Reinforced Concrete – Effective depth of a shallow beam: Identify the correct definition used for flexural design and detailing.
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ADistance between maximum compression and maximum tension fibres
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BDistance from neutral axis to maximum compression fibre
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CDistance from neutral axis to maximum tension fibre
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DDistance from maximum compression fibre to the centroid of tension reinforcement
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ENone of these
Answer
Correct Answer: Distance from maximum compression fibre to the centroid of tension reinforcement
Explanation
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:
1) Locate maximum compression fibre → top surface under sagging.2) Locate centroid of main tension steel → typically at bar centerline below neutral axis.3) Measure vertical distance between these two → this is the effective depth d.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