New build concrete slab for 10,000# lift?

[longez]

We're just about to pour the monolithic slab for my new construction 40'x60' shop; I'll be installng a 10,000# Rotary 2-post lift. Overall the slab will be 5-1/2" min of 3000 psi concrete, but I'm having a 48"x48" x12" deep footing centered under each post with a 24"x72" x12" "dog bone" tie between the two posts. Plenty of rebar in each 48 x48 tieing the footings to the rest of the slab and to each other through the "dog bone" conenctor.

I read through Rotary's installation manual, but am wondering if I'm doing the right thing or should be doing anything else/different?

Thanks in advance.

 

[Modoc]

I believe most of the lift manufacturers call for 4,000 psi concrete.

 

[jonjon1]

I would use a at least 40Kpsi. I know my forward 10K called for 5" of 4k,, rotary would most likely be the same...

Cost shouldnt be that big of a difference.. I always ordeer 4k psi with the fibermesh additive, I think its number 300? You will never have an issue and the price difference between the 4k fiber and 3k base is like $6 a yard, so if you should need about 42 yards for your slab and lift footings (extra for foundation/footings), so to have a much better floor it will cost you $250 extra, lol. Should be a no brainer...


ALSO, instead of doing the footings I would rather just do a 6 1/2" thick floor and have the entire floor strong, another 7 yards of material, and a little more digging, you will have a floor that will last forever and can put the lift anywhere.. I was never huge on putting footings for lifts. A friend of mine has a 15K or 18K# 2 post bendpak, I built the building for him, its 90x64, 8.5-9.5" thick slab, compacted really really well, welded bar, floor is 8 years old now and looks new, HE pulls in some heavy stuff, he did a bottom on my cat 320 and that is a small machine for him, lol. He lifts tow trucks with that 2 post and the floor is SOLID, no spiders, nothing...

Advice to all, DO not skimp on your floor, rent a compactor and compact it your self if you have to, get the ground HARD, Pour as thick as you can, and use the best material you can find, the floor is important, also- always make sure you have a south facing roof, just incase some day you want to install solar panels, some day in the future they will be cheap and everyone will be installing them, so may as well plan ahead... (I know that off topic, lol, but just trying to help...

 

[Ironcrow]

I'd bury a large I-beam in there and stop worrying about re-bar, footing depth, anchors pulling out, and concrete psi.

 

[wssix99]

We're just about to pour the monolithic slab for my new construction 40'x60' shop

Have you planned where you are going to put your saw cut and/or expansion joints? This will impact a lot of what you will need to do.

Overall the slab will be 5-1/2" min of 3000 psi concrete

This is fine. A lot of people will use 4000 psi concrete because the higher cement content makes for a nicer surface finish on the floor.

but I'm having a 48"x48" x12" deep footing centered under each post with a 24"x72" x12" "dog bone" tie between the two posts. Plenty of rebar in each 48 x48 tieing the footings to the rest of the slab and to each other through the "dog bone" conenctor.

I read through Rotary's installation manual, but am wondering if I'm doing the right thing or should be doing anything else/different?

^ This is very problematic and dangerous. In Rotary's manual, you will notice they said nothing about doing anything remotely like this. Building something like this could cause cracking around this dog bone and then the whole thing could then tip over once a load is in the air.

On a brand new slab, the lift is intended to be installed on a perfectly flat and uniform pad.

You may find instructions on how to build a "thickened repair slab." These are intended for applications where the original slab doesn't meet specs. It may seem somewhat like the dogbone idea because it has some reinforcing in it and its limited to the area of the lift posts. However, the thickened repair slab acts mechanically different because its poured separately (hence is is a physically separate structure) and it's pinned or keyed to the old slab. (This last part keeps the thing from tipping over on you.)

 

[wssix99]

It's common to assume that these 5 ton lifts need a "footer" because that's a lot of weight!

