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# [CIVIL]How do I Set Boundary Conditions to Avoid Unstable States?

## Question

How do I set boundary conditions to avoid unstable states?

For example, in a continuous beam where horizontal forces act on both ends with roller supports, if there are no boundary conditions in the horizontal direction, even a small load can result in an infinite displacement. This state is referred to as "unstable."

When the analysis model encounters such conditions, it issues "ABNORMAL" or "SINGULAR" warning messages as shown below

In such cases, to prevent warning messages and ensure the stability of the analysis, you need to add virtual horizontal boundary conditions. These boundary conditions are not real and should not create any reactions in the structure.

Let's consider an example. If the defined boundary conditions in your model already include the shear stiffness of the ground, there may be no need to add additional boundary conditions. However, if the boundary conditions do not include the shear stiffness of the ground, the boundary conditions for the underground structure will look like this

In this state, there are no horizontal boundary conditions, and the program will output a Singular Error. Such results may be unreliable. To prevent the Singular Error, you should add virtual horizontal boundary conditions.

Many users add DX-direction support to prevent Singular Error. While this is not incorrect, it may not be the best approach. It works well when there are only vertical loads and the structure is symmetrical. However, if there are asymmetric horizontal loads and the structure is not perfectly symmetrical, the applied boundary conditions can generate horizontal reactions that were not initially intended.

To prevent Singular Error while minimizing the impact on the structure, it's recommended to use Point Spring Supports. Input very low stiffness values to have minimal impact on the structure. Place them in the center of the structure or symmetrically to avoid generating asymmetry. For 2D analysis, you can add DX-direction only, and for 3D analysis, add low stiffness boundary conditions in both DX and DY directions. Even when using these boundary conditions, it's essential to check the reactions after analysis. These boundary conditions are not real, and the reactions should be close to zero or very small to have minimal impact on the structural system.

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