Sawcut Joint in Thickened Edge Slab

[pbc825]

Hi all,

I'm working on a project in which the client is proposing to excavate under a portion of the outside of an existing 8" reinforced slab structure with an edge thickened to 18" for 42" and tapered back into the slab at about 45 degrees. Piles are included at 20' O/C to support building columns. The drawing notes construction, expansion, and sawcut joints. The expansion joints are detailed with fiber board and sealer through with reinforcing steel continuous. Construction joints are not detailed, but I assume they're the same as expansion joints without fiber board/sealant and with a shear key. Sawcut joints are not detailed, but in the 8" slab I'd assume dowels mid slab and a cut to 2" and discontinuous top reinforcing steel (and maybe bottom??). Dowels at sawcut joints are noted.

What I'm struggling with is sawcut joints parallel to the thickened edge within the thickened edge. I've attached an image showing the detail and the sawcut is shown as a dashed line through the whole thickened edge. I'm presuming this would look like a 2" or so saw cut with discontinuous top bars and dowels roughly 4" under the top of the thickened edge while mid and bottom layers are continuous. I would think it's crazy to sawcut through as the line is shown. Can I get "reinforcement" or alternative thoughts on my assumptions.

Thanks in advance for any input.

 

[JAE]

That appears to be a reinforced structural slab - not sure why sawcut joints were even specified for that. Seems ridiculous.



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[SlideRuleEra]

I would think it's crazy to sawcut through as the line is shown.

I agree.

The entire design looks "odd".
Piles beneath columns that bear on an integral thickened slab on grade?
Saw cut joints though a reinforced concrete slab on grade?
Expansion joints with rebar continuous through the joint?

Maybe I don't understand the description...

I would stay away from this project, if that is an option. No matter what you do, if there are (unrelated) problems you will get the "blame".

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[allgoodnamestaken]

Can't you just ask your client or your client's architect to explain what they'd like?

 

[KootK]

I'm okay with the overall concept. I suspect that the odd jointing is probably coming from standard details that don't make sense for this project. I see no need for a joint parallel to the thickening as shrinkage over the width of the thickening should be minimal. I also see no need for perpendicular contraction joints as the new pours will be heavily restrained by their connection to the existing slab and reinforcing would be a better crack control strategy. And, you know, it would generally be nice if you could maintain the flexural integrity of the edge thickening which is a spanning element of sorts.

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.

 

[pbc825]

To clarify,

1) The construction was completed in the 1990's. Our scope is to provide recommendations on how to successfully excavate a portion of the slab to maintain services under the slab without risking maintenance personnel safety.
2) Definitely a slab-on-grade, and not structural slab. Although I agree it has atypical reinforcing for a slab-on-grade.
3) I agree the jointing is odd, but it was done in 1990.
4) These folks deserve a competent engineer to make informed decisions for their maintenance personnel. So I'm not going to decline them.

Unless there are more comments or opinions then I'll travel down to the site, dig out some joints, and report back.

 

[SlideRuleEra]

pbc825 - A sizeable excavation is likely ok. The reinforcing in the thickened portion of the footing will help it to act as a beam and span a reasonable distance. In addition to this "width" of the excavation, the "depth" (how far under the slab) and how close the excavation is to a piling will be considerations.

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