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h2. Why doesdo P-DeltaI analysisget notnegative convergestiffness forduring certain load casesP-Delta analysis?

*Extended Question:* When DoesI get thenegative inabilitystiffness toand convergeP-Delta indicateanalysis instabilitydoes andnot bucklingconverge, ordoes shouldthis Iindicate changeinstability certainand analysis parametersbuckling?

*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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