Post tension foundations - What do you think about this design?

[DOHC427]

Post tension foundations -I need some guidance..

I live in an area where we have some expansive soil conditions, so I decided to have a soil test done before building a garage.

Results came back and the engineer said ours wasn't too bad for this area (2.5 pvr) but recommended using 30" beam depth and a post tension design as a precaution. I've attached some shots of the designs.
Looking for others who have knowledge about post tensioned foundations.
Is this a normal design? Overkill? Is there another way to design?

 

[ard]

You should upload those images to THIS SITE.

Imgur doesn't play nice, we can't see them.

 

[DOHC427]

You should upload those images to THIS SITE.

Imgur doesn't play nice, we can't see them.

Thanks for the tip. I think you should be able to see them now.

 

[6768rogues]

Post tensioning is usually used where the depth of the beam is a concern. For example, a parking garage might have post tensioned beams and slabs. That would lessen the thickness of the beams and slabs, allowing cars to pass without excessive story height. The few times I have been involved with post tensioned construction, it was very expensive. Also, it is not recommended for places that undergo renovations because hitting a tensioning cable is a big no-no. I would ask what the difference in cost would be to use a standard rebar reinforced system as opposed to post tensioned, then make a choice based on ease of construction and cost.

 

[72Anthony]

Post tension slabs are typically used by production homebuilders in the Houston area to deal with the expansive clay soils. I understand the post tension slabs are more "efficient" and less expensive than traditional slabs...this is why the production builders use them. However, slabs using traditional rebar can be designed for these soil conditions.

 

[ard]

I was going to say it seems like overkill to me, but WTF do I know?

Hope ssdave sees it and comments......

 

[The Tool Tyrant]

They're used quite a lot in California too, I for one love them, I've seen new slabs with small cracks prior to tensioning, actually close back up post-tensioning.
Also great because California is the 'SO SUE ME STATE' and the vulture lawyers love to get all the HOA's to sign up for a class action lawsuit against the developer/ builder/ sub-contractor for cracks in their slabs.

 

[DOHC427]

Plan is to have a lift in the garage. So with post tension it sounds like I better know exactly where I want to put it...It seems like I would rather not have a post tension but I don’t want cracks, either.

 

[DOHC427]

Post tension slabs are typically used by production homebuilders in the Houston area to deal with the expansive clay soils. I understand the post tension slabs are more "efficient" and less expensive than traditional slabs...this is why the production builders use them. However, slabs using traditional rebar can be designed for these soil conditions.

Do you know how they build a traditional rebar slab for expansive soil conditions? More rebar? More concrete?

 

[GMCGarage]

Re: Post tension foundations -I need some guidance..

I live in an area where we have some expansive soil conditions, so I decided to have a soil test done before building a garage.

Results came back and the engineer said ours wasn't too bad for this area (2.5 pvr) but recommended using 30" beam depth and a post tension design as a precaution. I've attached some shots of the designs.
Looking for others who have knowledge about post tensioned foundations.
Is this a normal design? Overkill? Is there another way to design?

You could have the engineer redesign with rebar, but I would suspect that the cost of the additional concrete thickness will be more than the cost of the post tensioning.

 

[roguegts]

Plan is to have a lift in the garage. So with post tension it sounds like I better know exactly where I want to put it...It seems like I would rather not have a post tension but I don’t want cracks, either.

2 or 4 post lift? If you go 4 post then just don't bother bolting it down.

My house is post-tension and it's great in most respects but definitely has some shortcomings. The obvious being you need to X-Ray the damn thing if you want to drill a hole in it. Want redesign the kitchen and pull gas or water to the island? well now it's a HUGE ordeal.

If the day comes you need to do serious plumbing work under the slab

For a garage it wouldn't be my first choice, but it's a sure fire way to keep it looking great for a very long time. Me personally, i'd rebar the **** out of it, pour a 6" slab with the biggest footers you've ever seen and have no fear driving a forklift in there.

 

[K'ledgeBldr]

Plan is to have a lift in the garage. So with post tension it sounds like I better know exactly where I want to put it...It seems like I would rather not have a post tension but I don’t want cracks, either.

