Two post lift concrete mistake

[C-S-H]

The practices that apply to a slab on grade are different than for elevated structures, beams, columns, etc. The reinforcing on a continuously supported slab is completely different than other simply supported concrete.

I wasn't necessarily talking about elevated structural elements. A structural slab-on-grade must be designed for the loading it will receive, the same as for an elevated slab. A slab doesn't care whether it has distributed loads on the bottom and point and line loads on the top (on-ground), or distributed loads on the top and point and line loads on its bottom (elevated).

Slabs on grade also have a unique issue where they experience tension across their entire area due to shrinkage. These forces are concentrated in the middle of the slab and complicated by penetrations, features like reentrant corners, and inconsistencies in the cross section. (If not accounted for by proper isolation and control joint design, uncontrolled cracking occurs.)

The shrinkage stresses from ground restraint are not damaging unless poor practices make it so. The restraint from the temperature/shrinkage steel is of minor concern, too. Concrete stresses from temperature changes can be large, and cause cracking. This is where points of restraint need to be softened, and concrete mix designs made sufficiently cool.

 

[C-S-H]

I like the idea of a ceiling supported two-post lift. But it needs proper engineering design. And then the lift will be more of a custom design instead of a generic production design, and may be solving a problem that does not exist.

 

[Hobby_Man22]

Yeah I just had a 2post 10k lift installed and was worried about all the things I have heard about horror stories of lift installs... My Situation is 5" 3500 PSI Concrete with mesh and I have been put at ease by the Lift Manufacturer's engineers and the installer on several separate conversations. You are fine just get it installed and enjoy.

That's what my slab is. I did 5" because most lifts required 4-1/4" thick concrete. Mine has actual rebar like every 16" I think. 3500psi

 

[wssix99]

A structural slab-on-grade

There is no such thing. Reinforcing of a thin slab sitting on the ground is not structural and does not/can not resist loads.

The shrinkage stresses from ground restraint are not damaging unless poor practices make it so. The restraint from the temperature/shrinkage steel is of minor concern, too. Concrete stresses from temperature changes can be large, and cause cracking. This is where points of restraint need to be softened, and concrete mix designs made sufficiently cool.

None of this is incorrect. If you would like an intro on the mechanics of how this works, you can read up here: https://files.engineering.com/downl...e-4d16-a187-c2c672890a53&file=ACI_360R-06.pdf

I like the idea of a ceiling supported two-post lift. But it needs proper engineering design. And then the lift will be more of a custom design instead of a generic production design, and may be solving a problem that does not exist.

Maybe you should reconsider. Doing this creates two pinned joints at either end of the post. This would be caveman engineering. Something has to be free to move. ^ In installations where people are doing this, the ceiling structure is moving. <- Even a little bit of movement is bad. (Outside of a domed stadium with a removable roof and seismic features, I've never seen a ceiling structure designed to handle lateral movement.)

 

[dave*99]

Doing this creates two pinned joints at either end of the post. This would be caveman engineering. Something has to be free to move. ^ In installations where people are doing this, the ceiling structure is moving. <- Even a little bit of movement is bad.

Interesting thanks for that. It seemed to me that when the roof moves laterally from wind etc. that load transfers through the lift.

In looking at my 2 post lift, I see bolts in oval slots connecting the post top extensions to the main posts. Slots are horizontal. That assembly is there to take vertical loads from the lift cables. Not lateral loads.

The roof lateral movement from wind etc. is essentially worrying that connection back and forth.

 

[wssix99]

Interesting thanks for that. It seemed to me that when the roof moves laterally from wind etc. that load transfers through the lift.

A properly designed building would take wind loads through cross bracing or sheathing. If a lift post were to be attached to the ceiling, it wouldn't do anything (as a 2 pinned joint) unless bracing were to be added. ... But at that point, the lift post would be moving with the wind, which would be defeating the whole intended purpose of attaching in to the ceiling.

 

[firebirdparts]

I like the idea of a ceiling supported two-post lift. But it needs proper engineering design. And then the lift will be more of a custom design instead of a generic production design, and may be solving a problem that does not exist.

