My model is stiffened by tension members. The calculation is terminated with the message about instability. How can I finish the calculation?

Answer

The calculation can be terminated due to an unstable structural system for various reasons. There can be a "real" instability due to overloading the system, but the instability effects may also be caused by failing members.

In the calulcation parameters, you can deactivate the "Members due to member type" nonlinearity (see Figure 01). If the calculation is then possible without the error message, the problem is probably caused by failing members.

Figure 01 - Deactivating Nonlinearities

The complete failure of the tension members can be prevented using the option "Failing Members to Be Removed Individually During Successive Iterations" in the dialog box Global Calculation Parameters (see Figure 02). This should help in most cases. The number of possible iterations should be specified sufficiently large.

Figure 02 - Removing Failing Members Individually

An alternative method is to apply a prestress to the tension members to prevent the failure of them.

Keywords

Instability Tension member Failure Abort Nonlinearity Error message

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