Unstable Model

Tips & Tricks

1. The most common cause of unstable models represent failing member nonlinearities such as failure of tension members.

As the simplest example, there is a frame with supports on the column footing and moment hinges on the column head. This unstable system is stabilized by a cross bracing of tension members.

In the case of load combinations with horizontal loads, the system remains stable. However, if it is loaded vertically, both tension members fail and the system becomes unstable, which causes a calculation error. You can avoid such an error by selecting the exceptional handling of failing members under “Calculate” → “Calculation Parameters” → “Global Calculation Parameters.”

2. Another cause of unstable models are failing supports or foundations.

For example, the system of a building with support or foundation failure becomes unstable usually when calculating wind load cases.

You can avoid the error by calculating the individual load cases always together with the self‑weight in one CO, not separately.

3. Too weak systems can also cause the instability in a CO.

If the load for the specified cross‑section or surface thicknesses is too big that the program cannot find equilibrium in the deformed system, a calculation error occurs.

You can check this by selecting the Geometrically linear static analysis under “Calculate” → “Calculation Parameters” → “Load Combinations” tab for the problematic CO. The deformation according to the geometrically linear static analysis itself is usually too big and the dimension of the elements has to be increased.

4. If instability occurs despite compliance with the points specified above, the cause is probably errors in modeling.

In this case, click “Tools” → “Model Check” and check if there are identical nodes, overlapping lines/members/surfaces or crossing unconnected lines/members. Especially in the case of imported systems, it often happens that the elements are unconnected due to rounding differences. You should also check the boundary conditions such as hinges and supports.


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