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From the solution , it's clear that the top part of the beam undergo tension , while the lower part undergo compression .

But , i dont understand why it is so . When the moment is applied in the horizontal axis , the structure will slide become like this , right ? ( the top part will be slided into the book ) I cant imagine how the moment applied causing the top part to experience tension , while the lower part experience compression .

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  • $\begingroup$ I think that perhaps you are misunderstanding the problem statement. The moment is acting about an axis that is perpendicular to the longitudinal axis of the beam (that is to say, about the orange line in your image above). The hand sketch you included seems to indicate that the applied moment is a torsion of some sort. $\endgroup$ – William S. Godfrey- S.E. Dec 2 '16 at 15:38
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You are correct that the given bending moment will create compression on top and tension in the bottom.

What I don't understand is where you see the book state that the tension is on top. If you look at the calculation of the centroid $\overline y$, you'll see it is 3.4" from the top fiber. If you then look at the calculation of the stresses, you'll see that the $c$ value used in $\sigma_t$ is $(10.5-3.40)$. It is therefore equal to the distance from the centroid to the bottom fiber, meaning $\sigma_t$ is on the bottom fiber, as would be expected. For $\sigma_c$, $c = 3.40$, which means $\sigma_c$ is found on the top fiber.

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