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Njuneer
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Discussion Starter #1 (Edited)
Working on a pole building design and will need to also design the floor. I plan to use 6" in most of the floor but I might try to reduce some material in the lighter traffic area if I can.

Right now I need to learn where to gather quality, code accepted structural data for concretes with different reinforcements. I am new to crete design but have been around construction most of my life and now an engineer.. I have some guys saying they would just get fiber fill in the crete and forgo any steel but not sure I can buy that. I probably need to understand show basic crete engineering to learn about the shrinkage and internal stresses.

The floor will all have foam under it and radiant tubing. I have done some reading that indicates the stiffness and interactions of crete may be underestimated such that lower density foams could be just fine. I just need to be sure!

I need to estimate my dead load over the foam and live load interactions on the slab inside a pole building. I will have large equipment on the crete and need it to distribute safely. However, most of this equipment is currently standing on an old DIY 4" floor and never had an issue! At one point we had 40-50K on the front axle of a forklift, still did not break so I would question when people tell me I need a massive floor.

My equipment is mostly mid size CNC machines weighing 10-20K lbs but will have some forklift and tractor traffic as well. I am thinking of running some total load calcs to ensure the foam is good but when I do, even with 1% deformation on the foam, it rates at 15psi which is more than good to cover over 1M lbs on my slab.

I was thinking of using welded metal mesh panels in the floor with fiber but I really need to better calculate the floor performance. Due to the pole building style, there are no other structural considerations for the concrete to handle other than some minor compression from the pole deflection in the wind but those loads should be easy to consider.
 

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Master General ReEngineer
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I have some guys saying they would just get fiber fill in the crete and forgo any steel but not sure I can buy that.
Ayuh,.... Generally speakin', ya want rebar, 'n mesh screen to tie the radiant tubin' to, 'n hold it in place for the pour,....

I don't buy it at all,....
 

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Unless your CNC machines have special foundation requirements for the manufacturers, which some do, you are definitely trying to overkill this project trying to show off your engineering skills. Concrete except in extreme circumstances is not really something that requires deep engineering analysis. You really don't want to use fiber, this certainly isn't the application for that product, you definitely want rebar. Probably #4 on 12" centers should work very well. For concrete I'd probably go with 6" of 4000 psi concrete and use a surface hardness of some type for wear resistance. I've had good luck using EMERYPLATE although there are many others equally as good.

I also question the use of in floor heating. Have some advantages but have disadvantages as well and one of the disadvantages is that it weakens the concrete significantly. Also it causes heat variations within the slab which further stresses the concrete.
 

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Your slab is nothing out of the ordinary.

Pour it all the same thickness.

I would be WAY more concerned with the placement & finish.
 

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Go for a 6" thick slab over the entire area, since whatever you put in the area will eventually be moved in, around or out. If you change the thickness, the slab will crack at that point. You will want a uniform thickness and strength over the entire floor area. Fiber mesh is for micro cracking and rebar is far more effective. - Spacing the rebar very slightly further apart (maybe a size heavier) may make it easier to place and finish unless your placers have very small foot sizes, otherwise 12" o.c. is OK if you use chairs to keep the steel near the middle of the slab depth.

You did not mention the area to be poured. You probably will want control and/or construction joints, so plan on how you plan to pour, joint and finish the floor. Keep in mind that the spacing of the poles, that are placed first will affect the over-all and intermediate joint spacing.

Dick
 

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JOATMON
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Go for a 6" thick slab over the entire area, since whatever you put in the area will eventually be moved in, around or out. If you change the thickness, the slab will crack at that point. You will want a uniform thickness and strength over the entire floor area. Fiber mesh is for micro cracking and rebar is far more effective. - Spacing the rebar very slightly further apart (maybe a size heavier) may make it easier to place and finish unless your placers have very small foot sizes, otherwise 12" o.c. is OK if you use chairs to keep the steel near the middle of the slab depth.

You did not mention the area to be poured. You probably will want control and/or construction joints, so plan on how you plan to pour, joint and finish the floor. Keep in mind that the spacing of the poles, that are placed first will affect the over-all and intermediate joint spacing.

Dick
^^THAT^^

Remember, concrete expands the same in all directions....so your expansion joints need to be done properly. 10x10 is a good start
 

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Concrete & Masonry
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First off, I'm not an engineer, and being one yourself, I'm sure you understand why a structural engineer isn't going to be willing to throw numbers out at you about your specific situation on the internet. It's just irresponsible.

From my own experiences, I much prefer 25 psi fowm to 15 for a number of reasons: It holds pex staples MUCH better, I've seen 15 psi fail with my own eyes under two different concrete floors, and the price difference is only about 3-5% for us.

As for the heated floor, they're great to work on all day, but keep in mind, if you ever need to remove part of the floor, ie: install a machine pit, extend plumbing waste lines, etc., the pex will be a nightmare to deal with. I'm not sure of the scale of machining your planning on doing in the future, but there's one factory we do alot of work in where we've cut floor and replaced with machine foundations at least 35 times in the last 15 years. In the entire building, there's probably only about 60-70% of the floor that hasn't been replaced by a base, etc..

