Welcome to the The Garage Journal Board forums.

You are currently viewing our boards as a guest which gives you limited access to view most discussions and access our other features. By joining our free community you will have access to post topics, communicate privately with other members (PM), respond to polls, upload content and access many other special features. Registration is fast, simple and absolutely free so please, join our community today!

If you have any problems with the registration process or your account login, please contact contact us.

Go Back   The Garage Journal Board > The Garage > General Garage Discussion

Sponsored Links (Register now to hide all advertisements)

Reply
 
Thread Tools Search this Thread Display Modes
Old 05-23-2012, 09:13 PM   #1
samert111
Senior Member
 
Join Date: Feb 2010
Location: Rockford, Mi
Posts: 174
Default Concrete too thin for new 2 post lift.

I was one of the buyers of the 24 or so Rotary lifts out of Canton, Mi that the deal fell thru recently and after talking to the installer that Frank recommended to me, he came across a Forward DP97A - 9000 lb lift that he was going to remove from a Walmart in Southwest Mi. So I bought it based on a few photos, no $$ down. Its 12 years old like the Rotarys that Frank was going to get and apparently Walmart did not let their employees lift a vehicle more than 3ft off the floor for safety reasons I guess, so this lift has never had a vehicle up high. I now know this to be true because they had attached some mounting brackets for air lines or whatever about halfway up the columns and the self tapping screws went thru the column and would have been in the path of carriage if it went up that high.

So they brought it out today to install it and it looked real good. Well after drilling a test hole in the concrete it seems my concrete guy from 15 years ago F@&Ked me and only gave me 3 instead of the 4 I wanted, so the lift is now sitting off to the side of my building and I need to cut out and replace some concrete I guess.

I have a couple options and was wondering what would be the best way to go strength wise.
1. Cut out a 4 x 4 section where each column would go and pour a replacement concrete column 2 ft deep with underpinning to the surrounding old concrete floor.
2. Cut out a 7 ft x 14 ft section and pour a 6 deep replacement pad with underpinning to the surrounding old concrete floor? The reason I would go 7 ft wide is that would get me to a current crack control joint and also eliminate an uncontrolled crack in the middle of the floor in this area.

The amount of new concrete and overall cost is about the same for both options but Im a little concerned with the stability of the 4 x 4 x 2 column over the larger overall larger replacement pad. I was told drilling and pinning into the old slab is also a good idea but the guy that quoted cutting out the old concrete said that he was concerned it would compromise the strength of the old concrete due to it being only 3 thick.

So what say you all, especially you concrete guys?
samert111 is offline   Reply With Quote
Old 05-23-2012, 09:23 PM   #2
pattenp
Senior Member
 
pattenp's Avatar
 
Join Date: Jun 2008
Location: Virginia - USA
Posts: 5,830
Default Re: Concrete too thin for new 2 post lift.

All I can say is the instructions for my 2 post said to use a replacement concrete pad 166" x 47" x 12" thick keyed into the original slab. So I think the 7' x 14' is best.
pattenp is offline   Reply With Quote
Sponsored Links (Register now to hide all advertisements)
Old 05-23-2012, 09:24 PM   #3
64dragnwagon
Senior Member
 
64dragnwagon's Avatar
 
Join Date: Sep 2006
Location: Northeastern Tennessee
Posts: 462
Default Re: Concrete too thin for new 2 post lift.

My first thought is 2' deep is way overkill and unnecessary. It would help if you could include some pics of the saw cut and existing crack. I would still pin to the old concrete but drill the bar holes closer to the bottom of the slab than the top. The more sq footage you pour the safer I would feel. I called Bendpak for my lift before I poured the concrete and they said anything over 4" was unnecessary. I went about 8" deep around the post anyway, it just made me feel safer.
__________________
Here is a link to my build thread

http://garagejournal.com/forum/showthread.php?t=138158
64dragnwagon is offline   Reply With Quote
Old 05-23-2012, 09:27 PM   #4
64dragnwagon
Senior Member
 
64dragnwagon's Avatar
 
Join Date: Sep 2006
Location: Northeastern Tennessee
Posts: 462
Default Re: Concrete too thin for new 2 post lift.

