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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. Walls that have been modeled uising membrane type area element (never recommended) or shells with unrealistic thickness and or low stiffness modifiers can buckle and cause convergence issues. As an alternative, you may also refine the factors and/or loads applied during analysis.
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