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

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sanji02

sanji02

Structural
Oct 7, 2010
8
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.
 
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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
 
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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.
 
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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.
 
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@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?
 
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Sanji02 said:
Did a bit Googling, look like only EC2 requires minimum shear reo in two-way?
Click to expand...

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
 
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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!

 
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0! = 1 [ponder]

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

-Dik
 
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