**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 }*= 1.15

*V*;

_{t }·
Edge/Corner
Columns: *V _{eff }*= 1.4

*V*.

_{t }1. Check maximum shear at column face:

where *u*_{0 }is the Perimeter of the column.

2. Shear
stress at the critical section, 1.5*d *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 *£ 2*v _{c }*: Link
reinforcement may be used.

*v *> 2*v _{c }*: Alternative a Proven system to be used.

For preliminary
design, it is sufficient to pass Step 1 and to know that *v *£ 2*v _{c
}*at the

critical perimeter.

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

*v _{c
}*= 0.65 N/mm

^{2}

^{ }

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