RCC Structures Design MCQ Questions & Answers | Civil Engineering

In RCC stair design, the waist is the minimum thickness of the inclined slab measured perpendicular to the soffit of the stair.

For stairs spanning horizontally, the slab behaves like a reinforced concrete slab and must have sufficient thickness to safely resist bending moments and shear forces.

As per standard RCC design practice, a minimum waist thickness of 12 cm is required to ensure adequate strength, stiffness, and durability.

Providing a waist thickness less than this may lead to excessive deflection and structural inadequacy.

Therefore, the correct answer is Option D – 12 cm.

Reason: As per IS 456:2000, the basic span-to-effective depth ratio for a simply supported slab spanning in two directions is 35 when mild steel reinforcement (Fe 250) is used.

For HYSD bars: The value is multiplied by a modification factor of 0.8, giving 0.8 × 35 = 28.

Purpose: This limit controls deflection and ensures adequate stiffness of the slab under service loads.

Understanding Doubly Reinforced Beams and Neutral Axis
Let's break down this question step-by-step to understand what it's asking.

Key Concepts:
* Doubly Reinforced Beam: This is a beam that has steel reinforcement in both the tension zone (bottom) and compression zone (top).
* Neutral Axis (NA): This is the line within the beam where there is neither tensile nor compressive stress. It's the point where the stress changes from compression to tension.
* Actual Neutral Axis: The actual position of the neutral axis in a real beam under load. It's determined by the equilibrium of forces.
* Critical Neutral Axis: This is a theoretical position of the neutral axis. It's the depth where the concrete and steel reach their maximum allowable stresses simultaneously.
Maximum Stress:
Think of 'maximum stress' as the point where the material (concrete or steel) starts to yield or fail.
Explanation of Options:
* Option A: "Is greater than the depth of critical neutral axis, the concrete attains its maximum stress earlier"
* If the actual neutral axis is *deeper* than the critical neutral axis, it means the compression zone is larger.
* A larger compression zone implies the concrete is experiencing higher compressive stresses.
* Therefore, the concrete reaches its maximum stress (and potentially fails) before the steel reaches its maximum tensile stress.

* Option B: "Is less than the depth of critical neutral axis, the steel in the tensile zone attains its maximum stress earlier"
* If the actual neutral axis is *shallower* than the critical neutral axis, it means the tension zone is larger.
* A larger tension zone means the steel is experiencing higher tensile stresses.
* Therefore, the steel reaches its maximum stress (and potentially yields) before the concrete reaches its maximum compressive stress.

* Option C: "Is equal to the depth of critical neutral axis; the concrete and steel attain their maximum stresses simultaneously"
* This is the definition of the *critical* neutral axis.
* When the actual neutral axis matches the critical neutral axis, both the concrete and steel reach their maximum allowable stresses at the *same time*. This is the ideal balanced design.

* Option D: All the above
* Since options A, B, and C are all correct under their respective conditions, this is the correct answer.

Therefore, the correct answer is D: All the above.