FAQ 004632 | When using a line release, I get the error message saying that the model is unstable. Why?

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08/18/2020

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Question

When using a line release, I get the error message saying that the model is unstable. Why?

Answer

Figure 01 shows a model where Surfaces 1 and 2 should rest on Members 1 and 2. Members 1 and 2 should be supported by Nodal Supports 1 and 2.

Figure 01 - Structural System

For decoupling the members from the surfaces, a line release type shown in Figure 02 is defined. The displacement uz is only released in the case of a tensile force, the other displacements ux, uy and the rotation φx are always released.

Figure 02 - Line Release Type

Line Release 1 is defined on Line 5 and releases Member 1 (Figure 03).

Figure 03 - Line Release 1

Line Release 2 is defined on Line 14 and releases Member 2 (Figure 04).

Figure 04 - Line Release 2

Member 1 is now defined on Line 15 generated by the line release with Nodes 12 and Node 13, and Member 2 is defined on Line 16 generated by the line release with Nodes 14 and Node 15 (Figure 05). Members 1 and 2 have different nodes (13 and 14) at their intersection and are not connected to them.

Figure 05 - Lines and Nodes of Members 1 and 2

Furthermore, Nodal Supports 1 and 2 are defined at Nodes 5 and 10 and thus support the surfaces, but not the members (Figure 06).

Figure 06 - Nodal Supports

During the calculation, the instability message shown in Figure 07 is displayed because the members are unsupported.

Figure 07 - Error Message "The stiffness matrix is singular!"

The connection of Members 1 and 2 can be created by defining Node 13 generated by Line Release 1 as a definition node in Line Release 2 (Figure 08). Members 1 and 2 then share the same Node 13 and are connected to it.

Figure 08 - Line Release 2 with Definition Node

Furthermore, you have to define Nodal Supports 1 and 2 at the nodes of Members 1 and 2 (Figure 09).

Figure 09 - Nodal Support on Members 1 and 2

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Dlubal FAQ Line release Instability Definition node Frequently Asked Question FAQ about Dlubal Question and Answer about Dlubal

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  • Updated 09/15/2020

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