The most common causes of unstable models are failing member nonlinearities such as 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".
A calculation break‑off due to an unstable system can have different reasons. On one hand, it can indicate a "real" instability due to overloading of the system; on the other hand, the error message can result from inaccuracies in the model.
If the calculation of a member model according to the second-order analysis is terminated with an error message, this instability is often caused by failed tension members: As soon as compressive forces appear in a tension member during a calculation step, this member is no longer considered in the following iterations. Thus, the model can become unstable.
Shoring braces usually obtain the "tension member" type. There are a few specifics to note because in the case of uniform, symmetrical structures and solely vertical loads, an error message often appears as follows: "The model is unstable in node No. 20. Free movement around Y-direction."