And the longer the pad front to back is better. Most of the movement comes that direction and very little side to side force.
__________________
Here is a link to my build thread

http://garagejournal.com/forum/showthread.php?t=138158
64dragnwagon is offline   Reply With Quote
Old 05-23-2012, 09:27 PM   #5
Nighttrain
Senior Member
 
Nighttrain's Avatar
 
Join Date: Aug 2009
Location: Dripping Springs, Tx
Posts: 2,138
Default Re: Concrete too thin for new 2 post lift.

I would say go big. Pinning it to the 3" pad I would say would not give it much strength at all. I think any front to back movment would come right through the 3" cement. But I am also very nervous with the two post lifts.
__________________
My Garage Buildhttp://www.garagejournal.com/forum/s...301#post569301
Nighttrain is offline   Reply With Quote
Old 05-23-2012, 09:29 PM   #6
Nighttrain
Senior Member
 
Nighttrain's Avatar
 
Join Date: Aug 2009
Location: Dripping Springs, Tx
Posts: 2,138
Default Re: Concrete too thin for new 2 post lift.

Quote:
Originally Posted by 64dragnwagon View Post
My first thought is 2' deep is way overkill and unnecessary. It would help if you could include some pics of the saw cut and existing crack. I would still pin to the old concrete but drill the bar holes closer to the bottom of the slab than the top. The more sq footage you pour the safer I would feel. I called Bendpak for my lift before I poured the concrete and they said anything over 4" was unnecessary. I went about 8" deep around the post anyway, it just made me feel safer.

I would say thats a sales pitch. Look at how many sales they would loose if they said 6" is required.
__________________
My Garage Buildhttp://www.garagejournal.com/forum/s...301#post569301
Nighttrain is offline   Reply With Quote
Old 05-23-2012, 09:31 PM   #7
tlmartin84
Senior Member
 
Join Date: Apr 2012
Location: West Virginia
Posts: 225
Default Re: Concrete too thin for new 2 post lift.

What kind of reinforcement was in it?
tlmartin84 is offline   Reply With Quote
Old 05-23-2012, 09:33 PM   #8
OldmanB
Senior Member
 
OldmanB's Avatar
 
Join Date: Apr 2012
Location: Charlottetown,PE.Canada
Posts: 452
Default Re: Concrete too thin for new 2 post lift.

I had similar problems a few weeks ago with my slab. It was suppose to be 4",turned out as thin as 2" in places. I cut 4'x4' out, dug and underpinned the old, poured 5000psi, 10" thick with rebar and new wire mesh. I have using it for a couple of weeks now, had a 6500lb. truck on and not a problem in the world!
Brian
OldmanB is offline   Reply With Quote
Old 05-23-2012, 09:36 PM   #9
tlmartin84
Senior Member
 
Join Date: Apr 2012
Location: West Virginia
Posts: 225
Default Re: Concrete too thin for new 2 post lift.

Quote:
Originally Posted by 64dragnwagon View Post
And the longer the pad front to back is better. Most of the movement comes that direction and very little side to side force.
Not to contradict you but if the car is centered on the lift, the moment occurs where the lift contacts the car (between the posts) not towards the front and rear of posts. you have a downward acting force between the posts and and uplift to the outside if that makes since.

If there is rebar in the floor I would not cut it out. I would saw cut the area and jackhammer it out leaving the rebar intact in the 3" pad. Then dig down and repour.
tlmartin84 is offline   Reply With Quote
Old 05-23-2012, 09:39 PM   #10
tlmartin84
Senior Member
 
Join Date: Apr 2012
Location: West Virginia
Posts: 225
Default Re: Concrete too thin for new 2 post lift.

If I knew the 3" had rebar in it, and the subgrade was compacted well, I would be tempted to not do anything.......
tlmartin84 is offline   Reply With Quote
Old 05-23-2012, 09:40 PM   #11
64dragnwagon
Senior Member
 
64dragnwagon's Avatar
 
Join Date: Sep 2006
Location: Northeastern Tennessee
Posts: 462
Default Re: Concrete too thin for new 2 post lift.

