Plans to install 2-post lift - Incorrect slab poured

[skamp]

I have a post tensioned slab as well and had no issues installing my lift. I used a metal detector and my orginal photos to locate all of the PT cables and made sure all the holes were 12" or more away from any cable. Looking at your photo's it looks ike the slab is deeper than 4". I would drill a test hole (in the area without a tendon) and see what your depth is. If you have more than 4" I can't see how you would not be ok. If you have 6" you may want to use an epoxy anchor to make sure you have no issues with a wedge anchor bitting. You can also get a core sample done to detemine the exact strength if you are really that concerned. There are more low tech methods like the hammer test that will give you a ballpark.

I also taked with multiple post tension engineers and they all said cutting out a piece between the cables is fine if you needed to pour a deeper hole provided you are 12" from a tendon. The only thing they said was to not pin the new pad to the existing slab to prevent it from pulling down the slab if any settling happened from the heavier slab. Hope this helps.

Steve

 

[wssix99]

I have a post tensioned slab as well and had no issues installing my lift.

Without and engineer's blessing, this is kind of like having a pet rattlesnake and never being bit. Will it bite some day? Maybe, maybe not. If a load on a lift is perfectly balanced, then there is very little stress transferred to the slab. If that load is unbalanced, that's when the bending gets introduced to the slab. All of the design specs are detailed so that the lift will stand up in a situation where a load is badly unbalanced.

I also taked with multiple post tension engineers and they all said cutting out a piece between the cables is fine if you needed to pour a deeper hole provided you are 12" from a tendon. The only thing they said was to not pin the new pad to the existing slab to prevent it from pulling down the slab if any settling happened from the heavier slab.

That very well may be the case with the OP's slab but, unless the slab is a standardized design, only an engineer with specific knowledge of the slab can confirm. The grade beams that criss-cross the garage also need to be accounted for in addition to the slab.

 

[ConCretin]

I'm probably wrong when I say this, but I don't think those are tension cables; I think those are snap ties...used to hold the forms during the pour.
But again, I could be wrong on that.

I'll take the OP's word for it but you are right about the metal rods - they are not PT cables.

For those who haven't come across it, post tensioned residential slabs are pretty common in some parts of the country. Plastic sheathed steel cables are run between steel plates across the slab prior to placment. After the concrete has reached a predetermined strenth, you hook a special hydraulic jack to the cable and apply tension. Wedges hold the cable once you release the tool.

Drilling holes in a PT slab is serious business. Cut a cable and it will fly out and can do serious damage - plus you've lost your post tensioning.

EDIT; Sorry. I missed a whole page of responses before my post - most of this has been covered.

 

[skamp]

Without and engineer's blessing, this is kind of like having a pet rattlesnake and never being bit. Will it bite some day? Maybe, maybe not. If a load on a lift is perfectly balanced, then there is very little stress transferred to the slab. If that load is unbalanced, that's when the bending gets introduced to the slab. All of the design specs are detailed so that the lift will stand up in a situation where a load is badly unbalanced.

I am not sure why you state that. The issue with a post tension slab and a lift is not the reinforcement or strength of the concrete but the possibility of hitting a post tension cable and causing it to snap during the drilling. The PT cables serve the same purpose as rebar to provide the tension strength.

That very well may be the case with the OP's slab but, unless the slab is a standardized design, only an engineer with specific knowledge of the slab can confirm. The grade beams that criss-cross the garage also need to be accounted for in addition to the slab.

If you cut out sections in a PT slab for a lift you still need to meet the requirement of the lift vendor for a retrofit, ie. 4'x4' 12" 3000 PSI with rebar, etc. That is to support the lift and has nothing to do with the PT slab at that point. In addition it is highly advisiable to talk with the PT engineer that spec'ed the slab to make sure that those cutouts do not impact the slab for the structure.

I was lucky that I did not have to do any cutouts but after talking with the firm that built my slab they said it would not impact the slab to cutout 4x4 sections provided I was at least 12" from any tendon/beam. They also advised not to pin to the existing slab to prevent any sinking from pulling down the slab. They also mentioned that depending on the soil I may have to go down 24+" to hit solid ground as most of the time they bring in soil. These were all precautions to be safe.

Steve

 

[Kevin54]

Didn't read through every single reply, but if it was supposed to be one type, AND it is down in writing, AND if they poured something of lesser quality and you can prove it, the Contractor should eat the cost of having it ripped out and redone correctly. If he won't, then it would be that you hold monies back from him. Then it would be going to court to get things remedied.

 

[buening]

I question whether this slab was even designed by an engineer, so the OP should fill us in on that aspect as it will affect our recommendations. If it was designed by an engineer, then you MUST contact the engineer immediately to get this issue resolved. If it was designed by an engineer, then there are documents specifying the strength of concrete and he has the contractor by the balls and can make them rip it out. If it wasn't engineered and the concrete strength was a verbal request, it will be a fight to get anything done.

Much like a garage slab elsewhere (at least in my area), the slabs are not typically designed by an engineer but rather a contractor has a typical rebar size and spacing that he likes. These post tensioning strands are no different, they just take a bit more work but the end result is much better. Don't confuse these stressing stands as being structural like what is used on a bridge or suspended multistory slabs. This is a slab on grade and the strands are merely for temperature and shrinkage crack control and the strands were likely placed middepth of the slab. You don't want the strands placed outside the middepth, as this adds eccentricity to the compressive loads and will not control cracking as effectively. In fact, it could actually cause cracking if it is placed too close to either surface. A post-tensioned slab greatly reduces the control joint spacings and can often times eliminate the joints if the slab is small enough. Keep in mind there are no resistance to shrinkage cracks from the time its poured till the concrete has attained enough strength that the strands can be stressed. Proper mix design is crucial to minimizing cracking during this period.

The loads from a vehicle distributed on a slab on grade is generally a function of the slab thickness, subgrade modulus (controlled by the subgrade improvements most of the time), joint spacing, and concrete strength. Load resistance is not dependent on the reinforcement UNLESS it is a reinforced structural slab on grade, in which layers of reinforcement are placed at the top AND bottom of the slab. This is typically only used at industrial buildings. See signature for credentials.

Cutting into the slab is very serious, as cutting a duct will often cause injuries. I can't stress this enough. You should be able to track the ducts from the exposed edges of the slab as well as use a metal detector to mark the ducts. The 12" rule mentioned previously is a good rule of thumb!

 

[buening]

Here is a good reference for those building a slab with post tensioned strands: http://www.concreteconstruction.net/Images/Building Better Post-Tensioned Slabs_tcm45-356807.pdf

more good info: http://www.concreteconstruction.net...ost-tensioned-slab-on-ground-foundations.aspx

 

[wssix99]

The PT cables serve the same purpose as rebar to provide the tension strength.

In a non-tensioned, slab-on-grade situation, the reinforcement is not there for tension strength and is assumed as 0 strength in the design. It is only there to combat shrinkage cracking and is placed in the middle/neutral area of the cross-section. The stressed slab does have tension reinforcement, which is why its a different thing all together.

To your point, it does look like the tensioning is there more for the grade beams and the structure and not for the slab, itself. A slab on grade doesn't need any tension reinforcement if the cross section is deep/strong enough.