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Doweling equipment pad into existing slab - development length?

Doweling equipment pad into existing slab - development length?

Doweling equipment pad into existing slab - development length?

(OP)
We're doweling new 4" thick equipment pads into an existing concrete foundation slab. The dowels are #4 reinforcement that I equate to a 90° hook. I assume the horizontal portion of the #4 dowel is ldh as defined in ACI 318 fig. R12.5. My question is the necessary length of the vertical portion of the hook. Is it correct to assume the minimum embedment in the existing foundation is 12*db or is some of this 12*db allowed to be in the new pad itself thereby lowering the necessary embedment depth? We plan to specify Hilti HY-200 adhesive.

RE: Doweling equipment pad into existing slab - development length?

Are you designing for any uplift between the new pad and the existing concrete?

If not, you're only transferring shear and it's really there just to keep everything together. I wouldn't worry about developing the bars.

RE: Doweling equipment pad into existing slab - development length?

@detstru,

how do you determine when doweling to existing foundation if you will just design for shear or you need to design for moment (full development length)?

RE: Doweling equipment pad into existing slab - development length?

We're talking about a topping / housekeeping pad installed on top of the foundation, right?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Doweling equipment pad into existing slab - development length?

The horizontal portion of the hook is NOT ldh. For a #4 hook, the horizontal portion is always 8". ldh would be the distance from the top of the existing foundation slab to the hook--in your case, about 2". So you are not developing the bar very well.

That being said, the bar probably doesn't need to be developed.

I would embed it into the existing foundation slab the distance recommended by Hilti.

DaveAtkins

RE: Doweling equipment pad into existing slab - development length?

Let's start with what you're mounting on the slab. If it is heavy/tall/narrow equipment that can have significant lateral force during a seismic event it would require an uplift attachment to whatever it's anchored to. If you can't resolve that with the weight/stiffness of the pad you'd have to anchor that down to the slab/foundation below. In that case you have to develop some amount of tensile capacity between the equipment pad and the embedded rebar.

This is not typical.

My guess is you just have some piece of equipment that needs a pad so that it sits elevated from the slab. In that case the dowels are there just to tie it all together.

RE: Doweling equipment pad into existing slab - development length?

(OP)
DETstru: We're not designing for uplift but what would be done in that situation?

KootK: You are correct.

RE: Doweling equipment pad into existing slab - development length?

The purpose of dowels in shear is to pull the two concrete surfaces together - kind of like the normal force that creates the friction force. Like everyone has said, it's typical to just put something there and not worry about it for housekeeping pads. If you have real forces to resist, having 2" of development is just about useless.

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