For a simply supported T-beam in reinforced concrete design, what is the maximum permissible ratio of effective span to overall depth that is typically used for preliminary sizing to control deflection?

Introduction / Context:
In reinforced concrete beam and T-beam design, span/depth limits are used at the preliminary stage to keep service deflections within acceptable limits without doing full deflection calculations. For simply supported members, the basic limit is the most restrictive because end fixity is low and midspan deflections are relatively larger.

Given Data / Assumptions:

• Member behaves as a simply supported T-beam under service loads.
• Basic span/depth ratio is sought for preliminary sizing (before modification factors for tension steel, compression steel, and finishes).
• Normal exposure and common material grades.

Concept / Approach:
The span/depth ratio is a serviceability control. Codes and standard practice provide “basic” ratios for different support conditions. For simply supported members, the basic ratio is smallest (more conservative) because the support restraint is minimal. The T-beam flange enhances compression capacity but does not typically change the basic starting ratio for deflection control at the sizing stage; refinement comes later with modification factors and detailed checks.

Step-by-Step Solution:
Identify the support condition: simply supported.Adopt the basic span/depth ratio for preliminary sizing = 20.After preliminary sizing, apply modification factors as permitted (based on percentage of steel, stress levels, compression reinforcement, and finishes) to fine-tune the depth.Carry out a detailed deflection check at the final design stage if the member is sensitive (e.g., brittle finishes or long spans).

Verification / Alternative check:
Compare with other support conditions: continuous beams often use a higher basic ratio (less restrictive), while cantilevers use a much lower ratio. This confirms that 20 is the correct baseline for simply supported members.

Why Other Options Are Wrong:

• 10 and 15 are overly conservative and lead to uneconomical depths.
• 25 and 30 are more appropriate to stiffer, continuous systems, risking excessive deflection if used for simple spans.

Common Pitfalls:
Confusing beam and slab limits, forgetting to apply modification factors, or skipping a final deflection check when spans are long or finishes are crack-sensitive.

Final Answer:
20.