But, stepping back a bit, it's easy to see that a simple concrete slab is much stronger. When you drive a 5 ton truck on the concrete, it's not a problem - and that 5 ton truck is only supported by 4 wheels. 50 ton delivery trucks will also drive on your driveway without problem.

The secret is in the contact patch of the tires of the 5 ton truck. The area of the tires spreads the load out on the surface of the pad. The rotary lift has even larger base plates on the bottom that do the same thing. So, when you lift that 5 ton load up on to the lift, your concrete won't know the difference between that and the stresses of having the truck parked on it with no lift at all.

The significant stresses are the ones that cause the two post to want to "tip" over when a load is up on it. Having a continuous slab underneath (with no cracks) is what gives you strength here. Imagine the legs of the lift and the slab forming an upside-down "T." That is what resists the thing from tipping over.

 

[Gerald O]

... also- always make sure you have a south facing roof, just incase some day you want to install solar panels, some day in the future they will be cheap and everyone will be installing them, so may as well plan ahead... (I know that off topic, lol, but just trying to help...

You should also make sure to site it on high ground for the eventual rise in sea level. Maybe put in a bunker below the slab for the coming zombie apocalypse.

 

[dave*99]

I've read that the dog bone you describe actually creates a potential for cracking. There are stress risers created where the thickened sections meet the floor slab. When I put in my floor, I used a 4" thick slab and in the area where the lift posts go, I gently sloped the grade under the slab to get about 7 or 8 inches under the lift posts. I have a 2 post 10,000 lb. Rotary lift.
And even what I did was beyond Rotary's recommendations.

 

[Vegaman_Dan]

The dogbone link with rebar connecting them may indeed introduce cracking as rebar will bend and flex as it is designed to. It's not meant to prevent cracks so much as keep the broken pieces linked together afterwards.

BUT if you were to build up a structure using structural steel that linked both column bases, possibly with plates at surface level to bolt the lift to, then it won't be an issue. There's some very good build threads on this approach here on GJ.

The thing I'd do regardless of concrete choices is to plumb in a pipe between the two columns to hold the hydraulic hose you'll need to connect the columns. Maybe something like that going to the near wall where you can mount your controls / pump if you wish to put them more remotely. It's a good chance to also bring power out to the column with a surface outlet, along with air, though that might be better done from overhead for easier installation and modification.

 

[dave*99]

The thing I'd do regardless of concrete choices is to plumb in a pipe between the two columns to hold the hydraulic hose you'll need to connect the columns.

This is likely not needed with the usual SPOA10 Rotary lift. The hydraulic hose goes overhead and the pump attaches to a column.

I do agree some nearby convenience outlets are handy.

 

[wssix99]

There's some very good build threads on this approach here on GJ.

There are also a lot of really bad and dangerous threads like this. Probably more than the very good ones!

BUT if you were to build up a structure using structural steel that linked both column bases, possibly with plates at surface level to bolt the lift to, then it won't be an issue.

This will cause cracking, too. The web of the steel beam in the concrete divides the slab and creates the same effect as a saw cut joint. It eliminates concrete cross section and cracks will radiate out from where the end of the steel beam is.

A steel frame could probably be properly engineered as to not cause a problem, but I don't understand what the value would be. It's a lot of additional expense and unnecessary risk when a plan slab is more than adequate.

The thing I'd do regardless of concrete choices is to plumb in a pipe between the two columns to hold the hydraulic hose you'll need to connect the columns.

I thought most of these lifts now universally run that hose over the top bar?

 

[ct03911]

Longez,
I had the very same initial question and plan as you.
After talking to a few very old school contractors and lots of contradictory information here I just went back to basics.
Almost every lift I could find said 4" of 3000psi.
A few called for 4 1/2" and a few said 4000psi.

To cover all eventualities I poured 6" of 4000psi with fiberglass.
There is a vapor barrier underneath and mesh in the pour.

Get the loading slip from the driver so you know for sure what is being delivered.

