Beam Bending, beyond the basics, a little

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Discussion Overview

The discussion revolves around beam bending, specifically addressing the challenges of calculating moments of inertia (MOI) for various cross-sectional shapes beyond simple rectangles, such as channels and tubes. Participants seek clarification on methodologies and resources for these calculations.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Homework-related

Main Points Raised

  • One participant expresses confusion about calculating MOI for non-rectangular cross sections, such as channels, and requests assistance or resources.
  • Another participant questions what other factors are being changed aside from the cross-sectional MOI.
  • A participant states that for simple beam bending calculations, calculating the MOI for the section is sufficient, noting that the same beam equations apply across different shapes, though stress distributions may vary.
  • Mark acknowledges a misunderstanding in his approach to calculating MOI for a tube, indicating he was overthinking the process by trying to calculate MOI for each side separately.
  • A participant confirms Mark's approach is close to correct and provides a link to a resource for calculating MOI for common cross-sections, suggesting that formulas for these properties are already available for shapes like I-beams.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the best approach for calculating MOI for complex shapes, and there are multiple viewpoints regarding the methodology and resources available.

Contextual Notes

Limitations include potential missing assumptions about the specific conditions under which the beam bending equations apply, as well as the dependence on the definitions of cross-sectional properties.

Who May Find This Useful

This discussion may be useful for students or professionals dealing with structural engineering, particularly those interested in beam bending and the calculations involved with various cross-sectional shapes.

mechdesign2k4
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Ok, I am fairly comfortable with beam bending equations for simple rectangles. However, I am totally confused on how to approach other cross sections... like a channel can someone help? Or point me to a good online instructional resource? I have googled all I can think of...


-Mark
 
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What else are you changing other than the cross sectional MOI?
 
For simple beam bending calculations, all you have to do is calculate the MOI for the section. Whether its an I-beam or a circular tube, all of the beam equations are the same (although some stress distributions may change).
 
Thanks! I must have been over thinking it. I was sort of approaching it as taking each side of the tube, and top and bottom, calculating MOI for each...and adding them together. Thanks for the help!

Great forum!

-Mark
 
mechdesign2k4 said:
Thanks! I must have been over thinking it. I was sort of approaching it as taking each side of the tube, and top and bottom, calculating MOI for each...and adding them together. Thanks for the help!

That's pretty close to how you would calculate the MOI, see here: http://en.wikipedia.org/wiki/Second_moment_of_area

...but for common cross-sections like an I-beam, formulas for it's section properties have already been calculated in detail, you just have to find them. Here is one resource: http://www.efunda.com/math/areas/Common_Cross_Section_Index.cfm
 
Thanks again!
 

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