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{toc} h1. General FAQ h2. 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. h2. 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. h2. 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. h2. 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|P-Delta] and [large displacement|large displacements] 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): !Evaluate_buckling_by_nonlinear_analysis.png! h2. 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. {related-incident:no=25068|comment=Interpretation of buckling internal forces.}
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This page is devoted to frequently asked questions (FAQ) related to buckling.

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Table of Contents

Analysis

Why do my hand calculations not match linear-buckling results?

ANSWER: Structural objects should be meshed before running linear-buckling analysis such that the software may accurately capture the instability modes under the specified set of loading conditions.


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  • Incident 48668: simple buckling analysis problem

What happens analytically when a member reaches buckling capacity during nonlinear buckling analysis?

ANSWER: The analysis process typically experiences convergence problems at the buckling limit.

What is the process for buckling analysis of a structure with cables?

ANSWER: To analyze buckling in a structure with cables, nonlinear analysis should be run to determine the structural stiffness at the end of a nonlinear case. Buckling analysis may then be run, starting at the end of this nonlinear case.

Does buckling analysis include the effect of shear deformation?

ANSWER: Yes, buckling analysis includes the effect of shear deformation. For models in which shear deformation governs, this may keep the calculated buckling factors from matching the theoretical critical load. The influence of shear deformation on buckling behavior may be eliminated by setting large property modifiers to shear areas in directions 2 and 3.

Buckling factors

Why does lateral-force application not reduce the buckling factors of a cantilevered 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. Regardless of lateral-load magnitude, the same buckling load is generated though a smaller buckling load is expected. Am I making a mistake?

ANSWER: For a cantilevered-column model, linear buckling analysis would produce buckling factors independent from applied lateral load. This is because, in this particular case, lateral load does not affect the geometric stiffness of the structure.

The structural softening which occurs under lateral-load application may still be captured. However, nonlinear analysis must be run with P-Delta and Large Displacement effect. Lateral load may then be plotted against lateral displacement. Initially, this relationship will be linear, but then the structure will begin softening at a certain load. Response of a column with slight initial perturbation is shown in Figure 1:


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Figure 1 - Nonlinear buckling evaluation

Reporting

How should internal forces and reactions be interpreted for buckling analysis?

Extended Question: When performing buckling analysis for a cantilevered column, buckling mode shapes result in proportion to applied loading. When an axial load is applied to the column, the resultant axial force is zero for every mode shape, though moment and shear reactions are present. Could you please explain?

ANSWER: The internal forces and reactions reported for buckling load cases correspond to the buckled configuration of the structure. For a vertical cantilever model, the structure buckles laterally, generating internal moments and shears without axial force.


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  • Incident 48668: simple buckling analysis problem


Why am I getting a negative-stiffness error during P-Delta analysis?

ANSWER: Negative stiffness occurs during P-Delta analysis when structural objects buckle under second-order P-Delta effects. To avoid negative stiffness and buckling, object size should be increased, especially at columns and diagonals.


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Related Incidents:

  • Incident 47138: Ill condition structure