Flat slab design — location of the critical section for two-way (punching) shear (Measure distance from the periphery of column or drop panel; d = effective depth.)

Introduction:
Punching shear is a critical limit state in flat slabs around column supports. The correct position of the critical (control) perimeter directly affects the calculated shear stress and the need for shear reinforcement or thickening (drop panels).

Given Data / Assumptions:

• Flat slab supported by columns or drop panels.
• d denotes effective depth of the slab.
• We are evaluating the location of the critical section for two-way (punching) shear around a support.

Concept / Approach:
For two-way shear in flat slabs, the code defines a control perimeter located at a set distance from the column or drop panel edge. This perimeter captures the shear flow where diagonal tension cracking is most likely to form due to concentrated column reaction punching through the slab.

Step-by-Step Solution:
1) Identify support type: interior column/drop panel.2) For two-way shear, the critical section is taken at a uniform offset from the support periphery.3) Standard practice places this perimeter at distance d/2 from the column/drop panel edge.4) Use this perimeter length to compute punching shear stress v_u = V_u / (b_0 * d), where b_0 is the critical perimeter.

Verification / Alternative check:
Design examples for flat slabs consistently adopt d/2 offset. One-way shear (beam shear) is checked separately at distance d from the face; do not confuse the two checks.

Why Other Options Are Wrong:
Option B (d) corresponds to one-way shear, not punching. Option C (0) underestimates critical perimeter length. Option D (1.5 d) is unconservative and not code-based for punching shear.

Common Pitfalls:
Mixing one-way and two-way shear locations; forgetting to exclude openings within the perimeter; ignoring column capitals/drop panels when drawing the periphery.

Final Answer:
At a distance d/2 from the periphery of the column or drop panel.