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# Improper boundary conditions result in Singular Error.

## Question

Improper boundary conditions result in Singular Error.

When modeling a structure, if there is an error regarding the degrees of freedom or abnormal displacement that occurs in numerical analysis, then the user must check the following procedure at each modeling stage.

### 1. In the case of applying the Rotational Constraint on Solid Elements

If a Rigid Link is used to connect the Plate element and Solid element like in the example above, an error occurs when the rotational constraint of the Rigid Link is applied.

While the Plate element has both translation and rotational degrees of freedom, the Solid element only has translation degrees of freedom. Therefore, an error can occur due to the difference in degrees of freedom. In such a case, only the translation degrees of freedom should be constrained and connected with a Rigid Link.

When connecting a Solid element with another element using Rigid Links, only the translation degrees of freedom should be applied.

### 2. Initial Tangent Displacement for Erected Structures

The above shown is a model where the 'Initial Tangent Displacement for Erected Structures' option is applied in the 'Construction Stage Analysis Control'.

The initial tangent displacement is calculated based on the nodes that have already been created. If the substructure is created first and the superstructure is created later and they are far apart, then the initial tangent displacement cannot be calculated.

In this case, if you create the nodes of the superstructure at the same time as the substructure, the initial tangent displacement will be calculated without any problem.

### 3. Nodal Connectivity

The model shown in the upper left image is a structure where the 'Free Edge' of the structure is checked in the Structure tab > Check Structure group > Display Free Edge.

If a Free Edge occurs in a place where it should not occur, it can be seen that the node is detached. If a node appears to be attached together, but in reality, it is slightly apart or there are duplicate nodes created at the same location, errors may occur during analysis. In this case, you need to merge the nodes into one or modify the shape of that part.

The image in the upper right shows a common error that occurs when modeling Plate elements. In this case, you also need to modify it.

### 4. Compression-Only Type Elastic Link

The image shown above is a model that uses a Compression-Only Type Elastic Link to model the compression characteristics of a temporary vent.

In some cases, the support of the Elastic Link is constrained in the normal direction of the structure considering the characteristics of the temporary vent resists only in the normal direction of the structure.

However, this method causes 'Singular Error' due to insufficient constraints.

The compression characteristics of the temporary vent have already been reflected in the boundary conditions through the Compression-Only Type Elastic Link, and for the point at the bottom of the Elastic Link, all degrees of freedom should be fixed.

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