Post-Tensioned Slab-On-Grade

[S R P]

Hi,

I am new to this subject, but am designing a 125*100 foot rectangular foundation on expansive soil (with post-tensioned slab on ground with ribs in Houston, TX. PTI-DC10.1-08 gives several design examples for Extreme dry, Extreme Suction profile and wet climate. Do I have to do the design for all conditions or only one? If only one condition? which and how should I select it? Please advise.

Depth to water table is 9feet. Given constant suction, velocity of moisture flow, thronwaite moisture index, effective PI (29%)

 

[hokie66]

It seems, from previous threads on this site, that the ribbed and post-tensioned solution you are talking about is only used in Texas and maybe a few other states in the US southwest. The logic escapes me, as ribs restrain shortening of the slab, so the PT can't get into the slab. Crack control with that system would be minimal if any.

 

[txeng91]

I've done a lot of reinforced slabs on grade in the area. We usually use conventional reinforcing as it provides more redundancy and therefore rely less on the contractor installing it properly. Also I have encountered a number of PT slabs that have performed poorly while doing inspections on structures with excessive movement and cracking. You should design your beams based on maximum center lift and edge lift, which are products of soil expansion and shrinkage. The geotechnical engineer should provide you with parameters to calculate this using either PTI or BRAB method. In any case you should design your beam depth and spacing to resist deflection based on the building type and the finishes being used. We usually use min 30" grade beams spaced at maximum 15' on center each way with beams lining up with return corners. On the PT slabs I have designed I have used BRAB method to obtain beam depth and reinforcement and converted it to required PT tendon. Final designs have consisted of a tendon at the top and bottom of beams and at 3'-5' on center each way in the slab, with conventional reinforcing at corners and anywhere else it that might needed to provide continuity where PT can't.

 

[hokie66]

dnlv,

Sounds like you have it right with the conventionally reinforced, ribbed slabs. I wouldn't use PT in those situations, as the crack control provided would be close to nil.

 

[rapt]

Agree entirely Hokie.

Restrained unbonded PT is not much use for crack control. Bonded PT is not much better. The PTI logic that the PT slab is much stiffer only works if the concrete is uncracked. There is little chance of that, considering the low levels of P/A used and also the restraint that reduces this effectively to zero and the section is in tension. And if it is cracked without any bonded PT, it will be much less stiff than a conventionally reinforced section of the same shape.

Have not seen the other method mentioned.

Bonded reinforcement is required at relatively close spacings.

 

[dik]

With all the work, is it not possible to use a flat plate with friction piles on the interior giving a conventionally reinforced flat plate? Depending on the loads, friction piles at a 15'x15' grid and a 6" slab on void form over?

Dik