Standard fiber is not a replacment for steel re-enforcement, even the fiber manufacturer's have gone away from making that claim. As Dick mentioned above, it's purpose is primarily to control micro cracking in the first few days, and there's other ways to deal with that. 90% of industrial floors here are 6"+ thick, contain 6x6x6ga. welded wire mesh (sheets, not rolls), and are designed at a minimum of 4500 psi for surface wear.
 

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Concrete & Masonry
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^^THAT^^

Remember, concrete expands the same in all directions....so your expansion joints need to be done properly. 10x10 is a good start
The mot common Portland cement is Type I, and it does not expand, only contract from it's original size. It will never "grow" bigger than the day it was placed, so what you're refering to are actually called "control" or "contraction" joints.
 

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I have a 30 x 60 ft shop that regularly sees my dozer that weighs 15 tons, yes the weight is spread out over the tracks, I have a 6inch slab with synthetic fiber mixed in to the mix ..its been down about 8 years with no cracks and pouring in 1 solid slab, around the perimeter of the building is expansion joint so the concrete doesnt push on the poles, as it is a pole barn framing...do you need to bolt down any of you heavy machinery? if so radiant tube placement could be an issue if you need to drill the slab for anchors..take pictures of the tubing and measurements before pouring the concrete for reference on where the tubing is , so if you need to make an educated guess where the tubing is you have a start..they make special foam for going under a concrete slab that can take the compression pressure, dont cheap out on that...
 

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Your engineering education will be tested when you choose the soil the mix is poured onto, how well it was compacted and not so much on the mix and rebar. That's where a large percent of concrete jobs fail.
 

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Njuneer
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Discussion Starter #12
Your engineering education will be tested when you choose the soil the mix is poured onto, how well it was compacted and not so much on the mix and rebar. That's where a large percent of concrete jobs fail.

NAILED it! Concrete has awesome compression strength but none in bending so if a part of the slab is not supported, you put a bending moment into the concrete and POP!

I would ask for some basics or specs on subgrades if you can provide them. Ultimately, I will probably do a complete subgrade soil test and do a compression test on the subgrade before pouring. At a minimum, probably a sheeps foot over the area, then rock base with sand dusted over it, then a plate tamp.

Poured a lot of critical bridge jobs but a pole barn is not quite in that area. There is a 6000psi 'bridge mix" around here but I know I don't need that.

Thanks for your input!
 

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Njuneer
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Discussion Starter #13
The mot common Portland cement is Type I, and it does not expand, only contract from it's original size. It will never "grow" bigger than the day it was placed, so what you're refering to are actually called "control" or "contraction" joints.

Sounds like you have experience, and that is what I am looking for. Ultimately, I am simply researching what experienced pros are doing but no, I will not just pour the job based on a recommendation. Just like in any other engineering, one might say "we use this I beam for this application". What that does is tell me it is common but I would then go run numbers to make sure I like it.

I will admit that I am doing some learning in this area as I am not familiar enough with crete. I do know it will shrink and that value can be determined by the slump and mix ratios. This is the opposite of what would really help the rebar so I can see why post tension rods are popular but I am not going that route.

I am curious, with your WWM, is that over the rebar matrix?

Also, I am very curious about your foam failure! Can you tell me more specifics? usually the foam is rated to 1% compression deformation so if you use say 15psi under a slab that is 1000sf, you could expect the foam to handle 2,160,000lbs uniformly. The dead load from the crete at 6" thickness is 70,000lbs so uniform max load might be around 2M lbs. NOW, this assumes the building is not supported by the crete and a uniform load! This is why I need to determine the bending of crete as this would reduce the localized loading. Crete is not infinitely stiff but I need to understand that reaction better.

None of my machines will require an isolation pad really but I can tell you, having done them myself, that the pad is NOT to support the machine, it is to isolate the machine from the floor to stay absolutely level and insulate from vibrations in the floor. It is also to add mass to damp its own vibrations. Harmonic tuning if you will.

I am not trying to teach there but I have done in depth studies that proves a basic floor will easily handle the weight of a machine but the OEM has no idea where it will be installed so they spec a foundation to ensure it works right. For all they know, I am installing a high precision machine right next to a hammer forge. That might cause a hiccup in the finish:biggrin2:
 

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then howcum highways 'blow up' in the summertime ? huh ? tell me that, big guy :wink2: conc's a solid & (remember hyskul fizix ? ) solids expand & contract according to ambient temps,,, hi temps cause expansion & lo temps cause shrinkage,,, this's why joints are bigger in the winter than summer,,, now that you understand that, conc does initially shrink during hydration ( curing ) hence need'd 'contraction' jnts,,, after that, nuttin' changes as relative humidity/temp are usually constant inside buildings
 

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flexural strength is a far different aminal than compressive,,, typical bdge decks have 2 mats of rebar to increase conc's flexural strength while floors have 1,,, mass placement is much different than either & not relative to this thread nor ANY diy column,,, raising flexural strength relies on rebar size & placement so no wwm

slab size - typically 20 x (slab thickness in inches) expressed in feet - eg, 20 x 4" = 80 / 8.0" slab,,, admittedly this is old but so am i compared to when i 1st learned it 35 yrs ago,,, therefore, using that formula, a 6" slab would measure 12' x 12',,, i think today any floor designer would look for 15'+ as a goal but others will chime in i trust.

you haven't mentioned traffic which would raise the issue of load transfer devices in the slab
 

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Njuneer
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Discussion Starter #16
You know, you are right. I probably need to get in the books to better understand interaction between a single level of bar. Back when I was working on bridges, I did not fully understand what I was doing and why....


