Proportioning an R.C.C. beam section: A common preliminary thumb rule keeps the ratio of breadth b to effective depth d approximately equal to what value?

Introduction / Context:
At the preliminary stage of reinforced concrete beam design, dimensional ratios are used to obtain a practical section before detailed checks. The breadth b to effective depth d ratio affects shear capacity, bar placement, and deflection control. Many textbooks provide a simple starting ratio for ordinary beams carrying typical floor loads.

Given Data / Assumptions:

• Ordinary reinforced concrete beam, simply supported or continuous.
• Serviceability and constructability considerations typical of building work.
• Working-stress or limit-state preliminary sizing intent.

Concept / Approach:

Keeping b about one quarter of d (b/d ≈ 0.25) often balances shear web width, bar congestion, and economy, while allowing sufficient depth to control deflections. Final proportions must be verified by bending, shear, deflection, and detailing checks, but 0.25 is a standard rule-of-thumb starting point in many courses and older handbooks.

Step-by-Step Solution:

Verification / Alternative check:

Trial designs using b/d = 0.25 commonly meet shear and reinforcement spacing requirements; adjustments are made if serviceability or bar spacing dictates a wider web.

Why Other Options Are Wrong:

0.50, 0.70, and 0.75 produce excessively wide beams for a given depth in typical building floors, which may be uneconomical or create beam-slab conflicts. “None of these” is incorrect because 0.25 is the widely taught starting value.

Common Pitfalls:

Freezing preliminary ratios without later verification; ignoring minimum web width for bar placement and shear links; not coordinating with architectural beam widths.

Final Answer:

0.25