Post Tension Slab on Grade Prestress Values

[Stenbrook]

I am currently working on the design of a PT slab on grade for a small PEMB structure that is 60'-0" x 100'-0". I'm using the program PTISlab3.5 to run the analysis. Under the Prestress value for the slab tendons, I have always used 50 psi, however I have heard that 75 psi is becoming more common. Does anyone know more information about this and how much of a difference this really makes??

 

[Stenbrook]

whoops, I mean in the slab not the tendons.

 

[hokie66]

Not sure about the PTI program, as I haven't used it, but doubt that amount of prestress does much good. In my experience, real, bonded, post-tensioned slabs on ground will have in the range of 300 to 400 psi after losses.

 

[rapt]

Hokie,

Agree with your numbers. They will have nothing after shortening restraint losses etc. Probably why there are so many complaints about cracking in residential PT slabs in USA! And with unbonded PT, there will be no crack control either!

 

[hokie66]

Yep. The system they use, as I understand it, is just not logical. The edges are usually heavily restrained, thus preventing prestress from getting into the slab. And when cracking occurs, the cracks can freely open because there is no bond.

 

[rapt]

Unfortunately, that is what you get when you allow the industry trying to create a market and sell a product making the rules.

And then, from what I have been told, putting pressure on others in the industry to hide the problems!

 

[dcarr82775]

I have done several in the US and they perform ok if you actually follow the requirements. Most of the ones with trouble don't follow the requirements. I do agree the ribbed foundation system makes little sense when it comes to getting the PT into the slab, but I don't see putting several 100 psi into the slab instead of 50-100(after losses) does any better.

Is it primarily a flat slab type system in the southern hemisphere, or do you also do the ribbed ones? To me the ribbed ones are the only ones that can really be stiff enough to work.

 

[Ingenuity]

It is interesting that approx 50% (on a tonnage-installed basis) of all PT constructed in the USA is SoG to residential foundations. This is 1,000's of tons of strand. And it has been that way for decades. Definitely market and cost driven. From a technical standpoint it is very emperical "design" method, with design definitely in inverted commas.

dcarr82775: in the Southern Hemisphere, residential foundations are very, very seldom constructed with PT. In Australia specifically, where UNbonded PT is not permitted for suspended framing systems, SOG PT is usually only used on large warehouse and intermodal slabs, uniform slab thickness, and bonded PT.

 

[hokie66]

dcarr,
What Ingenuity said, as to PT slabs. We do use a lot of ribbed slabs in residential work, with conventional reinforcing, and they work.