Linear and non-linear buckling

Hi all,

When we do a non-linear buckling analysis, initially we introduce some imperfections (mode shapes of mode 1, 2 etc) from eigen buckling analysis. Then we multiply the load with eigenvalue of the first mode. This load is applied on the structure having imperfections. Now put on non-linear geometric option and run the analysis. At bifurcation point we get non-linear buckling load. Non-linear buckling load comes close to eigen buckling value.

Decription of my problem is as follows.

I have a big structure say 10 m length, 3 m width and 0.02 m (=20 mm ) thick. It is subjected to a complicated loading and constriants. When I do a linear buckling analysis, local buckling is taking place (mode shape is confined to an area of 1m X 0.2 m). Buckling factor is 1.3. I used the corresponding mode shape to introduce imperfection, mutiplied load by 1.3 and then arryied out non-linear buckling analysis. The load factor computed is 2.3. The whole structure (globally) is undergoing deformation.

My questions/statements are as follows.

1. When a local buckling takes place in a structure, the load factor obtained in non-linear buckling analysis is not in the close to the load factor obtained in linear buckling analysis. Correct or not?

2. If the above statement is not correct, what could be the probable mistake that I am commiting in carrying out non-linear buckling analysis?

3. What checks shall I carry out?

I request to give a thought on these issues.

Thanks in advance and regards.

- Ramadas