Quote:
Originally Posted by tlmartin84 View Post
What kind of reinforcement was in it?
Wire mesh with 4,000 psi concrete. I didn't want to put rebar in it because I was still not sure on the EXACT placement of the bolt holes and if you hit a rebar with a masonry bit you won't be able to drill through it. It was part of a complete new pour and therefore was all tied together.
__________________
Here is a link to my build thread

http://garagejournal.com/forum/showthread.php?t=138158
64dragnwagon is offline   Reply With Quote
Old 05-23-2012, 09:43 PM   #12
64dragnwagon
Senior Member
 
64dragnwagon's Avatar
 
Join Date: Sep 2006
Location: Northeastern Tennessee
Posts: 462
Default Re: Concrete too thin for new 2 post lift.

Quote:
Originally Posted by tlmartin84 View Post
Not to contradict you but if the car is centered on the lift, the moment occurs where the lift contacts the car (between the posts) not towards the front and rear of posts. you have a downward acting force between the posts and and uplift to the outside if that makes since.

If there is rebar in the floor I would not cut it out. I would saw cut the area and jackhammer it out leaving the rebar intact in the 3" pad. Then dig down and repour.
That is true but if you have a model with a horizontal bar tying the two post together as most do it will deflect most of that force. I am saying that there is MORE of the force front to back.
__________________
Here is a link to my build thread

http://garagejournal.com/forum/showthread.php?t=138158
64dragnwagon is offline   Reply With Quote
Old 05-23-2012, 10:02 PM   #13
samert111
Senior Member
 
Join Date: Feb 2010
Location: Rockford, Mi
Posts: 174
Default Re: Concrete too thin for new 2 post lift.

Lots of good questions so far and I'll try to answer them.

No rebar in current existing slab. Just fiberglass strand reinforced. It was the popular thing to do 15 years ago. Not sure what it adds to the concrete but they still offer it today. At 3" thick the installer would not put the lift in without a waiver.

The sub-grade is all sand here and it was compacted well before the original pour but I've go some concerns with subterranean digging vermin here undermining the base. Moles are everywhere and I'm killing them off as best as possible but I'm living in there environment so it's a loosing battle. The original concrete guy was suppose to pour a 12" "Rat" wall (footing) in the floor the entire perimeter of the building which is suppose to prevent these typed of little bastards from getting under the floor but I've since found out he didn't do that either.

This is a clear floor lift with a HEAVY walled square tubing top bar connecting the 2 posts so I'm not too concerned with the columns deflecting inward towards one another, just front to back.
samert111 is offline   Reply With Quote
Old 05-23-2012, 10:03 PM   #14
tlmartin84
Senior Member
 
Join Date: Apr 2012
Location: West Virginia
Posts: 225
Default Re: Concrete too thin for new 2 post lift.

Agreed, as long as there is a bar, the force should net zero.
tlmartin84 is offline   Reply With Quote
Old 05-24-2012, 07:14 AM   #15
wssix99
Senior Member
 
wssix99's Avatar
 
Join Date: Mar 2011
Location: Chicago, IL
Posts: 994
Default Re: Concrete too thin for new 2 post lift.

Quote:
Originally Posted by 64dragnwagon View Post
My first thought is 2' deep is way overkill and unnecessary.
This is not correct. Its absolutely required. A common misconception with these lifts is that the concrete is needed to support the weight of the car. In fact, the concrete slab sees no more vertical stress from the lift than it does with a car parked on it. (The car's weight is spread across the contact patches of the tires. If the lift has a base plate with the same surface area as a car's contact patches, then the bearing stress on the concrete will be the same.)

The big worry with concrete pad design for lifts is the rotational component. If you secure a lift to the pad and the weight is just slightly off-center, then the posts will induce a rotational force at their bases.

You can combat this rotational force by a deep footing, which essentially extends the lift posts underground. This resists the rotational force just as a fence post would if you tried to move it side to side. Or, you can use a (typically 4") pad. The pad creates a flat "beam" laying on the ground so the lift post and the pad make an upside-down structural "T." That concrete pad "beam" sees bending forces when the lift post tries to rotate, which is why the 4" of concrete is needed - to resist those bending forces in the concrete.


samert111 - I assume an engineer or the lift company gave you those options? If so, there's no structural difference. They are going to design the options to resist the same forces. Personally, I'd go with the deep footing as I think it would look nicer.

