I have a frame with lateral and gravity loads. I wish to run a P-delta analysis to account for second-order effects. When I do so I get the error "Matrix is not positive definite". To troubleshoot the issue, I have added a lateral restraint at the top of each frame. This would negate 100% of the sway. Robot gives me the same error. To troubleshoot further, I due a simple static analysis with the lateral restraints and the members are only lightly stressed. I do not understand how to get rid of the "Matrix is not positive definite" notice for P-delta analysis.
I am using RSA 2018 with the latest patch. I've included the model with the lateral restraints at the top of each frame and the P-delta analysis turned on.
Solved! Go to Solution.
Solved by Artur.Kosakowski. Go to Solution.
Hi @Anonymous
You cannot intersect truss bars as they have no out of their plane stiffness. Try to replace them with bars with releases.
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Thank you Artur,
Sorry, but I sent you the incorrect model. The model I intended to send did not have tension only bars. It did have compatible nodes. As it turns out, it will not converge using a P-delta analysis unless the compatible nodes are deleted. I'm sure that I've used compatible nodes with a P-delta analysis in previous versions of Robot. Has this feature changed in the 2018 version?
Best regards,
Carl
then send the correct "incorrect" model!!
Rafael Medeiros
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Hi @Anonymous
I'd say the definition of compatible nodes is such that some part o the model becomes unstable under the level of the applied load. It can be the same story - if you released all rotations in the 'middle' node rather than only these in the bracing plane ...
As @Rafacascudo wrote - we need to see the 'right' model.
Good day,
Here is the model using compatible nodes. I've been trying different options to get Robot to use P-Delta including tension only and axial releases on the bars. All of which work in non P-delta analysis, but not using P-delta.
Best regards.
Hi @Anonymous
If I managed to answer your question(s) press the Accept as Solution button please. This will help other users to find solution(s) much faster. Thank you.
As follow-up to this, we have successfully modelled this frame using compatible nodes and tension only members when using a P-delta analysis. We accomplished this by changing the method of solving the system of equations to "Multi-threaded solver" from "Automatic" under Tools -> Job Preferences -> Structure Analysis.
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