# Flexural stress calculation

1. Jul 19, 2012

### Aragorn49

From what I know the equation for calculating flexural stress of a beam with a rectangular cross section is:

3PL/2Bd2

I have two questions about the validity of this equation. I will post a pic I just made to show what I mean:

1st question, Does this equation work both for filled beams (A,) and dull beams (B,)?

2nd question, Does this equation work both for when the force is applied in the exact center of the beam (C,) and for when the force is applied to the edge of the middle area (D,)?

2. Jul 19, 2012

### PhanthomJay

The equation you listed applies only to a solid horizontal rectangular beam of negligible weight and of length L and pin supported at each end, with a concentrated load P applied at its midpoint. You don't want to blindly apply equations. In general, the flexural stress in beam at a given point is My/I.

3. Jul 19, 2012

### Aragorn49

Thank you for the help. But what does My/I mean? What quantities do the letters of that equation represent?

4. Jul 19, 2012

### PhanthomJay

I believe you posted on the wrong forum, since this does not appear to be a homework problem. M is the bending moment, y is the distance from the neutral axis, and I is the Area Moment of Inertia. For the beam in discussion, the max moment M occurs at mid point = PL/4, the moment of inertia I is bd^3/12, and the max stress at this section occurs at the outer fibers where y = d/2. Sound at all familiar?

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5. Jul 19, 2012

### Aragorn49

I apologize if I posted on the wrong forum, I'm new here. It isn't exactly a homework problem, more like going a bit ahead of the stuff I learn right now. After I finished highschool I had a few free years and now I want to get my physics knowledge back in shape cause I want to go studying structural engineering on a university. I forgot much of my highschool knowledge so I'm pretty much relearning it right now, though sometimes I get the diesire to learn some more advanced stuff which really interests me.

Btw, where is the section where questions like these should be posted?