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specifying cross section moment of area for truss buckling

Posted 2 févr. 2020, 09:18 UTC−5 Structural & Acoustics, Structural Mechanics Version 5.4 2 Replies

Christine Buffinton
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I am teaching a course in finite element analysis in a mechanical engineering department. It is mostly theoretical, but I am using COMSOL for lab exercises. We are starting with truss analysis, not such a common application in COMSOL but class is just up to 1D theory now. Question: in linear buckling analysis for a truss structure in 2D, how can I input I, the cross section 2nd moment of area? Cross-sectional area can be specified, but what does COMSOL use to calculate buckling factor? I cannot find anywhere to input I and no information in documentation. Thanks for any help.

2 Replies Last Post 3 févr. 2020, 09:12 UTC−5
Henrik Sönnerlind COMSOL Employee

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Posted: 5 years ago 3 févr. 2020, 03:58 UTC−5

Hi Christine,

The buckling analysis considers buckling of the structure as a whole, not local buckling of the individual members. The individual members are treated as straight.

If you want to investigate local buckling, you can write an expression. The utilization of a member can for example be written as

-truss.Nxl/(pi^2*truss.E*moment_of_inertia/h^2)

'h' is the built-in variable for element length, so it is assumed that the default mesh with one element per physical bar is used.

You would have to define moment_of_inertia as a variable defined with the same selections as the area itself. For circular cross sections, you can use (truss.area)^2/(4*pi) as moment of inertia.

Note that the Euler 2 buckling expression used above contains some assumptions about how the end of the bars are constrained.

Anyway, thanks for the comment. We will add some built-in variables for this in the next release.

Regards,
Henrik

-------------------
Henrik Sönnerlind
COMSOL
Hi Christine, The buckling analysis considers buckling of the structure as a whole, not local buckling of the individual members. The individual members are treated as straight. If you want to investigate local buckling, you can write an expression. The utilization of a member can for example be written as -truss.Nxl/(pi^2\*truss.E\*moment_of_inertia/h^2) 'h' is the built-in variable for element length, so it is assumed that the default mesh with one element per physical bar is used. You would have to define moment_of_inertia as a variable defined with the same selections as the area itself. For circular cross sections, you can use (truss.area)^2/(4\*pi) as moment of inertia. Note that the Euler 2 buckling expression used above contains some assumptions about how the end of the bars are constrained. Anyway, thanks for the comment. We will add some built-in variables for this in the next release. Regards, Henrik
Christine Buffinton
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Posted: 5 years ago 3 févr. 2020, 09:12 UTC−5

Wonderful, thank you very much!! I should have thought of that, but I was convinced that COMSOL had an internal parameter used in the evaluation. We (my class) will define the 2nd moment of area/"moment of inertia" as a parameter, and then make a plot group using, as you did, the ratio of the negative of axial force to the pinned-pinned Euler buckling load. Then the plot will show the multiple (positive number) of exceeding Euler buckling load, and negative numbers will not exceed the buckling load. We can also include a factor of safety in the expression. Thanks again, Christine

Wonderful, thank you very much!! I should have thought of that, but I was convinced that COMSOL had an internal parameter used in the evaluation. We (my class) will define the 2nd moment of area/"moment of inertia" as a parameter, and then make a plot group using, as you did, the ratio of the negative of axial force to the pinned-pinned Euler buckling load. Then the plot will show the multiple (positive number) of exceeding Euler buckling load, and negative numbers will not exceed the buckling load. We can also include a factor of safety in the expression. Thanks again, Christine

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