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For this question, I am having problem for part b. We know that the bending stress $$ \sigma = My / I $$ For part (b), the author only stated he wanted the bending stress at a section 2 m from A, but he didnt specify at which point of the beam (cross-sectional view)? So, how to do this question?

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  • $\begingroup$ Read the question: it asks for the maximum bending stress in that section. $\endgroup$ – Wasabi♦ Nov 5 '16 at 17:48
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As you've said, $$ \sigma = My / I $$

At a particular location (i.e. 2m from A), M and I are constant. The question asks for maximum bending stress at that location, so you need maximum y.

So, you need to work out where the neutral axis is, and whether top face or bottom face is furthest from it, and use y for that face.

(Maximum bending stress is very likely to mean maximum magnitude of bending stress, so while one face will give you a positive y and the other a negative y, just use the one with the largest magnitude. Keep the sign in order to determine whether it is compressive or tensile stress.)

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  • $\begingroup$ so , the centid that i found is 346mm away from the bottom , so , bending stress = (110x10^3)(346x10^-3 ) / moment of inertia ? $\endgroup$ – kelvinmacks Nov 8 '16 at 4:38
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    $\begingroup$ @kelvinmacks - Almost, except the question isn't worded too clearly and you've made a small mistake: it wants maximum stress from the moment at 2m from A, not the maximum stress anywhere on the beam (which would be from the maximum moment). $\endgroup$ – AndyT Nov 8 '16 at 9:27
  • $\begingroup$ I couldn't find my mistake.... What should be the correct one ? $\endgroup$ – kelvinmacks Nov 8 '16 at 10:06
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    $\begingroup$ You put "110x10^3" into your equation. From a quick glance at your workings in your question, this is the maximum moment. As I said in my previous comment, the question doesn't want you to use the maximum moment; it wants you to use the moment at 2m from A. $\endgroup$ – AndyT Nov 8 '16 at 10:23
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    $\begingroup$ @kelvinmacks - to calculate the maximum bending stress at 2m from A (which is what the question is asking for), you need the bending moment at 2m from A. The bending stress is equal to My/I, where y is the distance from the neutral axis to the point under consideration. The maximum bending stress at a location is hence made by maximising y. $\endgroup$ – AndyT Dec 1 '16 at 12:01
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You have all of the necessary information.

Look at the BM diagram to determine the magnitude and direction of the moment. The question is asking for the maximum stress in the section. (Hint S=0 at NA).

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  • $\begingroup$ How could bending stress = maximum at the neutral axis ? Bending stress has maximum value on the most upper part and the lowest part , right ? the most upper part is the compression stress , while the lowest part undergo tension stress? $\endgroup$ – kelvinmacks Nov 6 '16 at 0:03
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    $\begingroup$ This doesn't answer the question. Giving hints is not what this site is about. $\endgroup$ – AndyT Nov 7 '16 at 14:25
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    $\begingroup$ Yes it does. The "hint" I added was that the Stress = 0 at the Neutral Axis $\endgroup$ – Donald Gibson Nov 10 '16 at 21:25
  • $\begingroup$ we know that beding stress $$\sigma = My / I $$ , so to determine the max bending moment 2 m from A , we should take the maximum value of M from the BMD or take the value of moment which is 2m from A ? $\endgroup$ – kelvinmacks Dec 1 '16 at 8:00
  • $\begingroup$ @DonaldGibson - "You have all of the necessary information" does not answer "he didnt specify at which point of the beam (cross-sectional view)? So, how to do this question?". The only question which has an answer "You have all of the necessary information" is "Do I have all of the necessary information?". $\endgroup$ – AndyT Dec 1 '16 at 12:40

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