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Average Precompression

Average Precompression

Average Precompression

When designing a 2-way PT flat slab, I am having a small issue with the average pre-compression stresses at an area that has been thickened.

On the attached image, i am providing about on average 140 to 160 psi of pre-compression in the slab. (8" thick slab) However, in 2 of the design sections that are within an area of a thickened slab rib (14" thick), the pre-compression falls below the min 125 psi of ACI 318 due to the increase in area. My question is if i am still ok, or if i need to add more strands. It seems like since this is localized to a thickened region that i woulc be ok, and the design sections should be based on the areas outside of the thickened slab. If not, since the depth has almost doubled from 8 to 14", I would need to almost 40% more tendons just for this small area.

I can say that since i am relatievly new to PT design, i have been looking at other engineer's designs and replicating them in my software (ADAPT) to make sure i understand the principles of design and the software. Everything checks out except this small little design, which make me believe that it is not an issue, but wanted to know other opinions.......


RE: Average Precompression

You should be fine without upping your average precompression.

Much of the original testing done on PT slabs was done at a minimum of 125 psi and that level of precompresion was assumed in the punching shear requirements of the day. Nowadays, ACI punching shear provisions account for the beneficial effects of precompression directly so the 125 psi no longer serves that purpose.

Currently, ACI uses the 125 psi as a means of ensuring a minimum level of reinforcement is provided in each design section. That reinforcement can be any combination of prestressing steel or passive steel. Most international codes present this in what I consider to be a more rational form: simply specifying the minimum quantity of steel required (accounting for differences in Fy). I'd be pretty surprised if the 125 psi doesn't go the way of the dodo bird within a few more code cycles.

To be ACI code compliant in your situation, all you need to do is supply enough passive reinforcement in the thickened area that the effective total, including the prestressed reinforcing exceeds the standard temperature and shrinkage value of 0.0018. This is the approach that I'd recommend.

Note: I coulnd't fimd the sketch that you mentioned so I had to craft my response without the benefit of that information handy. Not sure if that matters.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

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