Bending Moment & Neutral Axis: Why Not Centroid?

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Discussion Overview

The discussion revolves around the concept of the neutral axis in beams subjected to bending moments, specifically questioning why the neutral axis does not coincide with the centroid of the beam's cross-section when maximum bending occurs. The scope includes theoretical aspects of structural mechanics and the implications of bending in I-beams.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant suggests that the neutral axis does not pass through the centroid due to the bending of the beams when subjected to maximum bending moments.
  • Another participant expresses confusion about the initial question and requests clarification, indicating a need for visual aids.
  • A participant points out that the distance of the neutral axis from the centroid is related to strain, implying that if the neutral axis were at the centroid, there would be no strain and thus no bending.
  • Another participant challenges the previous claim, stating that the relationship between strain and the centroid is misunderstood, emphasizing that strain pertains to elongation while the centroid is a geometric property.

Areas of Agreement / Disagreement

Participants exhibit disagreement regarding the relationship between the neutral axis, strain, and the centroid, with conflicting interpretations of the underlying mechanics involved.

Contextual Notes

There are unresolved assumptions regarding the definitions of strain and the neutral axis, as well as the implications of bending in structural elements. The discussion does not clarify these concepts fully.

knight92
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Hey guys, as the title says I can understand why the neutral axis does not pass through the centroid after reaching a point where the bending moment is max before the beam collapses. Say you have a straight structure supported by two double I-Beams and a weight in the middle. Why does the neutral axis not pass through the centroid, is it because the beams are simply bending ??

Cheers
 
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can't understand *
 


Your reference to "double I-beams" is not entirely clear. A picture would really help get your question across.
 


The strain is the distance of neutral axis from the line through centroid. If the neutral axis were at centroid, it would mean no strain and presumably, no bending.
 


"The strain is the distance of neutral axis from the line through centroid."

This statement is simply not true at all, and represents total confusion. It must be completely discounted. Strain describes the elongation of a fiber, while the centroid is a geometric property of an area.
 

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