You'd certainly have plenty of leverage there on the lift, but of course not so much strength in most buildings up above. I am definitely not above tying the lift to something. I don't think it's a bad idea at all.

 

[Hobby_Man22]

Most wire mesh lays in the dirt anyways because they don't use stands or pull it up

 

[C-S-H]

There is no such thing. Reinforcing of a thin slab sitting on the ground is not structural and does not/can not resist loads.

Of course there is. I design structural slabs for up to 90 ton wheel loads, including running over the edges, and for loads from all kinds of structural attachments.

None of this is incorrect. If you would like an intro on the mechanics of how this works, you can read up here: https://files.engineering.com/downl...e-4d16-a187-c2c672890a53&file=ACI_360R-06.pdf

Thanks. See Section 2.2.4 and Chapter 11 on Structural Slabs-on-Ground.

Doing this creates two pinned joints at either end of the post. This would be caveman engineering. Something has to be free to move.

No it doesn't. If the ceiling is designed for the lift forces, great. Otherwise don't do it.

 

[pa2000xxxx]

Made the first large mistake in my build of the garage. Could be a costly one to correct.

New slab poured yesterday. It is 4.5in or 4000psi concrete with an extra area of 3.5ftx3.5ft that is 8in deep where the 2 post lift is supposed to go. It seems I forgot to tell the contractor those areas needed rebar. I unfortuantely only have wiremess in the entire slab now.

The plan is to install a BendPak 10APX lift. https://www.bendpak.com/10apx/ It does clearly state #6 rebar is needed. My mistake on this one. I should have sent the lift specs to the contractor so that nothing was missed.

A call to BendPak actually yielded unsuspecting answer though. The gentleman I spoke with said it should be fine although they still suggest rebar. So my question to those here, am I playing with fire by not cutting out a section of the brand new slab and having two pads poured to the BendPak existing concrete install specs with rebar?

I see post is 2 years old, any updates? U still alive? my situation is almost almost exactly same...fresh pour, 7"/4500 psi at column mount, 4" everywhere else, monolith pour, but misread BendPak concrete requirement and didn't use rebar

 

[Beemer]

Unfortunately concrete has very low tensile strength and that relates to bending conditions too.

Two post lifts bother me due to overturning that results bending in the slab or footing so I'd say that would be driving the rebar recommendations. Rebar is highly effective in resisting tension (that results from the bending) so it is the complement to balance the compressive strength of concrete making for an overall solution.

That same overturning of the lift posts in an unbalanced loading condition would render a single sonotube solution not a great relief to my concerns as the problem could be more than just vertical load. That is possibly unless it's a hugh pier like some caissons I once saw under a large building.

I don't walk under loaded two post lifts unless I know what supports them.

 

[wssix99]

I see post is 2 years old, any updates? U still alive? my situation is almost almost exactly same...fresh pour, 7"/4500 psi at column mount, 4" everywhere else, monolith pour, but misread BendPak concrete requirement and didn't use rebar

Read the thread from the beginning. All your questions and concerns have already been addressed here.

 

[pa2000xxxx]

Read the thread from the beginning. All your questions and concerns have already been addressed here.

I'm asking for an update from original poster. That's not in thread!

 

[Jetfixr320]

I'm asking for an update from original poster. That's not in thread!

Can you place the lift in another area to negate the lack of rebar? My Atlas lift just recommended 4" concrete minimum.
The only reason they required rebar was cutting out a area if the concrete didn't meet minimums.

 

[alwaysFlOoReD]

I'm on gravel with a wood floor

I did add a croos piece to the top.

 

[dave*99]

I'm on gravel with a wood floor

I did add a croos piece to the top.

Is your life insurance paid up?

 

[alwaysFlOoReD]

I take it you didn't look at the pictures of the jack stands.

 

[wssix99]

I'm on gravel with a wood floor

... and you know that its so suspect that you need to use jack stands. This is good and we should all do it. Its absolutely essential in this case because you are trusting your life to balance of the load and bolted pieces of wood.

 

[racecougar]

Dumb ways to die...