The allowable bending stress in a member subjected to bending is calculated based on the following formula.

σbt  or σbc  =  0.66 fy

(Clause: 6.2.1)

where

σbt	=	Bending stress in tension
σbc	=	Bending stress in compression
fy	=	Yield stress of steel, in MPa

For an I-beam or channel with equal flanges bent about the axis of maximum strength (z-z axis), the maximum bending compressive stress on the extreme fibre calculated on the effective section shall not exceed the values of maximum permissible bending compressive stress. The maximum permissible bending compressive stress shall be obtained by the following formula: (Clause: 6.2.2)

where

fy	=	Yield stress of steel, in Mpa
n	=	A factor assumed as 1.4.
fcb	=	Elastic critical stress in bending, calculated by the following formula: $=k_1(X+k_{2}Y)\frac{c_2}{c_1}$
X	=	$Y\sqrt{1+\frac{1}{20}\frac{{\pi }^2}{r_{y}D}}$ in MPa
Y	=	$\frac{26.5{\left(10\right)}^5}{{(1/r_y)}^2}$
k1	=	a coefficient to allow for reduction in thickness or breadth of flanges between points of effective lateral restraint and depends on y, the ratio of the total area of both flanges at the point of least bending moment to the corresponding area at the point of greatest bending moment between such points of restraint.
k2	=	a coefficient to allow for the inequality of flanges, and depends on w, the ratio of the moment of inertia of the compression flange alone to that of the sum of the moment of the flanges each calculated about its own axis parallel to the y-yaxis of the girder, at the point of maximum bending moment.
l	=	effective length of compression flange
ry	=	radius of gyration of the section about its axis of minimum strength (y-y axis)
T	=	mean thickness of the compression flange, is equal to the area of horizontal portion of flange divided by width.
D	=	overall depth of beam
c1 ,c2	=	respectively the lesser and greater distances from the section neutral axis to the extreme fibres.