When entering the geometry of a ribbed timber plate, I always get the error message 30343: "The computed stiffness matrix is not positive definite." How can I define the surface?

Answer

In the case of using timber materials in RFEM, a message appears when creating the material, saying that the "isotropic linear elastic" material model is only correct for the calculation with members because the Poisson's ratio is very large. For surfaces, it is necessary to select a different material model.

Figure 02 - Material Definition for Timber

If you have assigned the material with the orthotropic material model to the surface, it is not possible to use certain orthotropy types. This material cannot be used for ribbed plates.

Figure 03 - Stiffness Parameters - Orthotropic Elastic Material Model

However, if you have assigned the material to the surface with an isotropic linear elastic material model, you can access all types of orthotropy.

Figure 04 - Stiffness Parameters - Isotropic Linear Elastic Material Model

When entering the geometry for the ribbed plate, Error No. 30343 appears due to the incorrectly assigned timber material for the surface: "The computed stiffness matrix is not positive definite."

Figure 05 - Error Message 30343

Keywords

Orthotropic surface Error 30343 Stiffness matrix Material model

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RFEM 5.xx

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Structural engineering software for finite element analysis (FEA) of planar and spatial structural systems consisting of plates, walls, shells, members (beams), solids and contact elements

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The structural engineering software for design of frame, beam and truss structures, performing linear and nonlinear calculations of internal forces, deformations, and support reactions

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