 

[longez]

Thanks to all who replied. I'm a mechanical engineer, and drew the free body diagram of a loaded 2 post lift. I know little of concrete, but don't understand how the dog bone connector between two footings with #5 rebar that carries from the slab across the footings and is cross tied to the rebar in the dogbone can cause cracking. ??

I can kinda' see how the discontinuity in profile from the 12" footing to the 5-1/2"slab might cause a stress riser, but when I review the free-body diagram with the load applied to the post pads and shear carried by the anchors I'm still missing something.

Maybe I should have studied civil engineering

Going to 4k psi concrete is easy - but have confirmed Rotary calls for 3000 psi.

 

[dynahoe]

settle on a lift then build the floor to manufacturers recommendations, anything else is money and time wasted.

 

[Trey T]

Thanks to all who replied. I'm a mechanical engineer, and drew the free body diagram of a loaded 2 post lift. I know little of concrete, but don't understand how the dog bone connector between two footings with #5 rebar that carries from the slab across the footings and is cross tied to the rebar in the dogbone can cause cracking. ??

I can kinda' see how the discontinuity in profile from the 12" footing to the 5-1/2"slab might cause a stress riser, but when I review the free-body diagram with the load applied to the post pads and shear carried by the anchors I'm still missing something.

Maybe I should have studied civil engineering

Going to 4k psi concrete is easy - but have confirmed Rotary calls for 3000 psi.

When we talk about concrete strength, we need to call for the curing duration (#day). Typical off the shelf concrete is 4000psi @ 28days and 3000psi @ 7days of curing.

Rotary calls for minimum of 3000psi but it does not specify the curing day, if it doesn't you should assume that it's 3000psi @ 7days.

3000psi vs. 4000psi is like the AC electrical voltage 110, 115, or 120V; it depends on how you define it.

 

[ScD]

12 inches of concrete is overkill. 5-1/2 inches is plenty thick to support your lift. Having said that I do think it's a good idea to make the concrete 8 inches thick where the lift will be. This extra thickness isn't for added strength but to have the rebar deeper so you won't hit it when drilling the concrete for the anchor bolts. I would also recommend you make the area with thick concrete plenty big in case you decide to place the lift in a slightly different place once the building is up.

 

[6768rogues]

I used 5000 psi concrete. We use at least 3500 in a regular garage or shed.
Concrete strength is typically in pounds of compression strength after 28 days. If it is any other period of time, it will be stated (but that is very rare). Sometimes for construction to continue, concrete needs to reach a specific strength earlier than 28 days. One job had a need for 3500# after 7 days to put up shoring for the next story. The engineer specified 5000# concrete as it would be at 3500# after 7 days. He did not specify 3500# at 7 days, he specified the 28 day strength as that is what the batch plants use.

 

[wssix99]

12 inches of concrete is overkill. 5-1/2 inches is plenty thick to support your lift. Having said that I do think it's a good idea to make the concrete 8 inches thick where the lift will be. This extra thickness isn't for added strength but to have the rebar deeper so you won't hit it when drilling the concrete for the anchor bolts. I would also recommend you make the area with thick concrete plenty big in case you decide to place the lift in a slightly different place once the building is up.

Doing anything that makes the slab non-uniform will increase the likelihood that it will crack because it introduces non-uniform shrinkage stress in the slab. Thickening the slab can be eased, somewhat, by having a gradual thickening and not an abrupt change but changing the positioning of the rebar will have a large adverse impact.

I used 5000 psi concrete. We use at least 3500 in a regular garage or shed.

Another thing to consider for the OP, since they are using rebar in the whole slab -> Higher compression strength concrete builds greater shrinkage stresses, which requires more reinforcing bar to be placed to have the same control of cracking.

 

[wssix99]

Thanks to all who replied. I'm a mechanical engineer, and drew the free body diagram of a loaded 2 post lift. I know little of concrete, but don't understand how the dog bone connector between two footings with #5 rebar that carries from the slab across the footings and is cross tied to the rebar in the dogbone can cause cracking. ??

