Is the Bending Diagram Accurate for this Beam?

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    Bending Diagram
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Discussion Overview

The discussion revolves around the accuracy of a bending diagram for a beam in a homework problem, focusing on the calculations of bending moments and stresses. Participants are examining the validity of the provided solutions and the assumptions made in the calculations.

Discussion Character

  • Homework-related
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant notes a discrepancy between their calculated maximum stress and the book's answer, questioning the accuracy of the bending diagram.
  • Another participant asserts that the bending moment (BM) diagram should be zero at both ends for a simply supported beam, suggesting an error in the calculations presented.
  • A third participant emphasizes the importance of verifying reaction forces and moments to ensure the correctness of the calculations, indicating that the moment diagram may be incorrect.
  • One participant claims their instructor confirmed the correctness of their reaction force calculation, although they acknowledge a possible minor error in another calculation.
  • Another participant challenges the accuracy of the instructor's assessment and suggests checking the centroids of the applied loads and reactions, indicating a potential inconsistency in the drawing.
  • There is a request for clarification on the definition of bending moment being used, highlighting the complexity of the topic.

Areas of Agreement / Disagreement

Participants express differing views on the correctness of the calculations and the instructor's input, indicating that multiple competing perspectives remain unresolved.

Contextual Notes

Participants mention the need for clarity on definitions and the importance of checking calculations, but specific assumptions and definitions remain unspecified.

togo
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Homework Statement


11uer2p.jpg



Homework Equations


moment = force x distance


The Attempt at a Solution


the book answer is different and doesn't show bending diagram. Can't tell what it should look like. Answer for maximum stress in book is way off:

m=60 k*ft, s=17.1 in^3, stress = m/s = 60000 lb*ft/38.1in^3 = 18900 psi

thanks
 
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It can't be right, because the BM diagram should be 0 at both ends if the beam is simply supported.

Your first line
M = -(25K)(6) + (12)(fb) ... etc
looks right. But on the next line you forgot the - sign and said
150 + 12 fb ...
not
-150 + 12 fb ...

After that, it looks like you did the right things, but the numbers are wrong.
 
As soon as you got the reaction force (incorrectly in this case) you should have checked it by concluding what the other reaction is AND taking moments about any point other than A, to check the correctness of your calculations so far. As you didn't do this, the rest is not valid. As your moment diagram is not correct, I would ask you to state which definition of bending moment you are using (there are two possible, each one derivable from the other).
 
Fine, but I checked the reactions with instructor and he said it was correct (Fb)

Calculation for Fa may be off by .5
 
Well, either your instructor is wrong or there is a mistake in your drawing. Incidentally, what is the length of the right hand unloaded length of the beam? So here's another check you can do. The centroid of the applied loads should coincide with the centroid of the reactions. (If they didn't coincide, what would be the implication?) Can you calculate those two positions? In your case they don't, even with a 0.5 variation. I asked you for the definition of bending moment for a very good reason, but you didn't answer my question. Why was that?
 

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