Z Reference

Hi

I entered the original reference page and I would have answered your query earlier but my Broadband went down yet again!

I am sure you know the equation well neglect the right hand fraction and re-arrange the other two so:-

If you look at this equation you can see that for any given cross section y mas a maximum value as it is the distance from the Neutral Axis to the "outside" of the section. Likewise I is a Constant for that section being bent in that plane. Hence I can re-write the above equation as:-

Actually it might be better to write this as:-

M in this equation is the Maximum Bending moment that can be carried by the given section for a given maximum Stress. It is called the Moment of Resistance

Now look at your Post of the 20Th Nov.

Y is actually the distance from the neutral axis to where ever you want to know the stress. Of course this is usually the "outside" of the section because that is where the stress is a maximum. You ask whether you should measure upwards or downwards. Well that depends upon what you want. On a simply supported beam subjected to a downwards load the upper surfaces goes into Compression and the lower Tension and the important one depends upon the relative maximum compressive and Tensile stresses.

You asked about a rectangular section and the neutral axis is central. Had it been a "T" cross section the neutral axis would no longer be central and its position would need to be calculated.

In the particular example you gave you are correct. I hope that I have sorted out your problems.