Question

PRINCIPLE AND WORKING OF I-SHAPED GRIDERS

Answer 1

Principal
This follows from Newton's laws: the forces must sum to zero for the beam to remain in equilibrium, and so an internal moment is generated in the beam, with the top half of the beam going into compression and the bottom going into tension.

Working
I beams have very high moment of inertia  for the same volume of the given material.  So they have high stability in case of bending moments.  The two horizontal parts (called flanges) of the I beam can bear high bending and shearing stress.That means they do not get twisted and tilted easily.   That is why they are used in girders, and as rails on the railway tracks.