punching shear

[happayday]

We have 9" flat cast-in-place concrete slab with concrete columns. Because the parking garage in the first floor and the basement, the column spacing has to go 29' apart. According to ACI, the minimum slab thickness should be l/30, which is about 12", but client doesn't want to use that much thickness slab, how I we solve the problem without failed punching shear?

 

[JedClampett]

You're going to have to clarify. Is the slab in place or are you designing it now?
To reduce the depth, you could add drop panels and edge beams. Or possibly a prestressed system might reduce the depth. But I would strongly advise not to violate the code provision on minimum thickness. You're likely to have problems and the client will conveniently forget what he requested.

 

[WillisV]

I believe you misreading the code - the l/30 value for minimum slab thickness only applies if you do not check deflections. If deflections are checked versus table 9.5(b) of ACI-02 then the slab can be as thin as 5" per 9.5.3.2 (a) no matter what the l/ value works out to be.

 

[happayday]

The design is just starting. The client is really not happy increase the thickness of the slab and suggested shearheads. Is that going to work?
Acturally the deflection is fine but the puching shear around the column fail after we considered the unbanlance moment.

 

[WillisV]

If punching shear is controlling than you can either add drop panels or use shearheads to help.

 

[JAE]

Or concrete capitals.

 

[Taro]

Or stud rails.

And you could post-tension the slabs to get them as thin as possible without having deflection problems.

 

[happayday]

Acturally the architect doesn't like drop panel or concrete capital idea cause he doesn't want to use ceiling to cover it. Now it leaves the shearheads. I'm not familiar with that, how much it can help, if we use shear head, then the column spacing can go 29.5' with 9" concrete slab? Is any site or books/papers regarding it can be suggested?

 

[happayday]

stud rails? Could you provide some information about it?
thanks

 

[Taro]

Headed studs (sometimes known by the trade name Nelson studs) are welded to a strip of steel plate to form shear reinforcing that is placed around the column. It is much easier to place than shearheads. I believe they were originally patented by Decon in Canada (

http://www.constud.ca/decon.ca/)

but their patent has expired and other manufacturers like Nelson now offer competing products.

I have done a lot of similar projects with 30' spans where 8" post-tensioned slabs with studrail reinforcing turned out to be the most economical solution.

 

[jen4950]

29' is very manageable with post-tensioning- envision around a 7"-9" PT 2-way flat plate. Of course you need to run your own numbers and ask for help if you are unfamiliar with PT.

If you have problems with punching shear, there are several products like Decon Studrails or Lenton Steel Fortress that can really help you out. Need to watch out for penetrations within 10 times the slab thickness IIRC- it reduces your punching shear perimeter and can cause a great deal of problems if you have an MEP engineer or architect that doesn't understand punching shear.

http://www.erico.com/products/steelfortress.asp

http://www.constud.ca/decon.ca/studrails/index.html

- as previously posted.

 

[NewImhotep]

Try this Report

ACI 421.1R-99 Shear Reinforcement for Slabs

http://www.normas.com/ACI/PAGES/421.1R-99.html

 

[rapt]

happyday,

On what basis did you calculate that the deflections are ok for an RC slab with a L/D ratio of nearly 40. It is a good depth for a PROPERLY DESIGNED PT slab. It will not work for RC slabs unless your end spans do not control.

As others have said, capitals or shear heads, but get the deflections right first.

 

[dik]

Drop panels and col caps are not uncommon with parking garages and normally don't require a ceiling to cover them. They can be exposed and expressed. It is often possible to use a 'banded slab' (a wide flat beam), but, it too would be exposed.

The alternative to caps/drops is to opt for a thicker slab (if normal reinforced) and pay the hit for additional shear and concrete. Post tensioning is an option, just a matter of care for joints and corrosion. You also take a bit of a hit due to the increased concrete cover.

Dik

 

[rt06]

When checking punching shear moment is always an issue. Try to use FE program such as RAM cincept or Adapt builder to obtain two-way moment and use them for design. Also, watch opening next to the column

 

[jen4950]

We use Adapt for production work, but PT Data is good for quick runs..