If you do the deep footing, I would not add any pins or rebar. If there is any settling or movement, it could crack your slab. IMO - Best to keep those two different structural systems isolated. If you do the thicker pad, I would pin the slab to tie the two together and keep a flat surface.
wssix99 is offline   Reply With Quote
Old 05-24-2012, 07:48 AM   #16
jonzer12
Senior Member
 
Join Date: Oct 2011
Posts: 164
Default Re: Concrete too thin for new 2 post lift.

You can also fab up base plate extentions out of heavy plate to extend the lifts's contact with the floor. A foot or 2 on either side great increases the pads ability to resist the bending forces as described above. I also agree with wssix99. You don't need a deep footing to support weight. When an off center weight is introduced on the lift your concern is it rocking forward or backward, in essence trying to bend or tear the floor. This is what the concrete needs to be able to deal with.
jonzer12 is offline   Reply With Quote
Old 05-24-2012, 08:11 AM   #17
samert111
Senior Member
 
Join Date: Feb 2010
Location: Rockford, Mi
Posts: 174
Default Re: Concrete too thin for new 2 post lift.

Quote:
Originally Posted by wssix99 View Post
Personally, I'd go with the deep footing as I think it would look nicer.

If you do the deep footing, I would not add any pins or rebar. If there is any settling or movement, it could crack your slab. IMO - Best to keep those two different structural systems isolated. If you do the thicker pad, I would pin the slab to tie the two together and keep a flat surface.
wssix99,
It was the lift installer that suggested the 2 separate deep concrete columns and I’ve also read where others have gone with a larger but thinner single pad pinned into the surrounding floor. Either seems to be acceptable, so I guess it comes down to what looks better since both options cost about the same.

I agree with your analysis on the deep concrete column resisting the rotational forces. But you also bring up a good point that I did not consider and that is settling or possible frost heaving of the individual columns from the rest of the floor. Here in Michigan we need to go down 40” with a frost wall or footing to get below the frost line and If I do the 2 separate concrete columns it’s possible they could move up or down independently of one another and that could cause the lift to become out of alignment and not function properly. The building is well insulated and will be heated down to 45- 50 when not in use but it’s post frame construction so there is not a frost wall around the perimeter and I suppose some ground freezing may be possible around the edges where one of the new concrete columns will be. If the lift were on 1 complete pad I’m thinking this would not be a problem.

Lastly, one of the lifts posts needs to be very close to the outside wall for alignment to the overhead door and also give me clearance to the adjacent bay on the other side. Therefore, this post will not be able to be centered in it's new concrete pad. So now I’m leaning towards the 7’ x 14’ x 6” deep pad which I think will look much better also.

Last edited by samert111; 05-24-2012 at 08:14 AM.
samert111 is offline   Reply With Quote
Old 05-24-2012, 11:50 AM   #18
samert111
Senior Member
 
Join Date: Feb 2010
Location: Rockford, Mi
Posts: 174
Default Re: Concrete too thin for new 2 post lift.

Quote:
Originally Posted by jonzer12 View Post
You can also fab up base plate extentions out of heavy plate to extend the lifts's contact with the floor. A foot or 2 on either side great increases the pads ability to resist the bending forces as described above. I also agree with wssix99. You don't need a deep footing to support weight. When an off center weight is introduced on the lift your concern is it rocking forward or backward, in essence trying to bend or tear the floor. This is what the concrete needs to be able to deal with.
I'm liking this idea more and more. I'm thinking of a 5/8" steel plate, 6"-12" wider on both sides of the original baseplate and pre-drill with the same hole pattern plus 4-5 more on the outer edge each side. Set it on the floor, put the column on top of it, align the holes up and drill the concrete. Then weld the 2 plates together around the entire perimeter. Would have twice as many anchors as original to combat the rotational forces and a bigger footprint also.

The only downside I see is the lift arms would be 5/8" up off the floor more than normal and the bigger baseplate to trip over occasionally.

This would definetly be alot less work and $$ also.
samert111 is offline   Reply With Quote
Old 05-25-2012, 09:41 AM   #19
wssix99
Senior Member
 
wssix99's Avatar
 
Join Date: Mar 2011
Location: Chicago, IL
Posts: 994
Default Re: Concrete too thin for new 2 post lift.