I can kinda' see how the discontinuity in profile from the 12" footing to the 5-1/2"slab might cause a stress riser, but when I review the free-body diagram with the load applied to the post pads and shear carried by the anchors I'm still missing something.

Maybe I should have studied civil engineering

Even within Civil Engineering, this stuff is fragmented. Wood, Steel, and Reinforced Concrete all behave very differently and are studied separately in a structural context. This is a pavement problem, which is a totally different animal because the structure (slab) is continuously supported by the ground. Simple statics don't apply and a lot of geotechnical voodoo gets introduced.

You don't have strength issues to worry about if you follow the slab specs from the manufacturer. The key thing you'll want to realize with your slab is that the rebar is not structural. In your slab-on-grade, its there (as one tool) to help you control cracking as the slab cures. If you want to get deeper in to the engineering of this stuff, the "ACI 360R Design of Slabs-on-Ground" by the American Concrete Institute is the place to go.


Your big complication (and why the dogbone will crack the slab) is that the cured concrete material is very different from the form it starts out as it leaves the concrete truck. As it undergoes its chemical reaction and cures, the material shrinks.


In general - If the slab were poured on a smooth sheet of glass - no problem. It would just shrink away from the four walls and be smaller than when it was first poured. However, in reality, the slab is interlocked with the base material and this friction acts to rip the slab apart as it cures. As a result, tensile stresses build up in the slab. Concrete is weak in tension and it subsequently cracks. So, we place saw cut joints every 10-12 feet in the slab so weak points are introduced. Cracks then form along these "control joints" so the end result isn't so unsightly. (Cracks can still form outside these lines - its all a game of probabilities.) When rebar is placed in the slab, it helps resist these tensile forces and allows one to place the saw cut/control joints further apart - or it can be used as extra insurance against cracks forming outside the control joints. (In some cases, with enough steel, the joints can be eliminated entirely.)

When you have a piece of steel, (like the dogbone) a penetration, or other foreign object embedded in the slab it is immobile and creates a discontinuity in the cross section of the concrete. (just as the saw cut joint does) So, as the slab cures, it divides at these points and "pulls" away from these areas, forming cracks that radiate out from them.

In situations where penetrations, pipes, steel columns, etc. can't be avoided in slabs, the saw cut/control joints are planned so that they intersect these objects.

 

[matt_i]

I would just add my opinion that the subgrade prep for the dogbone should be wrapped in vapor barrier just like you'd do on the normal floor setup., with a couple feet of lap on the plastic all around In my opinion this helps keep hydration in the concrete which is necessary for more easily achieved design strength. Let the water all leach out to either the top or the subgrade and the number of chemical reactions gets "shorted". The more reactions which can be completed, the higher the strength.

Personally I might go up to #5 rebar and hot-bend with an oxyacet torch, heating tip or small rosebud. #4 can be bent by a person with a receiver hitch or other stationary object, but with #5 I think most people of medium strength are going to come up short.

Concrete is going to be inherently weak near the corners, inside corners especially, but outside corners need consideration too. Those deserve a bent piece going as far in each perpendicular distance as can easily go.

 

[rburke65]

Just do 4,000 psi mix, and a uniform 5" thick floor, add your rebar, be aware of where your concrete saw cuts will be ( away from your lift posts) and you will be golden.

 

[longez]

Just do 4,000 psi mix, and a uniform 5" thick floor, add your rebar, be aware of where your concrete saw cuts will be ( away from your lift posts) and you will be golden.

Thanks, exactly what I will do and forego the footings.

 

[wssix99]

Thanks, exactly what I will do and forego the footings.

If you post back your maximum saw cut spacing, final thickness, and final psi of your mix, we can confirm the minimum rebar requirements.

(If your concrete company has something a combo they are used to working with, and you increase the slab thickness or concrete strength, the rebar requirements will go up.)

