Span-to-depth limit for two-way simply supported slabs: What is the maximum basic ratio of span to effective depth commonly adopted for slabs that are simply supported and span in two directions under normal service conditions?

Introduction / Context:
Control of deflection is essential in slab design. Codes and design guides specify span-to-depth limits as a quick serviceability check prior to detailed deflection calculations. Two-way slabs supported on all four edges typically permit a higher span/depth ratio than one-way slabs due to two-directional stiffness.

Given Data / Assumptions:

• Simply supported slab spanning in two directions.
• Normal serviceability, typical finishes and imposed loads.
• Basic (unmodified) ratio prior to applying adjustment factors for tension steel, drop panels, etc.

Concept / Approach:
Empirical limits (basic l/d ratios) are provided for preliminary sizing. For two-way simply supported slabs, a basic limit of about 35 is widely used. Adjustments may be applied later based on percentage of tension reinforcement, concrete grade, and other parameters.

Step-by-Step Solution:
Identify slab system: two-way action on simple supports.Adopt the standard basic ratio: l/d ≈ 35.Apply modification factors in detailed design if needed; ensure deflections are within service limits.

Verification / Alternative check:
Design examples confirm that two-way systems can span longer per unit depth than one-way due to bidirectional stiffness. The figure of 35 is commonly referenced as a starting point.

Why Other Options Are Wrong:

• 25 or 30: conservative for two-way slabs; more typical for beams/one-way slabs.
• 40: may be excessive without favorable modification factors.
• 15: unrealistically low for standard RC slabs.

Common Pitfalls:
Assuming basic limits are final; always check deflection especially with long spans, high service loads, or low reinforcement ratios.

Final Answer:
35