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h2. Why do I get negative stiffness during P-Delta analysis?

*Extended Question:* When I get negative stiffness, and P-Delta analysis does not converge, does this indicate instability and buckling?

*Answer:* Convergence problems may correlate with [P-Delta|kb:P-Delta effect] behavior. Inadequate member capacity and insufficient structural support may lead to instability and [buckling|kb:Buckling]. Excessivley large stiffness values may also disrupt convergence by causing [numerical problems|kb:Numerical problems] which adversely affect computation.

When a model fails under P-Delta, increase the size and/or quantity of columns and bracing to improve lateral capacity, and thereby avoid buckling. The Analysis Run Log will indicate which members have experienced instability. As an alternative, you may also refine the factors and/or loads applied during analysis.

It may be best to troubleshoot the model without P-Delta until you are confident that behavior is appropriate. If problems persist with geometric nonlinearity, it may be helpful to run, after the P-Delta [load case|kb:Load case], a [modal|kb:Modal analysis] analysis which uses Stiffness At The End Of This Stage. Modal analysis may then reveal the source and location of instability.

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*Related Incident:*
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Additional suggestions which may resolve negative-stiffness errors include:

* *Remove auto line constraints*. 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|kb:Constraint] and possibly resolve instabilities.

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*Related Incident:*
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