what the formular for finding the maximum bending stress of a I beam?

Answer 1

Colin's answer S=Mc/I is the general stress formula for any shape. The Moment depends on the type of beam and the type of load as well as its shape.
The shearing stress, the bending Moment and bending stress must be computed and there are deflection formulas listed for the various kinds of beam supports and types of load, such a simply supported, cantilever, etc., single concentrated load, continuous load, etc.
There are engineering tables showing the c and I values for the common structural shapes. Another , often tabulated factor, Z (Section Modulus) is I/c so that S=M/Z

Answer 2

SIGMA = (mc)/I

SIGMA= MAX BENDING STRESS
M=MOMENT
C= DISTANCE FROM NEUTRAL AXIS TO EDGE OF BEAM
I = MOMENT OF INERITIA