dowel spacing question

[schoolie]

Hello,

I have a dowel question for everyone. I am saw cutting an existing slab(150'x240') with a 9' wide x 220' long cut and putting back 18" of reinforced concrete to support some new loads. The existing slab is inside of a building and is also 18" thick; however, the existing slab does not have any reinforcing, which is why I am removing it. I need to epoxy a piece of rebar into both sides of the existing slab at the saw cut to reduce the bearing pressure of the new equipment and reduce the amount of concrete that has to be removed and repoured. I have roughly 1500-2000 lbs/ft of shear across each side of the joint. I planned on putting one dowel at mid-depth. I used SAFE to analyze the new slab with soil springs. Anybody have any suggestions on how to calculate spacing, size and embedment for my dowels? I don't believe shear-friction is an option as the sides of the cut will be nice and smooth.

Thanks in advance.

Schoolie

 

[a7x1984]

Appendix D of the ACI 318 on anchorage will tell you what you need to know. Powers and Hilti have free design software for adhesive anchors and they work very well (I like the Powers one, myself).

In Russia building design you!

 

[hokie66]

Taking out a strip of 18" concrete pavement, only to replace it with 18" concrete pavement, sounds strange. I know you indicated that the new strip will be reinforced, but why do you think that will help? What kind of loads are involved?

 

[ztengguy]

Seems like the load is around 4000lbs/ft down the length correct?

 

[JAE]

hokie66 - the way I read it the removal is a 9 ft. wide strip x 18" thick.

I would think shear friction would still be applicable - there is a factor in the ACI equation for smoothness-roughness with a smooth surface.

Alternatively you could shot blast the surfaces or hand chip them - perhaps below the top surface - such as leaving a 1 1/2" wide smooth strip at the top and roughening the lower 16.5 inches.

 

[dik]

Unless warranted by loading or condition, an 18" slab may not warrant reinforcing... may not contribute anything. Just because it's not reinforced, doesn't mean it's not any good.

Dik

 

[hokie66]

JAE,
Yes, I read it that way also. Like dik, I wonder why the existing slab is not good enough. I wouldn't necessarily take "SAFE's" word for it.

 

[schoolie]

Thanks for the comments guys. It will have column loads on it for a crane runway. The existing slab is sometimes poured in two part, sometimes it has reinforcing, sometimes it doesn't. The owner just wants to put the columns on the existing slab because they see it is 18" thick. The existing slab was obviously not engineered. The only way to support the columns for the crane runways in the middle of the building is to saw cut and pour back a reinforced slab. "Slab" may not be a good word here in a narrow sense. The width of the "foundation" for the columns is 9' wide(center to center of columns is 5', each frame is 20' on center), 18" thick, and runs 220' long. Each runway will support a 20T top running bridge crane. It is a typical metal building that we are installing crane runways on the interior. My concern was where I intended to saw cut would be a smooth edge. I can get the full tension in the rebar dowel using a 12" epoxy HY 150 embeddment into the existing slab and 12" into the new pour. My concern was with that smooth edge for shear friction calculations.

Per ACI
Concrete placed against hardened
concrete not intentionally roughened ................ 0.6λ

Do ya'll agree that this covers a saw cut edge in order to use shear friction? I need to transfer some of the load into the adjacent slab for soil bearing purposes.

Thanks,

 

[hokie66]

From your original post, I thought you were trying to transfer load from the new reinforced footing onto the existing unreinforced slab...probably not a good idea. But if you are just trying to dowel the pavement joint, then these would be just shear dowels and designed as with any other rigid pavement. Smooth dowels are normally used, sometimes square dowels or diamond dowels to allow differential movement parallel to the joint. This will "tranfer some of the load" in both directions, but I don't think shear friction applies in transferring load from a reinforced to an unreinforced section.

 

[Ron]

I'm not sure there's any good reason to provide dowels here. The foundation for the crane can be completely independent of the floor slab; however, since both are 18" thick, I am presuming some hefty loads are being transported across the surface of the existing slab and if those loads are to run onto the strip footing for the crane, then load transfer dowels as hokie66 noted would be appropriate. If loads not traversing across joint, then I'd keep them separate.

