Effect of increasing steel percentage in a singly reinforced concrete section: how does the depth of the neutral axis and the lever arm respond as steel percentage rises (keeping section size similar)?

Introduction / Context:
The depth of the neutral axis (NA) in a reinforced concrete flexural section shifts with the relative proportions of concrete compression and steel tension. Understanding how NA moves as steel percentage changes is vital for service stress calculations, curvature, ductility expectations, and ultimate strength behavior.

Given Data / Assumptions:

• Rectangular, singly reinforced section under positive bending.
• Concrete in tension is neglected in service calculations.
• Linear strain distribution (plane sections remain plane).

Concept / Approach:
When the percentage of steel increases, the tensile force capacity on the tension side increases. To maintain force equilibrium, a larger compressive force in the concrete is required. This is achieved by deepening the compression block, which means the neutral axis shifts downward (measured from the compression face), i.e., the depth of NA increases. With greater NA depth, the internal lever arm z between resultant compression and tension forces tends to reduce slightly for a given overall depth, affecting moment capacity and curvature.

Step-by-Step Solution:
Increase Ast (tension steel area).Equilibrium requires C = T; with higher T, the compression block must grow, pushing NA deeper.Result: neutral axis depth increases; lever arm generally decreases modestly because the compression resultant moves farther from the tension steel.

Verification / Alternative check:
Transformed-section equations show n increases as m*Ast increases; typical design charts also reflect a deeper NA with higher reinforcement ratios.

Why Other Options Are Wrong:

• Decrease in NA depth contradicts equilibrium for higher steel tension.
• Increase in lever arm is not typical; with deeper NA, z tends to reduce slightly.
• “None of these” is incorrect because the correct trend is known.

Common Pitfalls:
Assuming lever arm always increases with more steel; misinterpreting ultimate stress block behavior as service behavior; neglecting ductility trade-offs with very high steel ratios.

Final Answer:
Depth of neutral axis increases.