http://en.wikipedia.org/wiki/Bending Please have a look on the right most picture in "Stress in large bending deformation". The Stress distribution shows that the bending center is not locate at the middle of the cross-section. (which also happen at connection caculation) However, base on normal caculation on a beam under bending, the equation used(Stress=M*y/Ixx) is still assuming zero value at crosssection's center line. I wonder what is their relationship. Thank you very much in advence.