The Neutral Axis (NA) is the axis in the beam's cross-section where the bending stress is zero during flexure.

The neutral axis lies between the compression and tension zones — it divides the section into compressive and tensile regions.

In a balanced section, the NA is at such a depth that concrete reaches its permissible stress in compression and steel in tension simultaneously.

In under-reinforced sections, steel yields before concrete crushes — so the NA shifts downward (closer to the tension zone).

In over-reinforced sections, concrete fails before steel — hence the NA shifts upward toward the compression zone.

The position of the NA helps in determining whether the section is under-reinforced, balanced, or over-reinforced and aids in stress analysis.

In WSM, the NA is found using the equilibrium of compressive and tensile moments of the transformed section (moment of areas).

Higher strength concrete can carry more compression, which shifts the NA deeper into the section (higher depth).

Depth of NA < critical means the steel yields before concrete — this is an under-reinforced, ductile section.

Neutral axis depth is key in determining the internal lever arm, which directly affects the moment of resistance of the section.