The beams are supported by supports at one end or more. When the beams are subjected to transverse loads bending stress is induced in the beam due to the bending moment. Copy the link given below and paste it in new browser window to get more information on Assumptions in Simple Bending Theory:- http://www.transtutors.com/homework-help/mechanical-engineering/bending-stress/assumptions-in-simple-bending-theory.aspx