You are right about the double level of rebar. That could exponentially improve stiffness because during deflection, that bar acts just like an Ibeam, the bottom goes into tension, and the top goes into compression. Steel is strongest in tension, and the compression buckling issue is mitigated by full support of the crete.

I am sort of wondering if two smaller levels of bar would way out perform a single layer. If course I need to look at costs. At some point, I am sure just making the crete thicker is cheaper. On bridges, weight is a BIG issue so pouring 24" just isn't practical.

I am sort of left wondering what a single level of bar even does?? Would seem flats would do much better than rounds for this.
 

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Civil Engineer
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The OPS is proposing a six inch thick slab. Consider that ACI (American Concrete Institute) recommends a minimum cover of 2 inches on the side of the slab exposed to soil, and 1-1/2 inches on the top side. This essentially means that the steel reinforcing is close to the neutral axis of the slab, and thus is not going to supply much tensile strength. This is why most of the time 4 inch and 6 inch slabs are not reinforced, the steel does not supply much strength, it is mainly there for crack control.

Concrete is strong in compression, and if the bearing soil beneath the concrete is the correct material, and is properly compacted, the concrete will do very well in compression due to machine load. Vibration is an entirely different matter, as vibration can set up tension forces in the concrete. Various machine installations I have worked on for large machines used thick, heavily reinforced steel slab (up to 4 feet thick). The steel in those slabs carries essentially all of the tension load. Not really sure that you can get the steel bars in a six inch thick slab to act effectively due to the neutral axis problem, perhaps others have some insight on this question.
 

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holly crap, the guy is pouring a slab of concrete on solid ground, not 40 stories in the air...your over thinking and engineering it...put down 6 inches of rca, compact it down with a roller or plate compactor, put your plastic down, steel wire mesh and pour the concrete , level and then float it out...if your gona put radiant heat put high density foam over the rca then wire mesh and zip tie the pex to the wire mesh first...then pour , level and float..then you can seal the surface to help with grease and oil clean up..
 

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Concrete & Masonry
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I am curious, with your WWM, is that over the rebar matrix?

Typically, it's one or the other, not both. You'll still see recommendations to set mesh first and tie the tubing to it, but it's extremely rare around these parts any more, more like 25 year old technology. Now, everyone staples the pex directly to the foam, and the steel re-enforcement is placed above it.

Also, I am very curious about your foam failure! Can you tell me more specifics? usually the foam is rated to 1% compression deformation so if you use say 15psi under a slab that is 1000sf, you could expect the foam to handle 2,160,000lbs uniformly. The dead load from the crete at 6" thickness is 70,000lbs so uniform max load might be around 2M lbs. NOW, this assumes the building is not supported by the crete and a uniform load! This is why I need to determine the bending of crete as this would reduce the localized loading. Crete is not infinitely stiff but I need to understand that reaction better.


Both times were instances where the foam was laid over the floor ledge, and colapsed to ~1/2 of it's original thickness. Both times, it pushed the exterior foundation walls outward, requiring extensive repair. Again, the cost diffence is extremely minimal and the 25 psi is far superior at holding the pex staples.


None of my machines will require an isolation pad really but I can tell you, having done them myself, that the pad is NOT to support the machine, it is to isolate the machine from the floor to stay absolutely level and insulate from vibrations in the floor. It is also to add mass to damp its own vibrations. Harmonic tuning if you will.

I am not trying to teach there but I have done in depth studies that proves a basic floor will easily handle the weight of a machine but the OEM has no idea where it will be installed so they spec a foundation to ensure it works right. For all they know, I am installing a high precision machine right next to a hammer forge. That might cause a hiccup in the finish:biggrin2:
I understand how machine foundations work, we've done them up to 10' deep (yes, 120" thick) and in excess of 800 yards of concrete. My point is simply that if your business ever grows to the point of needing a large enough machine to require a base, are you willing to abort a loop of floor heat permantly?


then howcum highways 'blow up' in the summertime ? huh ? tell me that, big guy :wink2: conc's a solid & (remember hyskul fizix ? ) solids expand & contract according to ambient temps,,, hi temps cause expansion & lo temps cause shrinkage,,, this's why joints are bigger in the winter than summer,,, now that you understand that, conc does initially shrink during hydration ( curing ) hence need'd 'contraction' jnts,,, after that, nuttin' changes as relative humidity/temp are usually constant inside buildings
You already know the answer to that, "smarty-pants"......

Foreign debris enters the open joints in cooler weather and creates the bind when it gets very hot.............
 
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