Two-way slabs with corners held down (IS 456 design strips): According to IS 456, a slab spanning in two directions with corners held down is divided, in each direction, into a middle strip and two edge strips. What is the width of the middle strip in each direction, expressed as a fraction of the slab breadth in that direction?

Introduction / Context:
For two-way slabs, bending moments are distributed between a central “middle strip” and adjacent “edge strips.” IS 456 provides conventional strip widths to simplify distribution of design moments and reinforcement, especially when corners are held down to prevent uplift.

Given Data / Assumptions:

• Two-way slab.
• Corners held down.
• Use of IS 456 strip method.

Concept / Approach:
IS 456 divides the slab, in each direction, into a central middle strip of three-fourths of the panel width and two edge strips (one on each side) making up the remaining one-fourth (i.e., one-eighth each). This aids moment partitioning and bar placing without a full elastic analysis.

Step-by-Step Reasoning:
In any one direction, total width = 1.00.Middle strip width = 3/4 of the panel width.Edge strips = 1/8 on each side (total 1/4).

Verification / Alternative check:
Using these conventional widths, the detailing becomes straightforward: heavier steel in the middle strip (where moments are higher) and reduced reinforcement in edge strips, consistent with two-way action.

Why Other Options Are Wrong:
Half, two-third, four-fifth, three-fifth: Not the standard fractions used in IS 456 for corners-held-down slabs.

Common Pitfalls:

• Confusing corners “held down” vs “free to lift,” which affects moment coefficients and detailing.
• Misallocating steel between middle and edge strips.

Final Answer:
three-fourth of the width of the slab