 

[schoolie]

Hokie66/Ron

I am trying to transfer some shear across the joint from the new pour to the existing, unreinforced slab. Minimal shear, but some. On the order of 1k-2k/dowel at 24" O.C. I am using the dowels to reduce the soil bearing pressure on the new strip footing and to take care of any differential settlement between the footings. I think its prudent to make sure there is no tripping hazards. No moment transfer at the joint, just some shear. I have to keep the bearings below 1000PSF. Soils as not so good in the swamp lands of South Louisiana and they may settle at different rates if no dowels are used. Using 11.6 of ACI with the friction factor of 0.6λ, the #4 bar works fine if I can assume that this applies to saw cut edges. I could bush the edges down, but that is a lot of work. I appreciate the thoughts guys.

Schoolie

 

[hokie66]

Again, I don't see how shear friction applies into plain concrete. Shear friction depends on development of the bars in tension, and in plain concrete, there is nothing to resist this development.

You stated that you only need shear transfer, and shear transfer is achieved in slabs on ground by dowel action, not shear friction. I would definitely use larger bars than 1/2", but they would be smooth dowels.

 

[petertor]

I would not rely on the existing slab for dowel load transfer, and most certainly would not rely on shear friction for dynamic crane loads. We have a 20T crane and footings are 12'x12'x36" thick, 30-foot bays, 4000psf limestone ground.
I would bite the bullet and tell your client you have to cut out for a pad-and-grade-beam foundation sized for the
soils you have, maybe reinforced with 2" pin piles, then pour the foundation against asphalt fiber board on the old,
so there is zero load transfer. Not telling you want to do, just some advice from 30 years doing heavy industrial SE.
If you get differential settling from relying on that unknown slab, the annual trackage inspection on the crane will
tag it out, and then your client is gonna be robo-calling you to pay for the crane trackage re-adjustment every year.
You will be amazed at how much trackage inspectors and crane adjusters charge by the hour, ...and at their own pace.
How much money do you have in your wallet?

 

[Ron]

Agree with hokie66...just a dowel issue and 1/2" is not sufficient. They should be smooth dowels and unbonded on one side. Depth of penetration ((length of dowel)/2) will likely be on the order of 12 to 18 inches to prevent cracking at the end of the dowels.

 

[schoolie]

Thanks Ron/Hookie66/petertor. I appreciate it.

On the shear friction issue, I'm not sure I agree with hokie66's comment that, ..."and in plain concrete, there is nothing to resist the development." Hopefully, ya'll can enlighten me. You can resist tensile forces from an anchor or a piece of rebar in plain concrete. The shear friction method uses the two concrete contact surfaces to transfer the shear across the joint.. ACI says, "For concrete cast against hardened concrete not roughened in accordance with 11.6.9, shear resistance is primarily due to dowel action of the reinforcement and test indicate that reduced value of 0.6λ specified for this case is appropriate."

Vn = Avf*phi*fy*u, u= 0.6λ(friction coefficient)(ACI 11-25)
2kips*1.5 = Avf*0.9*60ksi*0.6*1.0
Avf = .083in2 or a #4 bar

I can see the dynamic effect petertor is referencing.

So, using Hilit HY 150 max, #4 with 14" embedded into existing, plain concrete develops the full bar, at which point, the shear is transferred through the face of the concrete shear planes in contact. I would roughen the saw cut side and provide additional dowels at the column locations. I know I can develop the bar on the new 9' wide foundation side.

ACI 11.6.8 says "Shear friction reinforcement shall be appropriately placed along the shear plane and shall be anchored to develop fy on both sides of the embedment, hooks, or welding to special devices."

Sorry to rehash, and I'm sure you guys may be right, but I don't understand where this logic fails?


In any case, if ron and hookie66 are correct, could you guys point me to any reference to help me design these smooth dowels for shear, greased up on one side?

Many thanks,

schoolie

 

[a2mfk]

Petertor had the same idea as me- micropiles. Great situation to use them is in an existing building when you need to add multiple columns. They only need to cut about 1' by 1' holes in the slab depending on the type of pile, and you can get some impressive capacities depending on the soil and distance to bedrock. Helical piers may be an option depending on how deep the organic soils you mentioned are. You may end up saving your client a LOT on concrete demo and replacement.

My 2 cents..

 

[schoolie]

good idea a2mfk, I proposed something similar. The owner said that it is cheaper to run the saw down the hole path on each side