Short Answer:

Punching shear in footings is a type of failure that happens when a concentrated load from a column tries to push through the footing slab, creating a punch-like effect around the column. This failure appears as a cone-shaped crack around the loaded area and is very dangerous because it occurs suddenly without warning.

Punching shear usually happens in flat slabs or isolated footings under heavy loads. To prevent this, the footing must be properly thickened or reinforced around the column area to safely resist the vertical force and avoid sudden failure.

Detailed Explanation:

Punching shear in footings

In structural engineering, punching shear is a critical failure mode that affects slabs or footings carrying heavy point loads from columns. Unlike regular bending or flexural failures, punching shear is a local shear failure that happens when a column punches through the slab or footing, breaking away a portion of it like a hole made by a punch.

This type of shear is especially important in the design of isolated footings, flat slabs, and raft foundations, where concentrated loads are applied over a relatively small area. If not properly addressed in design, punching shear can lead to sudden collapse, making it one of the most serious forms of structural failure.

How Punching Shear Occurs

When a vertical load from a column is transferred to a slab or footing, the load is spread into the concrete. If the footing thickness is insufficient or the concrete fails to resist the shear force around the column perimeter, a conical or pyramidal section breaks out around the column base.

This failure does not give warning signs like visible cracks or deflection before collapse. Therefore, special attention is given to punching shear checks in structural design.

The critical section for checking punching shear is located at a distance d/2 (where “d” is the effective depth) from the face of the column, measured all around the column perimeter.

Factors Affecting Punching Shear

1. Column load: Heavier column loads increase the shear force at the footing base.
2. Footing thickness: Thinner footings have less shear strength and are more vulnerable.
3. Column size: Smaller columns concentrate the load more, increasing shear stress.
4. Concrete strength: Higher concrete grades resist punching better.
5. Reinforcement detailing: Shear reinforcement (like stirrups or shear heads) helps resist the punching effect.

How to Prevent Punching Shear

• Increase footing thickness: Thicker footings provide more concrete area to resist shear.
• Enlarge column base: Increases contact area and reduces shear stress.
• Add shear reinforcement: Install stirrups, dowels, or shear heads around the column.
• Use higher-grade concrete: Stronger concrete increases shear capacity.
• Distribute loads: Try to reduce heavy point loads by adjusting layout or using combined footings.

In design, punching shear is checked using formulas given in IS 456:2000, where the shear strength is calculated and compared with the actual applied shear force. If the applied force exceeds capacity, the footing must be redesigned.

Conclusion:

Punching shear in footings is a sudden and serious failure caused by heavy column loads acting on a small slab or footing area. It forms a punching cone around the column and can lead to collapse if not checked. Proper footing thickness, reinforcement, and design checks are essential to prevent this. Safe structural design must always include punching shear verification for all column-supported slabs or footings.