How do i know how many buckling modes should i introduce as imperfections in Riks?

[Dzonni]

Hello. I am modeling a simply supported beam and, as a first step, i had buckling procedure where i found eigenvalues and buckling modes. I have introduced the first mode as an imperfection, but i get that the Load Proportionality Factor is greater than the eigenvalue for the same load magnitude. Am i doing something wrong or i just need to introduce some more modes as imperfections? If so, how do i know how many modes do i need and which percentage of each mode do i need to include?

 

[TGS4]

Don't compare the results of a fundamentally linear elastic analysis method with a complicated non-linear one. Perhaps compare your non-linear to test results. And concern yourself with the magnitude of the perturbations more so than the number of modes.