Reinforced Concrete – Effective depth of a shallow beam: Identify the correct definition used for flexural design and detailing.

Introduction:
Effective depth is a key geometric parameter in flexural design. It directly influences the lever arm, moment capacity, and deflection predictions in reinforced concrete beams.

Given Data / Assumptions:

• Shallow, singly reinforced beam.
• Tension steel placed near the soffit (bottom) with adequate cover.
• Compression at the top fibre under sagging moments.

Concept / Approach:

By definition, effective depth d is measured from the most compressed fibre to the centroid of the tension reinforcement. This standard definition is used in design equations for ultimate and serviceability checks.

Step-by-Step Solution:

Verification / Alternative check:

Section analysis formulas for moment of resistance use d, not overall depth or NA distance, confirming the definition's role.

Why Other Options Are Wrong:

A and C mix up fibres or use NA; B uses NA which is not the definition; E is incorrect since a standard definition exists.

Common Pitfalls:

Confusing overall depth with effective depth, or measuring to bar surface instead of its centroid.

Final Answer:

Distance from maximum compression fibre to the centroid of tension reinforcement