Steel beam theory is used to find the approximate value of the moment of resistance of a doubly reinforced beam specially when the area of compression steel is equal to or more than the area of the tensile steel.

## Steel beam theory

### moment of resistance of a doubly reinforced beam

The moment of resistance of a doubly reinforced beam consists of :

(i)        Moment of resistance of compression concrete and the corresponding tensile steel (Ast1) i.e., moment of resistance of balanced section (M1).

(ii)       Moment of Resistance M’ of the compression steel (Asc) and the additional tensile steel (Ast2).

In the steel beam theory :

(i)        Concrete is completely neglected.

(ii)       The moment of resistance is taken equal to the amount of the couple of compressive and tensile steel.

(iii)     The permissible stress in compressive steel is taken as equal to the permissible stress in tensile steel.

$\therefore M _{r}=\sigma _{st}.A _{st}(d-d’)$

The Transformed or equivalent section as per steel beam theory is shown in Fig. 2.10.