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Concrete Pedestal Temperature/Shrinkage Reinforcement vs. Interface Reinforcement Requirement

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JD P.E.

JD P.E.

Mechanical
Oct 17, 2021
79
Designing a motor foundation with rough dimensions of 104" long, 39" wide, and 11" tall. Designing as plain concrete pedestal with T&S reinforcement of .0015Ag. The pedestal is on a concrete slab so interface minimum steel area is .005Ag. Using 26 #8 dowels for that, but wanting to get around using it for T&S to avoid the hassle of bending 1" dowels for top face. My proposal is to use the dowels to meet interface area requirements and attach #4 ties to them to meet the T/S reinforcement. I think it will be easier to construct in field. Thoughts on this? See attached.
 
 https://files.engineering.com/getfile.aspx?folder=478c1c46-e9c8-4f1d-9f8a-2506f1fe78a8&file=Capture.PNG
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Is that really considered a "pedestal" and needs 0.005Ag? Maybe someone technically better than me can answer the question. Small pump block foundation usually only has #5@12" U bars all sides and certainly dont meet 0.005Ag.

#8 bar can easily be bent in the field if the construction contractor has the proper bar bending equipment. If they don't, either they buy the #8 bars bent already or #5 or possibly #6 is the max they can field bent.
 
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So you’re suggesting a similar setup, but just use #5 dowels to tie into slab and bend those at top?

I agree meeting .005Ag is much overkill.
 
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#5 can easily be bent in the field if that's what you are concerned about if you don't know your construction contractor have bar bending equipment. I work in the plant and talk to construction people and watch them work so in my case I know their capability or can easily ask them what's easiest for them.
 
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That makes sense. Would you do a similar tie in to what’s drawn? Plan on expoxing the dowels into slab as shown.
 
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I'd follow my pump foundation design guide which is #5 U bars on all sides to prevent surface cracking. So I'd probably use #5 U bars as "ties", though #4 would still probably work.
 
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Is it possible that guide would be something you would share or some information from it?
 
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I'd likely do something like... same size (dimensions) dowels, two pieces the same size, not 3 pieces (2 the same size). 15M (#5) shown. Because they are the same size, you can accommodate slight dimensional differences: [pipe]

Eng-Tips07_nnztrd.png


So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik
 
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What about the side of the long dimension for reinforcement? Are the top faces and front faces all that really require reinforcement in this instance?
 
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You can have the same thing with the splice located in the middle, or can have two different length dowels and alternate the splice ends... both work OK... bars will only be about 8' long, max. I'll quickly modify my housekeeping pad detail

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik
 
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Something like... I'll attach the *.dwg file, too. I usually stretch to suit and adjust rebar... Often for the longitudinal bars, I don't dowel them into the slab. I've shown the dowels 12" into the slab; I often use 6" or even 4". The horozontal bars in the face can be straight; I don't use 'hoops' for those. Forgive the layer system; it's one I've modified using the AIA layering system, as a guide... it's not official of any kind, but comprehensive. [pipe]

Housekeeping_Pad_lzrwbh.png


[URL unfurl="true"]https://res.cloudinary.com/engineering-com/raw/upload/v1659302529/tips/Housekeeping_Pad_ow3cdx.dwg[/url]

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik
 
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Dik - Thanks for taking the time to do that. It helps a lot! Would you just connect the straight bars via wire tie?
 
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It just took a few minutes, and becomes part of my cad 'collection' for future jobs. Yup, can use straight bars... The longitudinal bars can be straight as well as the face bars... and with Hilti Hit-Hy 200 V3, 4" embed is likely as much as you need. [pipe]

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik
 
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So would you consider following the .005Ag interface to slab requirement even if it’s not really necessary? Or just go with #4 or 5 bars and go?
 
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I'd not normally worry about TS... the dimensions are too small to matter. If you have high uplift loads caused by vibration or something of that ilk, you might want to avoid replacing the embedded dowels with straight bars; the embedded bars can get large and developing them in a thin housekeeping pad can be problematic. It's unlikely that you need to go to larger bars than 15M (#5)... you could likely use 10M (#4) unless you have loading issues. I'd not use #3s.

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik
 
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FYI... I used my housekeeping drawing and stretched it to suit your dimensions. The CAD drawing is to scale. You can stretch the embedded dowel length to 4" or 6" from the 12" that it's drawin. Rebar are metric sizes, and bends are standare 90deg bends.

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik
 
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Dik - Thank you again!! Can you elaborate a little more on higher loads causing tendency to avoid replacing embedded dowels with straight bars? I am not following what you are saying there.
 
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If you have very high concentrated loads or high uplifting loads (without defining 'high') then you look at the housekeepins slab as a small structure by itself. The above is likely adequate for 95% of housekeeping slabs. Others can fill in their experience, and hopefully it will be similar. I should have added, first thing is you locate the existing rebar and use the locations to locate your dowels in a fairly uniform pattern. It may even help if you can move the housekeeping pad a few inches if rebar interference is an issue. Also, use headed anchor rods to secure your equipment, if you can. [pipe]

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik
 
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