 

[longez]

Even within Civil Engineering, this stuff is fragmented. Wood, Steel, and Reinforced Concrete all behave very differently and are studied separately in a structural context. This is a pavement problem, which is a totally different animal because the structure (slab) is continuously supported by the ground. Simple statics don't apply and a lot of geotechnical voodoo gets introduced.

You don't have strength issues to worry about if you follow the slab specs from the manufacturer. The key thing you'll want to realize with your slab is that the rebar is not structural. In your slab-on-grade, its there (as one tool) to help you control cracking as the slab cures. If you want to get deeper in to the engineering of this stuff, the "ACI 360R Design of Slabs-on-Ground" by the American Concrete Institute is the place to go.


Your big complication (and why the dogbone will crack the slab) is that the cured concrete material is very different from the form it starts out as it leaves the concrete truck. As it undergoes its chemical reaction and cures, the material shrinks.


In general - If the slab were poured on a smooth sheet of glass - no problem. It would just shrink away from the four walls and be smaller than when it was first poured. However, in reality, the slab is interlocked with the base material and this friction acts to rip the slab apart as it cures. As a result, tensile stresses build up in the slab. Concrete is weak in tension and it subsequently cracks. So, we place saw cut joints every 10-12 feet in the slab so weak points are introduced. Cracks then form along these "control joints" so the end result isn't so unsightly. (Cracks can still form outside these lines - its all a game of probabilities.) When rebar is placed in the slab, it helps resist these tensile forces and allows one to place the saw cut/control joints further apart - or it can be used as extra insurance against cracks forming outside the control joints. (In some cases, with enough steel, the joints can be eliminated entirely.)

When you have a piece of steel, (like the dogbone) a penetration, or other foreign object embedded in the slab it is immobile and creates a discontinuity in the cross section of the concrete. (just as the saw cut joint does) So, as the slab cures, it divides at these points and "pulls" away from these areas, forming cracks that radiate out from them.

In situations where penetrations, pipes, steel columns, etc. can't be avoided in slabs, the saw cut/control joints are planned so that they intersect these objects.

Ahhh, obviously a very informed post! Thank you for helping me understand why Statics 101 doesn't apply. Like many things, what at first appears simple isn't. I'm glad I posed the questions here

 

[longez]

If you post back your maximum saw cut spacing, final thickness, and final psi of your mix, we can confirm the minimum rebar requirements.

(If your concrete company has something a combo they are used to working with, and you increase the slab thickness or concrete strength, the rebar requirements will go up.)

Will do - the wx looks good next week to pour the slab I'm in podunk Montana (pop ~2,500); there's only one concrete company here so I'm hoping they know some of what you do.

 

[Diesel Dan]

10,000lbs is really not that much weight.

X2 with what rburke65 said.

I contacted Hilti directly about placement of anchors near saw cuts, basically not an issue they are only concerned with the edge of the slab. The saw cuts are not considered an edge.

 

[wssix99]

Will do - the wx looks good next week to pour the slab I'm in podunk Montana (pop ~2,500); there's only one concrete company here so I'm hoping they know some of what you do.

All the information to spec this stuff isn't readily available to finishers. They will probably have a combo (thickness, psi, reinforcing) that works well for them - which is fine. The thing to watch out for is changing one of those variables necessitates reactions in the others, which is not usually well understood.

Before your company comes out, I'd suggest having the conversation with them about your saw cut joints, reading your lift instructions, and confirming where they need to go. I expect that you won't have much room to compromise in their placement, which will dictate what you need to do with the rest of your slab.

When we did the slabs for our house and garage, having these joints pre-planned saved 4 hours of unnecessary debate when the crews showed up (unprepared with tools and time) and ready to negotiate on taking short cuts!

