Post-Tensioned SOG for Large Custom Home

[AELLC]

In my area, we have moderately expansive clayey-sand soils and we use uniform thickness 7.5" thick PT slabs (not ribbed), with a 12" wide x 12" deep perimeter turndown on the exterior. Typically, we have PT tendons from 2-4" to 5-0" on center.

What I have is a very large custom home, with the plan being in a sort of "W" configuration. Obviously, it would be impossible to run tendons in an orthogonal grid system.

Would it be feasible to separate the plan with construction joints, meaning the slab would be poured in 3 or 4 different stages, with the tendons stressed at each succeeding stage, and each stage of the slab connected with #4 rebar dowels at 24" o/c? The slab would be thickened to 12" at those construction joints?

 

[JAE]

Yes feasible.
A couple of concerns:
1. With a "W" layout you would have re-entrant corners. Even if you broke the "W" up into rectangular pieces there might still be long term creep in the concrete with cracking at those re-entrant corners.
2. A lot of my old mentors never believed that PT slab-on-grade even worked like they should - the idea being that the turn downs resisted the realization of compression from the PT stressing and that the typical house wan't long enough to allow for effective prestressed compression in the concrete at the tension levels that homebuilders used. Not sure if that is still a concern today or not.
3. Any concentrated loads from interior walls all placed on a slab? That bothers me a bit - I wouldn't want my home to be like that.

 

[hokie66]

JAE highlighted one concern I have with these systems in general. The edge thickenings restrain the shortening of the slab which would otherwise occur due to the PT. If the tendons are unbonded, as I believe most of these type slabs in the US are, any cracks are then free to open. This would probably be a bigger problem in an irregular shaped slab as described here.

 

[XR250]

AELLC,

How deep does the plastic clay go? Are you in Texas?
I am in the triangle area of NC and we have those same soils but we typically just dig thru them and build a crawlspace foundation.

 

[AELLC]

JAE,

1) We use a #5 x 8'-0" at all re-entrant corners.

2) Not certain of the theory here - perhaps 1/2" Styrofoam on the inside face of turndown would help allow the turndown edge move a bit inward.
PT foundations here seem to work better than conventional footings. We don't get the uneven settling of wall footings as with conventional.
The tendon lengths vary from 10' to 78' in this house with the slab in 3 sections - would be 128' if slab was all one pour.
I specify double live ends on tendons > 80' length.

3) There is a calculation for allowable concentrated load on interior slab, based on soil K and other variables - for a value of K=90 pci, about 8000 lbs maximum. If that is exceeded, we provide an integrally cast square footing 18" thick by area required for 1250 psi bearing of the entire concentrated load.

 

[AELLC]

Excel,

This is the area known as Southeast Valley near Phoenix AZ. It was formerly agricultural fields. Some acreage still exists, mostly alfalfa and sorghum crops.

The soil is mostly sand with clay and silt. It is not nearly as bad as some areas of Texas. Most houses are still being built with conventional foundations, but PT is used more in custom homes, and in the areas of more expansive soil.

Most houses are ranch style, slab on grade. Occasionally an owner wants a basement.