About Beams and Cross-Section Data
A beam is a slender structure that can be fully described by cross-section properties like area and moments of inertia. In COMSOL Multiphysics, there are two physics interfaces for analyzing beam structures, one in 2D and one in 3D. There are also two similar Pipe Mechanics interfaces, targeted at piping systems.
Approaches for Obtaining the Cross-Section Data
The values for the cross-section properties can be found in different ways. By:
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looking up tabulated values,
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inserting dimensions into handbook formulas,
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using the built-in common sections in The Beam Interface, and
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using The Beam Cross Section Interface.
For the common built-in cross sections available with the Beam interface (Rectangular, Box, Circular, Pipe, H-section, U-section, and T-section), using the Common sections option in the Cross-Section Data settings provides you with the most efficient input alternative. See Common Cross Sections for details.
For nonstandard cross sections with nontrivial shapes, using the Beam Cross Section interface is the only realistic alternative. This physics interface also provides you with more accurate data than the other methods, since it does not rely on common engineering approximations.
The cross-section data obtained from the different approaches can, in general, differ somewhat. Two common sources for this difference is that the influence of fillets in the geometry can be excluded, and that approximations such as assuming thin walled sections can be used.
Stress Computation
Since a beam interface is only aware of the cross-section properties and not of the true stress distribution over the cross section, the stresses computed there can only be approximate. The approximations used by COMSOL are conservative in the sense that all stress contributions are assumed to interact in the worst possible way.
In the Beam Cross Section interface, the true stress distribution can be computed given a set of section forces. This can be important in cases where the stresses obtained in the Beam interface or the Pipe Mechanics interface exceed allowed values. Also, from a pedagogical point of view, it is an advantage to be able to display the full stress distribution.
With the Beam Cross Section interface in 2D, you can plot the stress distribution in a certain cross section. In 3D, the beam and its results can be plotted using a solid representation.