 

[longez]

The weather finally warmed to 28* to allow us to pour the monolithic 40'x60' slab for my new workshop on January 6th. It's amazing the tricks the concrete guys have to pour concrete when it isn't seemingly good weather! Hot gravel, hot water, hot sand, blankets, etc. The footings are 30" deep x 18" wide, and the slab is 6" of 4kpsi with a lot of #5 rebar. No heavy footings for the 10,000# lift were included per the good advice earlier in this thread. The sonotube footings for the 8x8 columns to support the 12' wide x 75' long x 12' high "wing" on the East side (where the excavator is in the pic) of the shop will be set once we get a little further along.

This is the view looking South


...and the view looking North; quite an art to this >>





and another looking South



There will be 12'Wx 14'H door on each end, with a 12'W x10'H door on the East side opening to the wing. Ceiling height will finish at 15'10". I have 200A of power pulled from the 600A service we have on the new house. The shop is about 100 yards from the new house here >>



The Mrs is a little torqued I'm working on the shop when the house isn't quite finished, and we can't move in for 4-5 more weeks

 

[wssix99]

WOW! Very nice!

 

[Corsair4360]

Looking good longez. May I assume you are at least into airplanes, longez is a famous Rutan designed canard rear engined craft. I am in northern Utah, near Idaho, and planning on building a new shop / garage this year, assuming I can get through all the city / state nonsense. This thread has been helpful to me on the shop floor, thanks.

 

[lakeroadster]

One question I have always had for folks using rebar & a anchored lift: How do you prevent drilling into the rebar when you drill for the two post lift anchors?



Lay the rebar out so it isn't in the area of the posts?

 

[longez]

One question I have always had for folks using rebar & a anchored lift: How do you prevent drilling into the rebar when you drill for the two post lift anchors?



Lay the rebar out so it isn't in the area of the posts?

I wondered the same thing, and decided I couldn't figure this out ahead of time and will take my lumps if necessary when installing the lift.

Yes, I've built 3 Rutan LongEZ's over the last 35 years The new shop will allow me to build a Vans RV-14 without banishing the cars out of the new 3-car garage.

longez

 

[wssix99]

One question I have always had for folks using rebar & a anchored lift: How do you prevent drilling into the rebar when you drill for the two post lift anchors?

With a good hammer drill and the proper bit, you may not even notice if you hit rebar. After building my concrete house, I've done this several times and using a larger bit on the #4 and #5 rebar, like we'd have in a slab, is nothing. The larger rebar and using smaller drill bits is a little less "fun." When I am ready to drill for my lifts, I will be investing in a brand new bit. (for reinforced concrete)

Shameless plug - My Milwaukee 5262-21 Rotary Hammer has been a champ and the best investment ever. https://www.milwaukeetool.com/power-tools/corded/5262-21 When we are done with the house, I'm getting another one and my original will be gold plated and then put on the mantle for display. (BTW - the Mrs. feels the same way about it.)

 

[wssix99]

BTW - The rebar is non-structural and just there for crack control, so cutting in to it at a single point isn't that big of a deal.

 

[lakeroadster]

BTW - The rebar is non-structural and just there for crack control, so cutting in to it at a single point isn't that big of a deal.

I wasn't worried about the rebar itself. I've never hit steel when drilling concrete with a concrete bit. With the "typical" concrete bits I've used, I do believe they wouldn't come close to drilling through steel.

With a good hammer drill and the proper bit, you may not even notice if you hit rebar.

What bit are you using?

 

[applevalleyjack]

delete

 

[wssix99]

What bit are you using?

Just the stuff on the shelf at the "Orange Satan": http://www.homedepot.com/p/Bosch-1-2-in-x-4-in-x-6-in-SDS-Plus-Hammer-Drill-Bit-HCFC2081/202242773

The Bosch bits on the peg hooks here come in two flavors, for the SDS+ bits that fit my drill. One type for "regular" concrete and another for "reinforced" concrete (with rebar). I've been sure to get the reinforced concrete bits, which (if I recall correctly) have an extra little spiral in the bit.

Anchoring in places where I hit rebar has been nice, actually. The anchors always really bite well and right away in those locations!

 

[lakeroadster]

wssix99... good to know. Thanks