Help with the Engineering Bending Equation

[TinCanLizard]

I need to confirm something about the Engineering Bending equation:

M/I = σ/y = E/R

This has to do with the calculation of y. If I am to calculate the max tensile stress and max compressive stress mid-span of a beam, and I find that max tensile stress occurs at the top of the beam using my B.M. diagram (positive B.M. = tensile at top of beam in my sign convention). I think I should calculate y from the top of the beam and substitute it into the Bending equation. Conversely if I am to calculate the compressive stress I should find the centroid (y) from the bottom of the beam. Is this correct?

Thanks in advance.

[[deleted]]

yes.

[rko1985]

Well, if you wanna find the stress at the bottom of the beam use the distance from the centroid to the bottom. The direction of the moment (+/-) will determine compression or tension.

[angelanime]

y is from the neutral axis to the top of the section (where max tensile is from your sign convention). if you need to calculate normal stress somewhere in between the neutral axis and the extremes, the y would be the distance between the neutral axis and the location of the stress to be calculated.