In a singly reinforced concrete beam, the effective depth d is measured from which reference to which location?

Introduction / Context:
Effective depth is a key geometric parameter in flexural design because it determines the internal lever arm between compression and tension resultants. Accurate d directly affects moment capacity calculations.

Given Data / Assumptions:

• Singly reinforced rectangular or flanged section.
• Compression at the top (compression edge).
• Tension steel placed near the soffit.

Concept / Approach:
By definition, effective depth d is the distance from the extreme compression fibre (compression edge) to the centroid of the tensile reinforcement. It is not measured to the neutral axis or to the free tension edge; what matters is the actual steel centroid since that is where the tensile force acts.

Step-by-Step Solution:
Locate compression edge (usually top surface for positive bending).Locate the centroid of tension steel (distance from soffit equals cover + 0.5bar diameter for a single layer).Measure d between these two points.

Verification / Alternative check:
Design charts and code equations for moment resistance use lever arm z ≈ jd, where d is as defined above. Any mis-measurement leads to incorrect steel area estimates.

Why Other Options Are Wrong:

• Tensile edge: Not necessarily coincident with steel centroid.
• Neutral axis: Moves with loading/material; not the definition of d.
• Longitudinal central axis: A geometric convenience, not relevant for resisting couple.

Common Pitfalls:
Ignoring multiple steel layers (centroid shifts); failing to account for bar diameter and clear cover in computing d.

Final Answer:
tensile reinforcement (centroid of tension steel)