What is Punching Shear? | Punching Shear Design | LCETED

WHAT IS PUNCHING SHEAR?

The column rested on the footing tends to punch through the footing  due to the shear stress that acts around the footing, the fracture forms a  truncated pyramid-shaped failure section.

 

Punching Shear Design

The design to prevent punching shear failure proceeds as:

1. Check if the concrete is strong enough alone; 

2. If not, check if the amount of reinforcement is reasonable; 

3. Design reinforcement if reasonable, if not, change the form of structure.  

Changing the form of structure includes deepening the slab,  making the column larger, introducing drop panels or flared column heads. There is also the possibility to adapt foreign codes of practice that are more liberal! 

The reinforcement put in is usually vertical and traverses the potential failure line. Of course,  we  don’t  know  where  the  failure  plane  might  be,  so  we  must  reinforce  each  possible failure plane as shown: failure  plane  might  be,  so  we  must  reinforce  each  possible failure plane as shown:

Note that bars may be common to two failure planes and that as we move away from the column the (shaded) area over which the load is spread increases and so the stress reduces. Eventually, we reach a point where the concrete is sufficiently strong alone.

Preliminary Design

The column reaction, V_t, is modified as follows to take account of moment transfer:

·       Internal Columns: V_eff = 1.15V_t ;

·       Edge/Corner Columns: V_eff = 1.4V_t .

 

1. Check maximum shear at column face:

where u₀ is the Perimeter of the column.

2. Shear stress at the critical section, 1.5d from the face of the column:

 

 

where a and b are the plan dimensions of a rectangular column and m is the  perimeter multiplier of d: in this case, m = 1.5 . If:

v £ v_c : No shear reinforcement required.

v £ 2v_c : Link reinforcement may be used.

v > 2v_c : Alternative a Proven system to be used.

 

For preliminary design, it is sufficient to pass Step 1 and to know that v £ 2v_c at the

critical perimeter.

For preliminary purposes for the design of flat slabs buildings in 40N concrete, take:

v_c  = 0.65 N/mm²

 

 

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