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Natural Frequency Extraction in conjunction with UMAT in ABAQUS


We are using the user subroutine UMAT in ABAQUS for varying the material properties in a thin plate. For any static analysis this subroutine is working perfectly fine. However, while using UMAT during natural frequency extraction, ABAQUS is not completing the analysis and is giving a very vague following error message in the log file:

ABAQUS/CAE - How can I model a two-storey shear frame with rigid floors and LUMPED masses?

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This would seem like a simple task. Anyhow, I am having some trouble.

 Problem description: I want to model a two-storey shear frame with rigid floors and lumped masses? I.e., in all its simplicity, a two-DOF system ignoring the mass of the columns and lumping the mass at the two rigid floor beams. A typical example in all books on structural dynamics.

 The columns are modelled with B23-elements while the floors are chosen as analytically rigid parts. I have used 'special - intertia - point masses' to simulate the lumped mass at the reference point of the floors.

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