. How to debug a model? Run the model with –CHECK option to check out the model and to generate all the fatal and warning message. Fix the model to eliminate all fatal and warning messages. Fix all seriously bad elements. Run a free-free eigenvalue analysis to make sure there are 6 clean rigid body modes. GROUNDCHECK turned on and check out whether or not the model passes the ground check criteria If Massless Mechanisms exist, use OSDIAG,46,1 (It will be renamed to MECHCHECK for 11.0) to locate massless mechanisms. Remove OSDIAG,46,1 and get the correct results
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27. The NASTRAN PBAR has no torsional rotational inertia. This can lead to massless mechanisms. This issue does not exist when using the PBAR in RADIOSS.
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29. Difference between RADIOSS and NASTRANCoarse mesh NASTRAN: 0.238 RADIOSS: 0.236 Fine mesh NASTRAN: 0.246 RADIOSS: 0.246
![Agenda ,[object Object]](https://image.slidesharecdn.com/radiossanalysisqualityht-110227203431-phpapp02/85/radioss-analysis-qualityht-1-320.jpg)
