Span-to-overall-depth limit for one-way slabs: For a simply supported slab spanning in one direction, what is the basic maximum span/depth ratio used for preliminary design (before modification factors)?

Introduction / Context:
Span-to-depth limits provide a rapid serviceability check to control deflections without full calculation. For one-way slabs, codes give basic ratios depending on support conditions that are later adjusted by modification factors for tension steel and compression reinforcement.

Given Data / Assumptions:

• One-way slab behavior, simply supported at ends.
• Basic ratio sought prior to applying modification factors.
• Normal reinforcement and concrete grades assumed.

Concept / Approach:
Simply supported slabs are the most flexible among common support cases and therefore have the most restrictive (lowest) basic span/depth ratio. Adopting the basic value streamlines sizing, with further refinement via percentage steel, stress levels, and finishes.

Step-by-Step Solution:
Identify slab as simply supported, one-way.Adopt the basic limit span/depth = 20 for preliminary sizing.Apply modification factors later if needed to raise or lower this limit based on reinforcement and stress.

Verification / Alternative check:
Comparing continuous slabs (often 26) or cantilevers (often 7) confirms that simply supported cases fall at 20, aligning with common code tables.

Why Other Options Are Wrong:

• Higher values such as 25, 30, or 35 correspond to stiffer support or modified conditions and risk excessive deflection for simple supports.
• 15 is overly conservative and uneconomical in many ordinary cases.

Common Pitfalls:
Using beam limits for slabs, ignoring modification factors, or not checking detailed deflection when finishes/partitions are sensitive.

Final Answer:
20.