7.3 One-Way, Two-Way, Punching, and Interface Shear Effects

Shear describes distinct structural actions. A wide slab can fail across a one-way section, punch around a column, or slip along a construction joint. The geometry and units reveal which model applies. For July 2026, use the April 2024 PE Civil: Structural specification, the current PE Civil Reference Handbook, and ACI 318-14. Do not use the April 2027 references.

Begin With the Failure Surface

One-way shear is beam action. A crack and resisting section extend across member width, and demand is commonly expressed with effective width b_w and effective depth d. For common nonprestressed ACI 318-14 cases with loads applied so compression develops toward the support, the critical section can be located a distance d from the face of support. The code conditions and exceptions matter: concentrated loads near the support, tension-side support conditions, brackets, deep members, or unusual geometry can require evaluation at the face or another model.

Two-way or punching shear surrounds a concentrated load or reaction. An interior column can push a truncated punching surface through a slab or footing. Under the common ACI 318-14 model, the basic critical perimeter b_0 is placed d/2 from the column, pedestal, capital, or loaded area and is shaped to minimize the perimeter. Edges and corners shorten it; nearby openings can reduce the effective perimeter under the governing rules. Drop panels or capitals change geometry and may require additional perimeters.

Interface shear acts along a defined plane, such as a construction joint between placements, a precast-to-cast-in-place interface, or a corbel interface. Transfer may rely on shear-friction reinforcement crossing and anchored on both sides of the plane, aggregate interlock, cohesion or friction permitted for the prepared surface, and other mechanisms recognized by the controlling provision. Surface condition and reinforcement detailing are design inputs, not afterthoughts.

One-Way Demand Workflow

1. Draw the support face, effective depth d, applied loads, and reaction.
2. Locate the code critical section for the stated conditions.
3. Cut the free body and calculate factored shear V_u crossing that section. Loads between the support and section affect that demand according to the controlling rule.
4. If a stress comparison is required, compute v_u = V_u/(b_w d).
5. Compare total force with total capacity or stress with stress capacity—never cross the formats.

For a 12 in design strip with d = 8 in and factored shear V_u = 18.0 kips at the applicable one-way critical section,

v_u = 18,000 lb/[(12 in)(8 in)] = 187.5 psi

A capacity reported in kips for that strip can be compared directly with 18.0 kips. A capacity reported in psi must be compared with 187.5 psi. A capacity in kips per foot requires converting either demand or capacity to the same width basis.

Worked Punching Calculation

An interior 16 in square column supports a slab with effective depth d = 8 in. The problem gives factored punching shear V_u = 180 kips at the basic critical perimeter and states that no opening, edge, drop panel, or capital modifies the perimeter. At d/2 from each column face, the perimeter has side length c + d = 16 + 8 = 24 in. Therefore,

b_0 = 4(c + d) = 4(24) = 96 in

v_u = V_u/(b_0 d)

v_u = 180,000 lb/[(96 in)(8 in)] = 234.4 psi

The 180 kip total punching force and the 234.4 psi average stress are two representations of the same demand. They are not interchangeable without multiplying or dividing by b_0d. The one-way value of 187.5 psi above came from a different critical section and stated force; the lower number does not eliminate the punching check.

For a footing, the punching free body commonly uses the column load minus upward soil pressure acting inside the critical perimeter. One-way footing shear uses a straight section across the footing and a different enclosed soil area. Drawing the correct free body prevents subtracting the wrong pressure region.

Interface Shear and Shear Friction

At a construction interface, first calculate the shear that must cross that plane from equilibrium. Then apply the ACI 318-14 interface or shear-friction provision required by the problem. Reinforcement intended to provide clamping must cross the plane and develop on both sides. The coefficient or permitted mechanism depends on whether the surface is monolithic, intentionally roughened, clean but not roughened, or otherwise classified by the code. Strength-reduction factors, material limits, and maximum nominal shear limits still apply. Do not take friction credit from an unlabeled joint.

Units are a frequent trap. If 40 kips must transfer uniformly across a 48 in joint, the average line demand is

q_u = 40/48 = 0.833 kip/in = 833 lb/in

A supplied design resistance of 0.90 kip/in would exceed that line demand, while a supplied resistance of 0.90 ksi is a stress and cannot be compared until the effective interface area or thickness converts the demand to stress. Likewise, a reinforcement calculation producing total nominal force must receive the applicable strength-reduction treatment before comparison with factored V_u.

Keep the Checks Separate but Coordinated

A slab-column region can require one-way shear, punching shear, flexure, moment transfer, and detailing checks at the same time. Unbalanced slab moment can change punching stress around the perimeter; the simple uniform V_u/(b_0d) calculation is not a complete treatment when moment transfer is specified. A beam cast in stages can require both ordinary beam shear across a vertical section and interface transfer along a horizontal joint.

On exam day, label each sketch one-way, punching, or interface; mark the critical section before calculating V; and write its units beside both demand and capacity. Confirm whether values are nominal, factored, allowable, per unit width, or total. Most category errors become obvious when the failure surface and units are visible.