Consider a thin walled I section beam. At some intermediate c/s the shear force is V. The shear stress direction in flanges and web will be as shown.
The textbook says that, the vertical shear stresses in the flange (as shown in green) can be neglected and
"This is because the flange is thin, and the top and bottom surfaces of the flange are free of stress"
How having zero stresses at the top and bottom surfaces of flange result in negligible vertical shear in flange?