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# Two way thick slab (>750mm) and minimum shear

## Two way thick slab (>750mm) and minimum shear

(OP)
This is somewhat a continuation of discussions under:

- Requirements For Transverse Shear Reinforcement - Minimum Shear Reinforcement
and
- Size effect factor in thick slabs

The latest AS3600-2018 amendment 2 clarified that minimum shear reo is require for one-way slab or beam when depth >750mm. This requirement is addressing size effect as I understand? I read somewhere experiments have shown deep slab, the top half may separate from bottom half during experiments (sorry for my crude description, I can't find the info) thus shear capacity was different from theoretical.

Why is it then two-way thick slab, say >750mm like deep beam/one-way slab, does not need minimum shear reo to account size effect? And another point, the D>=750 for one-way and beam is a geometry requirement, where minimum shear reo is required irrespective of the shear capacity vs design shear force.

### RE: Two way thick slab (>750mm) and minimum shear

Can the slab be 739mm? I often use conc beams that are 29-1/2" deep because at 30" they require intermediate reinforcing.

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

### RE: Two way thick slab (>750mm) and minimum shear

I believe even a two-way slab at 750 mm thick would require the minimum shear reinforcement. The wording (from memory) is that if the slab can fail across a substantial width then it should be treated as a beam. If the slab is supported along beams at the edges then I think this would require it.

### RE: Two way thick slab (>750mm) and minimum shear

The logic I think was that a 2way slab has a lot of redistribution capacity due to the 2way action in the case of a shear crack forming.

Whereas there is no redistribution capacity in a 1 way slab or a beam.

I think the initial version of AS3600-2018 or the Draft actually had the 750 limit for all members but we were able to get 2way slabs removed from the limitation in one of the Amendments.

### RE: Two way thick slab (>750mm) and minimum shear

(OP)
@rapt:
yes, the AS3600-2018 amendment 2 has made it clear the minimum shear is for 1-way slab and beam with depth greater than 750mm.

@gusmurr:
I agree, but the current As3600-2018 is saying minimum shear reo is not a requirement.

Beside design & construction issues, I thought one of the outcome from De la Concorde overpass collapse is minimum shear reo for two way?

Did a bit Googling, look like only EC2 requires minimum shear reo in two-way?

### RE: Two way thick slab (>750mm) and minimum shear

#### Quote (Sanji02)

Did a bit Googling, look like only EC2 requires minimum shear reo in two-way?

From Eurocode 2, 6.2.1(4):
The minimum shear reinforcement may be omitted in members such as slabs (solid, ribbed or
hollow core slabs) where transverse redistribution of loads is possible.

Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/

### RE: Two way thick slab (>750mm) and minimum shear

I am amazed at how people think they can read and interpret a code via googling!

Have you even thought of the construction consequences of minimum shear reinforcement in any slab with depth > 750mm independent of loading.

The new AS3600 rule for minimum shear reinforcement requirements increasing with depth mean that in a 600 or deeper flat slab, minimum shear reinforcement will be required in areas where V* > .5Vuc, same as for any beam, instead of the old rule based on V* > Vuc.

You want to change it to any value V*, even V* = 0!

### RE: Two way thick slab (>750mm) and minimum shear

0! = 1

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

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