Obtaining compressive and tensile stresses in beams

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SUMMARY

The discussion focuses on determining compressive and tensile stresses in beams using the equations σ = My/I and σ = -My/I. It is established that compression is always represented as a negative value, while tension is positive. Participants clarify that when calculating maximum stress, the positive form of the equation should be used, and the negative sign is added only at the end for compressive stress calculations. This approach helps avoid confusion in stress analysis.

PREREQUISITES
  • Understanding of beam mechanics
  • Familiarity with stress and strain concepts
  • Knowledge of the moment of inertia (I)
  • Basic proficiency in using equations of equilibrium
NEXT STEPS
  • Study the derivation of the bending stress formula σ = My/I
  • Learn about the significance of the moment of inertia in beam design
  • Explore examples of calculating maximum tensile and compressive stresses in beams
  • Investigate the effects of different loading conditions on beam stresses
USEFUL FOR

Engineering students, structural engineers, and professionals involved in beam design and analysis will benefit from this discussion, particularly those seeking clarity on stress calculations in structural applications.

hashman
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Homework Statement



In calculating stresses, how do we determine when we have a negative in front of the question and when we do not have it. I am really confused, meaning I have not understood the concept, but the examples I am looking at some have got σ= My/I whilst others have σ= -My/I.

Homework Equations



σ= My/I

σ= -My/I

Hopefully they get uploaded properly. The first two use the first equation, whilst the the other two use the second equation

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Untitled1.png


Untitled2.png


Untitled3.png



Thanks for the help.
 
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Just understand that:
  • Compression is always Negative
  • Tension is always Positive

I always just keep my numbers positive in calculation until the end and if I am solving for a compressive stress I add the negative.

Hope this helps!
 
LabGuy330 said:
Just understand that:
  • Compression is always Negative
  • Tension is always Positive

I always just keep my numbers positive in calculation until the end and if I am solving for a compressive stress I add the negative.

Hope this helps!

Ah right okay. So basically if they ask for max or anything always use σ=My/I , and then if it is compression, add the negative sign to the final answer. Cheers mate
 
Yup you got it!
 

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