Flexural design basis — the quantity of main reinforcement in a slab should primarily be based on which structural demand?

Introduction / Context:
In reinforced concrete slabs, the principal steel (main reinforcement) is placed in the direction of the largest bending moments to resist tensile stresses. Correctly identifying the governing internal action ensures adequate strength and serviceability without uneconomical over-reinforcement.

Given Data / Assumptions:

• Slab is designed primarily for flexure in one or two directions depending on support and aspect ratio.
• Shear and torsion checks are also required but usually do not dictate the main bar quantity in typical floor slabs.
• Question asks what governs the amount of main reinforcement.

Concept / Approach:

Determination of main steel area As is based on the design bending moment envelope. Codes also require minimum temperature and shrinkage reinforcement orthogonal to main bars, but the main bar area itself must satisfy the maximum design flexural demand to meet ultimate limit state requirements.

Step-by-Step Solution:

Verification / Alternative check (if short method exists):

Check that As also exceeds minimum crack-control steel; verify deflection and crack width criteria. Shear check is performed, but in ordinary slabs shear rarely controls bar quantity (shear is often satisfied without shear reinforcement).

Why Other Options Are Wrong:

“Minimum bending moment” would under-design; “maximum shear” or “minimum shear” do not set main bar quantity for flexure; torsion may affect edge strips or corners but does not generally dictate the main bar quantity across the slab panel.

Common Pitfalls (misconceptions, mistakes):

Confusing distribution (secondary) steel with main steel; ignoring local peak moments near openings or discontinuities.

Final Answer:

maximum bending moment