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- Standard solver provides details on the number of digits lost and the location of the joint and degree-of-freedom (DOF) associated with the computation
- Advanced solver This is the program's default solver option and indicates that a significant number of digits were lost but does not provide details on the number of digits or the location
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The Multi-threaded solver should only be used after the stability of the model has been determined by previous analyses. If the Advanced solver indicates instabilities, re-run with the Standard solver for more details, or consider the methods below to check the model. If the Standard solver or other checks indicate sufficient stability and accuracy, then warnings from the Advanced solver can be ignored.
All three solvers report the number of negative stiffness eigenvalues found during linear equation solving ("number of eigenvalues below the shift"). This should always be zero except when performing an eigen modal analysis with a positive shift, which is not common. If the number of negative stiffness eigenvalues is not zero, a serious problem exists. This could be due to instability, very poor conditioning, or to buckling under P-delta.
A model should always be checked for stability and ill-conditioning without P-delta effects first. If the lost digits or negative stiffness eigenvalues occur only when performing P-delta analysis, please refer to the following information: P-Delta FAQ and troubleshooting.
Certain models may exhibit negative stiffness values even for linear cases in this case Select all deck/shell objects in the model, then select Assign > Shell Elements > Auto Line Constraint and uncheck the Apply to Full Structure option. This will remove auto line constraints and possibly resolve instabilities.
Regardless of the solver used, it is always recommended to carefully review the deformed shape for the gravity load case and the first few eigen-modes of vibration to make sure they are as expected. This can help determine problematic areas. Some situations where numerical instability may occur include:
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- Once the shape of the mechanism or rigid-body mode is determined, correct the connectivity or other contributing problems and check the model again before continuing with further analysis and design.