Deflection control criterion: For a simply supported reinforced concrete beam under normal conditions, the span-to-overall-depth ratio should not exceed what value to satisfy basic stiffness limits?

Introduction / Context:
Serviceability (deflection) control is essential in beam design to prevent cracking, ponding, and functional issues. Many codes present basic span-to-depth ratios as starting limits for stiffness, which may be modified for tension steel percentage and stress levels. This question recalls the basic limit for simply supported beams in traditional teaching practice.

Given Data / Assumptions:

• Simply supported beam.
• Normal service conditions and materials.
• Overall depth D used in the ratio; basic (unmodified) limit considered.

Concept / Approach:

A common baseline limit is L/D ≤ 20 for simply supported beams. Continuous beams often allow higher limits due to continuity (e.g., 26–28 baseline), while cantilevers are more restrictive (e.g., 7–10). Modifications are later applied based on steel stress and compression reinforcement, but the unmodified benchmark is 20 for simply supported members.

Step-by-Step Solution:

Verification / Alternative check:

Elastic deflection calculations with typical spans and reinforcement confirm that L/D around 20 is a practical starting point to meet serviceability in ordinary floors.

Why Other Options Are Wrong:

10 and 15 are overly conservative; 25 or 30 are too slender for a baseline limit without favorable modification factors and may lead to excessive deflection.

Common Pitfalls:

Confusing effective depth d with overall depth D in the ratio; applying continuous-beam limits to simply supported beams; skipping modification checks at high service stresses.

Final Answer:

20