Stress-strain response

[Heady]

Hi,
I tried to get the stress across a section of a beam using element recorder ( stress-strain response ). The beam is a rectangular one. So, the C.G. is at midpoint along the depth. Hence , at the midpoint along the depth of the section the stress should be zero. But, in the output it is not coming zero. Why is it so ?

[fmk]

if 0 loads or if steel and pure bending the stress will be 0 .. otherwise NO.

[Heady]

Thanks.
The stress-strain outputs changes with the number of subdivisions ( fibers ) in the section.
How can I know at what number of subdivisions ( fibers ) the outputs will be correct ?