Design of Slabs MCQ with answer and solution

Concept

Yield line theory needs ductile (under-reinforced) slabs so that plastic hinges (yield lines) can form and rotate fully.

Step-by-step solution

1. Yield line theory is a plastic (upper bound) method for slabs. 2. It assumes steel yields along lines and the slab rotates about them like plastic hinges. 3. Large rotation is possible only if steel yields before concrete crushes. 4. This happens only in an under-reinforced section. 5. So the slab must be under reinforced. Answer is (C).

Explanation

Yield lines are lines where the steel has yielded and the slab can rotate without breaking. Steel yields first only in an under-reinforced slab, so it gives enough ductility for the full yield line pattern to form. Over-reinforced or balanced slabs fail suddenly by concrete crushing.

Why the other options are wrong

A: Over-reinforced slab fails by sudden concrete crushing, with no ductile rotation. B: Shear reinforcement does not control flexural yield lines, and slabs usually do not need it. D: A balanced section has steel yielding and concrete crushing at the same time, so there is not enough rotation capacity.

Shortcut

Any plastic or yield method needs ductility, and ductility means under-reinforced.

Common mistake

Choosing balanced reinforced, thinking it is the most economical design.