In the notes , I dont understand that flange is thin , and the top and bottom surface are free of stress , can someone help to explain please?
If you are at the face of an element, there cannot be any internal shear stress, because there is nothing to balance it out. The bottom horizontal force I added cannot be balanced by another force since there is no more material at the face of the flange. The thinner your element, the less it will resist in shear and more it will resist in bending.
If you recall from practicing finding Vy when you make cuts along a beam, you have a shear force (Vy) on each side of the cut, each opposite and equal to the other. Now imagine cutting nothing off the surface of a body. What is the shear force of the surface? Nothing! Why? Because the shear force of the nothing you cut off is...nothing. Equal and opposite, remember?