ACI 318-19
ACI 318-19 Section 22.6 | Two-Way Shear (Punching)
Punching shear is the tendency for a heavily loaded column to punch entirely through a concrete slab or footing. The resisting perimeter is located at a distance of d/2 from the column face.
Governing Formula
Variables
Nominal shear stress provided by concrete
psi or MPa
Ratio of long to short side of column
unitless
Constant based on column location (40 interior, 30 edge, 20 corner)
unitless
Perimeter of critical section
in or mm
Quick Example
1
Find critical perimeter
Calculate b0 for a 12x12 interior column with effective depth d = 8". The critical section is 16x16.
2
Calculate limiting stresses
Compute the three limits. The standard 4√(f'c) usually governs for square, interior columns.
3
Check total capacity
Multiply the governing stress by the perimeter and depth.
When does this govern?
Punching shear is almost universally the governing design criterion for flat plate floor slabs without beams and isolated spread footings.
Common Mistake
Using the wrong αs value for columns that are near an edge but act like an interior column because the edge distance is further than the d/2 critical section.