Dear stress senior people,
pls see enclosed file,
i have seen it somewhere,is it right or not?
regards
jozm(old ozm)

Evaluation of the governing equations.

Bending Stress - M / Z where Z = I / c
I - Moment of Interia = pi/64.(do^2-di^2)
c - distance from the neutral axis (centre line of pipe) to the point of maximum bending stress (OD / 2)

Now, section modulus is a measure of a 'pipes' resistance to bending. Higher section modulus makes a 'pipe' more stiffer. (increases Z), and for the same applied Moment M, results in lower bending stresses.

Pluging values into your equation and studying interms of a constant 'M', you will realize the relation

Quite right John.

A pipe is actually a poor section in terms of bending resistance, a fact that we use to introduce flexibility in some cases. It's the distance of 'stuff' from the neutral axis that is is important, max OD/2 of course for a pipe. Compare that to where most of the material is located in an I-beam, lots of it as far from the neutral axis as possible, thus great resitance to bending.

Cheers.