Question

The neutral axis of the reinforced beam passes through ____

• A. Centre of gravity of the concrete section
• B. Meta-centre of the concrete section
• C. Centroid of the transformed section
• D. Centroid of the concrete section

Hint:

For RCC sections, \[ \boxed{ \text{Neutral Axis} \rightarrow \text{Centroid of the transformed section} } \] where steel is converted into equivalent concrete using the modular ratio.

Answer 1

The Correct Option is C

Step 1: recognise the problem as one of an equivalent homogeneous section.
Concrete and steel have different stiffness, so in working stress design we convert the steel area into an equivalent concrete area using the modular ratio $m = \dfrac{E_s}{E_c}$, giving one single transformed cross section made entirely of an equivalent material.
Step 2: apply ordinary bending theory to that transformed section.
Once the section is transformed, plain bending theory says the neutral axis is the axis about which the first moment of area is zero, which by definition is the centroid of that section.
Step 3: state where the neutral axis actually sits.
Since the transformed section already accounts for both concrete and the equivalent steel area, the neutral axis passes through its centroid, not through the centroid of the plain concrete area alone.
\[ \boxed{\text{Centroid of the transformed section (option C)}} \]