Where a flat slab bears directly on a column with no beams in between, the failure mode to fear is the column punching a truncated cone out of the slab. ACI 318 checks this two-way shear on a control perimeter b0 drawn at d/2 from every column face, where d is the effective slab depth — an interior 400 × 400 mm column with d = 200 mm gets b0 = 2(600) + 2(600) = 2400 mm, while edge and corner columns lose sides of the perimeter and with them capacity.
The demand is expressed as a stress, vu = Vu/(b0·d), and compared against φvc with the shear reduction factor φ = 0.75. The concrete stress limit vc is the least of three expressions: a baseline 0.33√fc, a reduction for elongated columns through the aspect ratio β, and a reduction for large perimeters through αs·d/b0, with αs = 40, 30, or 20 for interior, edge, and corner positions. In the worked case the baseline governs at 1.808 MPa, giving a 651 kN capacity against a 400 kN demand — 61% utilisation.
The concentric check is the floor, not the ceiling, of the problem: unbalanced moment transferred between slab and column raises the peak stress through a γv factor, and shear studs or stirrups can add capacity — both effects sit outside the basic concrete check and demand a more detailed analysis.