Saw joint location

[JStructsteel]

Transitioning from a 12" slab to a 4" slab, what is the best place for the saw joint? Where i show it, or up at the top of the transition at the 4" slab?

 

[phamENG]

The crack will occur where stress is greatest. That's more likely to be in the 4" segment than the 12" segment.

 

[JStructsteel]

Ok, thanks.

Would you extend those top bars out into the 4" slab?

 

[phamENG]

If I wanted to be sure of both the crack location and no differential settlement, I'd put a construction joint there and dowel them together. That, or not have the joints and reinforce the slab accordingly.

 

[JStructsteel]

The reason they want it continuous is they want to put in floor heating tubes.

 

[XR250]

I put floor heating tubes in the slab at my house with construction joints and it worked fine.

 

[phamENG]

If you're doing heating tubes than saw cutting is a risky proposition anyway. Unless the tube is properly restrained, it may try to float up. Nick one of those tubes, and the whole thing has to come out. Careful detailing and even more careful construction will be necessary.

But if you're going continuous, I'd just reinforce it to control cracks. What kind of finish for the slab?

 

[JStructsteel]

It will be a garage, so I assume a trowel finish.

 

[BridgeSmith]

I'd definitely reinforce the 4" slab. I'd also reconsider transitioning to the thinner slab so close to a rigid support. If it were me, I'd extend the top reinforcement all the way across the 4" slab, extend the 12" slab at least a couple feet beyond the wall, and then taper it down over 2-3 feet more.

 

[TLHS]

If you're worried about the tubes, can you wrap the tubes in something with some flex (foam or similar for a foot or so on either side of the joint and then do an isolation joint? Then you can have your structural slab / non-structural slab split but also have some play for differential movement

 

[hokie66]

I assume the 4" slab is on fill. The fill will continue to consolidate, so I would use a structural slab to span over the inevitable void close to the wall.

 

[Euler07]

I've probably done 50 of these, and the company I used to work for would have done at least a thousand. The rule of thumb there was to extend the suspended slab longitudinal top steel into the ground slab and lap over the ground slab mesh. The distance of the extension would min of 1.5m, or one-third of the suspended slab back span (for the purpose of preventing cracking from the hogging moment). The ground slab mesh would typically be SL62 (i.e. 6mm 500MPa at 200c/c) and 100mm ground slab. Don't use unreinforced in this situation otherwise it will crack at the end of the suspended slab reo.

i.e. If the suspended slab spans 6m then extend the reo 2m into the ground slab and lap over the ground slab mesh. Crank the ground slab mesh down to suit.

Hope this helps.