PT Slabs on Grade

[Engrman]

I have a condo project coming up that the owner has asked about using PT slabs as the fdn. PT slabs have been used only recently here and not very often, mostly with out of state contractors. I'm in an area where the frost depth ranges from 30" to 36" and we have some highly expansive soils on the site. I have no experience using this system. If anyone out there knows of some good resources for the design and detailing for this please respond. Some questions to be answered:

A. Normal practice here is to either over-ex and replace with structural fill, then use spread ftrs, or to use grade beams and drilled piers. With PT slabs is some over-ex or soil treatment still required?

B. Is there still a requirement for perimeter frost walls or turndowns? Or does the raft slab concept eliminate those concerns?

C. The floor plans are not simple rectangles. There are many protrusions and returns. The units vary in size from duplex to 12 plex. The units will be built as sales dictate, not all at once. The builder has no experience with the system either, and is having trouble determining costs. Beyond saying they can provide design parameters, the Geotechnicals have been no help. Is there a point where economies favor one system over the other? Above and below a certain square footage or lineal foot of wall?

Any and all comments would be appreciated.

 

[Rjeffery]

pls check with

http://www.foundationperformance.org.

they will have the resources you want.

 

[RG88]

Try the Post Tensioning Institute (PTI). A search can find it on the web. Their phone number is (602)870-7540. They have a good bit of information available (for a small price) for slabs on grade, including a design software package (I haven't used their software). A phone call to them can point you in the right direction for your specific site's needs and to help you find a source for costs.

For the application I worked on, the edge thickness of the slab was sufficient for frost depth. I don't know about the frost depths you indicate. If the slab can move, you can hit a big problem with the plumbing. I wasn't working with expansive soils either, just a contractor who could do PT cheaper than conventional.

As far as details, PTI has some information in Chapter 6 of the 'Post Tensioning Manual 5th ed'. It is primarily applicable to floors as opposed to slabs, but the general information is there, anchors, tendon sag, tension head, plan view of tendons, tension call out, etc.