Modal analysis may be used to troubleshoot the convergence failure of a staged-construction load case. If the model fails to converge in stage X of a staged-construction case named Staged, modal analysis may indicate the convergence problem through the following process:

Create a copy of the staged-construction load case and name it Staged-Copy.

From Staged-Copy, remove all stages which follow stage X.

Remove all loading assigned in stage X of the Staged-Copy load case.

Create a modal-analysis load case which uses stiffness at the end of Staged-Copy.

Run modal analysis, then check for instabilities while reviewing results (modal shapes, periods, etc.). Instabilities are clearly indicated by animating mode shapes.

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To locate disconnected elements, try assigning unit mass to all DOF and running a modal analysis. The modal shapes will identify disconnected elements that cannot support load and prevent convergence in nonlinear analysis.

NOTE: A variation of this process may be used to troubleshoot convergence problems which occur during time-history analysis. When a certain step fails to converge, run analysis until that step, then run modal analysis using stiffness at that step to check for instabilities.

Technical Knowledge Base

Using modal analysis to troubleshoot convergence failure

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