Dowel Connection (Slab to foundation Wall)

[Vrpps EIT]

Hi all,

In case we need to support a concrete slab (patio) and considered it to be a structural slab. Should the dowel bars be at the top of slab and then into the foundation wall or the dowel at the bottom of slab then into the foundation wall. Attached an image of it, there is no bearing available to rest the slab on top of the wall. so dowel is the structural element that is going to connect the slab into the wall and its going to bear the weight of the slab and transfer it into the foundation wall. Appreciate if anybody could help me understand this!
Thank you

 

[KootK]

My preference:

1) Keyway on the bottom for positive shear transfer / bearing.

2) Dowel on the top to reflect where the bending stresses will be.

Effectively this becomes a shear friction-ish connection with the keyway representing the roughening.

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[Vrpps EIT]

Thanks KootK, you mean like this---->

 

[KootK]

No sweat. And yeah, just like that.

Consider playing chess with me on the Social Chess app at iTunes. Same handle. Fear not, I suck.

 

[Vrpps EIT]

But, will it also be a positive option to move the dowels at the bearing/ shear transfer (to the bottom) & provide top bars just within the slab to resist the negative moment at the ends of the slab?

 

[phamENG]

Veerapps - if your dowels aren't developed in the top of the slab (and into the wall), you won't have any rotational restraint and no moment there anyway. You'd end up with a simply supported slab.

 

[KootK]

That's not an approach that I'd entertain. Without the top side connection to the wall, there's no negative moment at the end of the slab and, thus, no reason for the top bars and less clamping of the joint. Additionally, as I understand it, contractors pefers to keep the couplers out of the keyways for constructability.

Consider playing chess with me on the Social Chess app at iTunes. Same handle. Fear not, I suck.

 

[Vrpps EIT]

PhamENG, correct me if I am wrong by what you say is to provide the bearing (dowels at the bottom) and consider the slab as to be a simply supported slab is that right? So then I don't need to worry about the hogging moment?

 

[KootK]

Let's try it this way: why do you NOT want to put the dowels at the top of the slab?

Consider playing chess with me on the Social Chess app at iTunes. Same handle. Fear not, I suck.

 

[phamENG]

Absolutely not. I was telling you what would happen if you failed to provide top bars into the wall as KootK sketched out.

KootK - I like the use of couplers there. I've had similar details where the coupler idea would have solved lots of headaches and RFIs.

 

[Vrpps EIT]

Hi Kootk, I was actually more concerned with how the bearing is going to work out, like how the slab is to be supported to the foundation wall. I understand that you told about keyway, but in this site it's too complex to play with forming, my thinking was If I could check my slab for simply supported end condition (& consider the dowels as shear transfer/bearing (checked for shear it worked out) and if it satisfies bending, shear checks of the slab as a whole then I will feel relieved . To mention its an 8" thick slab w./ 15M bars @12"O.C(Main bars) & 10M bars@12"O.C (Transverse reinf.)+ it's a one way slab!!

 

[phamENG]

Veerapps - What about stopping the wall at the level of the bottom of the slab? You can pour the slab, and then come back and pour your little stem. You can make all the reinforcing continuous through the joint, just break up the pours. It adds a little time, but if fancy formwork isn't an option and/or workmanship is going to be questionable it may be your best route to get positive support.

 

[KootK]

If it's me, I'm pushing hard for a key-way, a ledge, a platform pour, or a corbel. If absolutely forced to do it without positive bearing, I'd be looking for field verified roughening of the wall and developed dowels top & bottom.

Consider playing chess with me on the Social Chess app at iTunes. Same handle. Fear not, I suck.

 

[Vrpps EIT]

PhamENG, LOL that was my last option to go with to have a shelf angle of 3.5" bearing in that wall and then create a step and pour that remaining upstand or curb!

 

[phamENG]

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1576695509/tips/1_page-0001_kxfo6x_hvqesj.pdf[/url]

 

[phamENG]

I get it - not ideal from a schedule perspective. But it is mostly fool proof if you doubt the contractor's abilities.

 

[Vrpps EIT]

Actually this is the exact scenario

 

[Vrpps EIT]

Beneath the slab, we will be having the grade so typically its a S.O.G but with this site lot of leagal issues going on and they want it to be counted as a structural slab

 

[phamENG]

Yikes. That changes things a little. How much room do you have on top of the existing foundation wall? What about a turndown onto the top of the wall? Either way you'll need a moisture barrier between your concrete and the floor framing/exterior finishes.

If you're really designing this as a structural slab, be sure to look at the eccentric loading on the foundation wall as it will create a nice little moment that it may not have been designed for. It needs to be able to do that, or retain the additional surcharge loading from this new patio.

 

[Vrpps EIT]

Yes moisture barrier is going to be there, its 4" at top of wall. Surcharge load considering due to patio wasn't substantial check on that, plus we are the thought process of to go with U-fill as backfill in order to eliminate the compaction load & typical earth backfill