RE: ***WARNING - INSTABILITY AT JOINT 2 DIRECTION = MX

Vishal Girase — Tue, 19 Dec 2017 03:14:47 GMT

Thank you for your prompt reply.

I will go through the details of 0 stiffness.

Regards,

Vishal

RE: ***WARNING - INSTABILITY AT JOINT 2 DIRECTION = MX

Sye — Mon, 18 Dec 2017 19:45:46 GMT

Instability warning along a particular degree of freedom does not depend on whether you actually have a destabilizing force along that degree of freedom or not. The warning is aimed at making the user aware that there is a potential for an instability at a certain node along a certain dof. In your case, if you do not have a force that would cause a Mx moment to develop, the warning would not have any effect on the results. The warning would still be there though. If you would like to get rid of the warnings you may change of supports to FIXED BUT MY MZ. It would not affect your results but would just take care of the warning.

Instabilities related to MY and MZ are not developed when you define PINNED supports at the two ends of a member. It just means that the ends of the beam would not develop any MY or MZ moments but the beam can still resist the moment through bending. The span moments would just be larger. There is a clear load path for the moment in this case. However if you release MX at both ends of a member, any torsion would not have a load path and it would simply get lost. Hence the instability warning related to MX.

Lot of details related to zero stiffness and instabilities are available at the page below

https://communities.bentley.com/products/ram-staad/w/structural_analysis_and_design__wiki/2046/staad-pro-instability-and-zero-stiffness-faq