My knee-jerk reaction would be No; to the PTC. It's a garage. PTC for a residence obviously makes sense. However, at the same time it would probably stand to reason to weigh the costs of a PTC slab versus a rebar reinforced slab of the same strength.

 

[Moose97]

In the area of Texas I am in we have extremely expansive soils. (Eagle Ford Shale for those in the know) Upwards of 19" of upward thrust in a given year. Almost all the residential slabs I see are post tension, designed very similarly to what you've shown. They work great. I built my mother a house 12 years ago. Post tension slab. Hasn't moved a bit. No cracking. No sticking doors. The only thing we did different was widen the exterior beams a little bit (16" I think) and deepen them to about 36". We did install corner rebar as well. I think 2 #4's top and bottom, 5' each way.

 

[DOHC427]

In the area of Texas I am in we have extremely expansive soils. (Eagle Ford Shale for those in the know) Upwards of 19" of upward thrust in a given year. Almost all the residential slabs I see are post tension, designed very similarly to what you've shown. They work great. I built my mother a house 12 years ago. Post tension slab. Hasn't moved a bit. No cracking. No sticking doors. The only thing we did different was widen the exterior beams a little bit (16" I think) and deepen them to about 36". We did install corner rebar as well. I think 2 #4's top and bottom, 5' each way.

Wow! I was told about an area that had 7-9" of PVR. 19" is really extreme.
Glad to hear the Post Tensioning worked for you.

Really makes me wonder if 2.5" PVR really needs post tensioning or if an overbuilt rebar slab would really do the trick?

 

[DOHC427]

Follow up:

I’ve done a good bit of reading about the post tension foundations and building in our area. I think I like the idea of using one as long as it doesn’t get too expensive. Discovered our house has a post tension foundation. It has a nearly 700 sf attached garage that doesn’t have a single crack in the floor and the house is nearly 20 years old.

The post tension design in the drawing above calls for 30” x 10” beams and 4” thick foundation. I’m thinking about asking for a 5” thick floor if it doesn’t cost much more.
I’m also going to run this design by a 3rd party structural engineer that I know.

Here’s a good article on post tension foundations.

http://www.senecastructural.com/articles/CommonConstructionPTissues%20article.pdf

 

[benjamintmiller]

So you paid an engineer to design a slab for you, and now you're asking a bunch of internet randoms if you should take his advice?

Take his advice!

 

[DOHC427]

So you paid an engineer to design a slab for you, and now you're asking a bunch of internet randoms if you should take his advice?

Take his advice!

Benjamin Miller, I agree... That would seem to be the obvious answer.....
Are you an engineer? My Dad is an engineer and this sounded just like his advice. Lol.


The reason I’m asking on this site is because I got an initial bid to do the whole building from two contractors. The chosen contractor hired a structural engineer and I’ve never actually spoken with the engineer directly.

This is not going to be an inexpensive project so I recently decided to bite the bullet and have a soil test done. The tests came back and there were a few foundation options. The contractor immediately had the engineer create a post tension design.

It may be the right answer but I want to make sure. I have a third party engineer that I also trust but wanted to learn more and get some opinions before I questioned any of the professionals. I have gotten some amazing tips and advice from the “internet randoms” on this site. There didn’t seem to be much about post tension designs on here so I decided to share my experiences.

I was thinking that I would also share the costs involved in a standard rebar vs post tension design, too. I couldn’t much info on that anywhere.

 

[benjamintmiller]

Yes, I am an engineer, but not a civil engineer. I would still trust this engineer, and I think he would have designed a rebar slab if it were more appropriate. You could always send the results to your own engineer for review.

Did he stamp these plans, or would he if you selected this slab?

I know a little bit about post-tensioned concrete:
- It is usually stronger for a given volume than traditional concrete with rebar.
- It is very common in areas with expansive soil.
- It is often used in critical applications such as skyscrapers and bridges.
- It is usually 10-25% less expensive than a larger slab with rebar, if you can find contractors experienced in it. In my state, it is not common and would likely cost more.