In IS 456 design for reinforced concrete beams, the span-to-depth ratio limits are specified primarily to ensure which serviceability criterion is respected?

Introduction / Context:
Serviceability governs how a structure performs in use—particularly deflections and crack widths. IS 456 specifies span/depth (L/d) limits for beams to control excessive long-term and short-term deflections without detailed calculations, ensuring occupant comfort, finish integrity, and line-of-sight performance in floors and roofs.

Given Data / Assumptions:

• Reinforced concrete beam spans and depths are design variables.
• Code provides L/d limits with modification factors for tension steel percentage and compression reinforcement.
• We are asked for the primary reason for these limits.

Concept / Approach:

The code’s basic L/d limits act as a prescriptive deflection control. While crack width has separate detailing provisions, and shear failure is addressed by shear design, the span/depth limits directly address the risk of excessive deflection under service loads, including creep and shrinkage effects.

Step-by-Step Solution:

Verification / Alternative check:

IS 456 tables list basic L/d values (e.g., 20 for simply supported beams, etc.) modified for reinforcement percentages, confirming a deflection-control intent.

Why Other Options Are Wrong:

• Crack width is managed via bar diameter/spacing and tensile stress limits.
• Shear failure is prevented by shear design (Vc + Vs), not by L/d limits.
• Allowable stresses relate to limit state checks, not prescriptive L/d limits.

Common Pitfalls:

• Ignoring long-term deflection from creep and shrinkage when near the limit.

Final Answer:

deflection of the beam is below a limiting value