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.

Using modal analysis to troubleshoot convergence failure

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