Quote:
Originally Posted by samert111 View Post
wssix99,
It was the lift installer that suggested the 2 separate deep concrete columns and Ive also read where others have gone with a larger but thinner single pad pinned into the surrounding floor. Either seems to be acceptable, so I guess it comes down to what looks better since both options cost about the same.
I'd suggest as an added measure of safety, reaching out to the manufacturer and see what they say. There have been other posts from folks here where the manufacturer gave them engineer-blessed options.


Quote:
Originally Posted by samert111 View Post
I did not consider and that is settling or possible frost heaving of the individual columns from the rest of the floor. Here in Michigan we need to go down 40 with a frost wall or footing to get below the frost line and If I do the 2 separate concrete columns its possible they could move up or down independently of one another and that could cause the lift to become out of alignment and not function properly. The building is well insulated and will be heated down to 45- 50 when not in use but its post frame construction so there is not a frost wall around the perimeter and I suppose some ground freezing may be possible around the edges where one of the new concrete columns will be. If the lift were on 1 complete pad Im thinking this would not be a problem.
Great point. If the posts heave, then you could end up with them becoming leaning towers if Pisa.

Another issue you have is that the soil 2" below your pad will NOT have enough bearing strength to support your lift fully loaded! So, if you loaded the lift to the full 9000 lb. capacity, it could compress the dirt below and sink. This is also not good...

The bearing strength and frost heave can both be solved by going deeper, but I would not go this way unless you had a licensed engineer bless the calculations. My (educated) guess is that a 2X2X40" caisson would be sufficient, but I'm not sure what factor of safety you'd be left with. A local engineer familiar with your soils and who does foundation designs should be able to confirm that.


Quote:
Originally Posted by samert111 View Post
Lastly, one of the lifts posts needs to be very close to the outside wall for alignment to the overhead door and also give me clearance to the adjacent bay on the other side. Therefore, this post will not be able to be centered in it's new concrete pad.
That may not be a problem, but an engineer could confirm. We have this issue all the time for foundations in the city that abut property lines. An engineer can design for this.


Quote:
Originally Posted by samert111 View Post
I'm liking this idea more and more. I'm thinking of a 5/8" steel plate, 6"-12" wider on both sides of the original baseplate and pre-drill with the same hole pattern plus 4-5 more on the outer edge each side. Set it on the floor, put the column on top of it, align the holes up and drill the concrete. Then weld the 2 plates together around the entire perimeter.
This is potentially very dangerous and could fail (causing the lift to tip) without warning. (You'd at least hear the crushing of the concrete before a pad failed. A metal plate can just "snap" on you.)

In order to resist the rotational forces, you need a "beam" that can resist both compression and tension effectively. (The concrete caisson acts as a vertical beam and the concrete pad acts as a horizontal beam.) Using a larger plate is problematic because you'd be asking the plate to behave as a beam. This can be done with a plate thick enough, but is 5/8" thick enough? I don't know, but again, you'd need the manufacturer or an engineer to confirm its safe.

Quote:
Originally Posted by samert111 View Post
Would have twice as many anchors as original to combat the rotational forces and a bigger footprint also.
Remember, the issue is the strength of the floor/beam that the lift is attached to. The stock anchors are sufficient for joining the two together.
wssix99 is offline   Reply With Quote
Old 05-25-2012, 10:55 AM   #20
dstaley
Junior Member
 
dstaley's Avatar
 
Join Date: Oct 2007
Location: Peoria, IL
Posts: 15
Send a message via Yahoo to dstaley
Default Re: Concrete too thin for new 2 post lift.

I was thinking a 1" or so thick plate, wider than the lift and at least 1/2 as long as a car would be a strong base on top of your concrete. It might be overkill, but this would be a good place for overkill. You could drill & tap the plate to accept the mounting bolts and anchor the plate to the floor. 1" of steel plate is stronger than the missing 1" of concrete.

Last edited by dstaley; 05-25-2012 at 12:36 PM.
dstaley is offline   Reply With Quote
Reply

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is On

Forum Jump

Sponsored Links (Register now to hide all advertisements)


All times are GMT -5. The time now is 11:10 PM.