General FAQ

How are structures with cables analyzed for buckling?

Run nonlinear analysis to determine the stiffness of the structure at the end of a nonlinear case. Then run buckling analysis starting at the end of this nonlinear case.

In nonlinear P-Delta Large Displacement analysis, what happens to the solution if a member reaches its buckling capacity?

The analysis will typically run into convergence problems.

Does the buckling analysis include the effect of shear deformations?

Yes, the buckling analysis included the effect of shear deformations. For models in which shear deformation govern, this may cause the calculated buckling factors not to match the theoretical [critical load]. The user can eliminate the impact of shear deformations on the buckling behavior by setting large property modifiers for shear areas in 2 and 3 directions.

Why applying lateral force does not reduce buckling factors for a cantilever column?

Expanded question: I modeled a simple cantilever column and determined its buckling load. Then, in a different analysis, I applied lateral load to the column and determined the buckling load at the end of lateral load analysis. I got the same buckling load (no matter how large is the lateral load). I expected a smaller buckling load. Am I making a mistake?

Answer: This behavior is expected for the model and loading you have described. For your particular model, the linear buckling analysis would yield buckling factors independent of the applied lateral load. This is because the lateral load in this particular case does not affect the geometric stiffness of the structure.

You can still capture the softening of the structure due to applied lateral load. However, you would need to run nonlinear analysis with [P-Delta] and [large displacement] effects and then plot the applied load against the lateral displacement. The relationship will be linear at the beginning, but the structure will start softening at certain level of the applied load. The plot of the anticipated response is shown below (for a slightly different model of initially crooked column):

How should I interpret internal forces and reactions obtained from buckling analysis?

Expanded Question: When you perform a buckling analysis of a simple frame column, you get the buckling mode shapes, associated to a factor of the original loads you applied. In my fixed cantilever model, I just applied a vertical load of 1000 to the free tip. For every mode shape, even if there is a factor different from zero, the axial force is always zero. I assumed it would be no problem if the purpose would be to evaluate only the shapes and the factors. However, there are global reactions in the joints, in moments and shear, that I do not understand how can they be present. At the same time the internal forces of the applied force is not present. Could you please clarify?

Answer: The internal forces and reactions reported for buckling load cases correspond to the buckled shape of the structure. For your cantilever model, the structure buckles sideways, which generates internal moments and shears, but no axial force.