Preliminary proportioning — for an initial estimate during beam design, a reasonable starting assumption for beam width (breadth) as a fraction of span is:

Introduction / Context:
Preliminary proportioning offers quick dimensions before detailed analysis and code checks. For singly reinforced beams, rules of thumb relate span to depth and width for economical, serviceable sections. While the effective depth is driven by deflection and bending, the width must accommodate bars, cover, spacing, and shear stirrups without congestion.

Given Data / Assumptions:

• Singly reinforced, simply supported beam as a starting point.
• Normal reinforcement ratios and standard bar sizes.
• Objective: pick a reasonable width-to-span rule of thumb.

Concept / Approach:

Common heuristics: effective depth d ≈ L/12 to L/18 (depending on load/deflection criteria), and width b is often taken between L/25 and L/30 as a starting estimate so that detailing remains practical and the beam does not become excessively narrow. Among the presented options, 1/30 of span is a widely acceptable starting value.

Step-by-Step Solution:

Verification / Alternative check (if short method exists):

Check that chosen b allows required stirrup legs and bar layering. Adjust during final design to satisfy shear and serviceability checks.

Why Other Options Are Wrong:

1/15 and 1/20 often lead to overly wide beams; 1/40 may be too narrow for practical reinforcement layout; 1/25 is acceptable in many cases but 1/30 is a more conservative starting point among the given choices.

Common Pitfalls (misconceptions, mistakes):

Locking into preliminary dimensions without later checks; neglecting architectural or formwork limitations.

Final Answer:

1/30 of span