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  • Jim Rogers

Q & A on Residential Post-tensioned Concrete Slabs

Question -

In general, the design of P/T slabs on grade is based on tensioning the strands prior to the slab being loaded. How much does it affect the design if the strands are not tensioned until after the building is framed and a majority of the design dead load is in place?

Answer -

In general...not long as there is no apparent damage due to slab/soil movement before the tendons are stressed. Best case, we would like to see the tendons stressed as soon as possible after the concrete reaches strength (2500 - 3000 psi). At that point there is typically some rough framing in place. Worst case, we would like to see the tendons stressed before the roof is loaded. Framing doesn't matter much, but once they are loading the roof that generally also means they have installed drywall and the combination of the two means that most of the dead load is in place. Absolutely worst case...they just forget and stress the tendons after the house is almost done and the slab is completely loaded. In this case, they have greatly increased the slab/subgrade friction and it may be questionable as to weather they can mobilize the slab enough under that increased load to actually apply compression across the entire slab width. The designer could actually check this by recalculating the residual compressive force in the slab and adding the design dead load of the structure to the weight of the slab in their calculation. When they run this calculation in the design phase, they are supposed to provide AT LEAST 50 psi in the middle of the slab after all factors including long and short term losses and slab/subgrade friction. If they went with the absolute minimum in their design of 50 psi, then by the time you increase the dead load, you are no longer going to have the required minimum in the center of the slab. If they used something more reasonable (in my opinion) of 100 - 125 psi, then they might be okay depending on slab size, geometry, building weight, etc. The problem with checking this after the fact as described above, is that there is very little that can be done at that point (other than tear down and start over!).

My opinion on this is... The slab will carry the design dead loads and will transmit them to the soil without the use of the post-tensioning tendons. The tendons are there to stiffen the slab in order to provide additional flexural capacity due to potential soil movement. IF the slab has not experienced any movement due to soils shrinking and swelling, then probably all you can do is stress the tendons and walk away. You will provide compression and stiffness in the outside edges of the slab, which is where the "zone of activity" (soil movement) occurs anyway. You may have very little compressive force as you move towards the center of the slab, so maybe you loose some of the advantage of keeping shrinkage cracks closed in the middle of the slab, but that's probably not that big of a deal and probably not why the designer used post-tensioning anyway. On the other hand, if the slab has experienced some distress due to soil movement, then I think it would probably need some additional analysis (I would not expect the tendons to just fix the problem at that point). Note that this opinion is valid for residential slab-on-ground foundations designed and otherwise constructed according to the "PTI design method" for expansive soils, and for wood framed houses that are "typical" production home size and shape (I would have to think about this opinion on a something like a large apartment slab that was 200' long x 50" wide for example).

{Reprinted from my old Blog - JR}

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