Why does P-Delta analysis not converge for certain load cases?
Extended Question: Does the inability to converge indicate instability and buckling, or should I change certain analysis parameters?
Answer: Convergence problems may correlate with P-Delta behavior. Inadequate member capacity and insufficient structural support may lead to instability and buckling. Excessivley large stiffness values may also disrupt convergence by causing numerical problems which adversely affect computation.
When a model fails under P-Delta effect, increase the size and/or quantity of columns and bracing to improve lateral capacity and avoid buckling. 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 effect 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, a modal analysis which uses Stiffness At The End Of This Stage. Modal analysis may then reveal the source and location of instability.
