Stair design: For stairs spanning horizontally (waist slab spanning between supports), what is the minimum waist (minimum structural thickness along the slope) generally provided for safe and serviceable construction?

Introduction / Context:
The waist of a stair slab is the minimum structural thickness measured perpendicular to the soffit along the slope. Providing an adequate minimum waist ensures strength, stiffness, durability, and sufficient cover to reinforcement for stairs spanning horizontally between supports.

Given Data / Assumptions:

• Reinforced concrete stair flight spanning horizontally.
• Conventional residential or institutional usage with standard loads.
• Minimum practical thickness targets based on common code-aligned practice.

Concept / Approach:
Practical design balances structural capacity with constructability. Too small a waist can lead to inadequate cover, difficulty in placing bars, excessive deflection, and vibration. Common practice adopts a minimum waist of about 80 mm for general stairs; higher values (100–120 mm) are used for longer spans, heavier loads, or vibration control.

Step-by-Step Solution:
Identify the design goal: safe, serviceable minimum thickness.Review typical code guidance and practice: minimum waist about 8 cm for standard spans and loads.Select the smallest value satisfying cover, bar placement, and stiffness requirements: 8 cm.

Verification / Alternative check:
Serviceability checks on deflection and vibration for typical tread-riser geometry indicate 8–10 cm waist is adequate for common spans; 8 cm is widely accepted as a baseline minimum.

Why Other Options Are Wrong:

• 4 cm, 6 cm: Insufficient for cover and stiffness; impractical to reinforce.
• 10 cm, 12 cm: Safe but not the minimum; chosen when spans/loads demand extra stiffness.

Common Pitfalls:
Ignoring cover requirements and bar diameters; using too thin a waist can compromise durability and cause cracking.

Final Answer:
8 cm