After the analysis is over, i noticed warning within the analysis output file that the structure is unstable or ill-conditioned and that "number of negative stiffness eigenvalues should be zero for stability). And after performing nonlinear buckling analysis, i found that some modes actually have negative eigenvalues. I had resized the elements' cross sections many times but nothing changed. So, What could be the solution to this problem?
Have you supported your structure properly? Try doing a modal analysis and see if there are any eigenvalues with zero or near zero frequencies...
Yes, the structure is well supported. I've carried out a modal analysis and there are no eigenvalues with zero or near zero frequencies.
The yellow elements are struts and the green elements are cables. The cables are subject to pre-stressing force to create a state of self-stressed equilibrium before loading the structure.
I found a table demonstrating the significance of the value of buckling load factor (BLF) in some reference.
Hallo Abdelrahman,
i do not know the software SAP2000. However the negative eigenvalues tells you something about your structure!. They represent buckling modes which may occur when you change the loading direction! So try to do the same simulation with reversed load direction and you get positive ones!.
Regards,
Tamer
yeah. The negative sign refers to the reversed direction of loads.
Thanks so much Dr. Angus Ramsay and Dr. Tamer Qaimari for your help.
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