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

σ_{bt }or σ_{bc }=
0.66 f_{y}
| (Clause: 6.2.1) |

where

σ_{bt} | = | Bending stress in tension |

σ_{bc} | = | Bending stress in compression |

f_{y} | = | 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)

${\sigma}_{bc}=0.66\frac{{f}_{cb}{f}_{y}}{{[{\left({f}_{cb}\right)}^{n}+{\left({f}_{y}\right)}^{n}]}^{1/n}}$ | (Clause 6.2.3) |

where

f_{y } | = | Yield stress of steel, in Mpa |

n | = | A factor assumed as 1.4. |

f_{cb} | = | 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 | = | $\frac{26.5{\left(10\right)}^{5}}{{(1/{r}_{y})}^{2}}$ |

k_{1} | = | 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. |

k_{2} | = | 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 |

r_{y}
| = | 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 |

c_{1} ,c_{2} | = | respectively the lesser and greater distances from the section neutral axis
to the extreme fibres. |