What is the difference between precompression and compression at transfer stage in Post-tension sla 2

[JDG3718]

thread507-312949 In this thread there was a talk about precompression maximum value, can anyone explain to me what is the difference between precompression and compression in the upper fiber that is checked in the transfer stage ? (ACI 318/318R-8 clause 18.4.1 states that at transfer we mus check the extrem fiber stress in compression with 0.6*f[sub]ci[/sub][sup]'[/sup] and other checks) but why do we need to make an upper bound for precompression if at transfer the code permit the extrem fiber in compression to be less than 0.6*f[sub]ci[/sub][sup]'[/sup] ? Are these two different ?

 

[rapt]

JDG3718

Precompression is P/A, not extreme fibre stress.

It is about time you read a good book on PT design. It is good that you want to learn about PT design, but this is not the forum for the level of learning you are looking for. Remember, this site is not for student questions.

 

[JDG3718]

actually I am reading a book called "prestressed concrete a fundamental approach by Edward G.Naway" but it doesn't tells the difference between the two and I was confused that if P/A is force/Area that means stress if i limit the stress in concrete to 400 PSI as recommended by the thread in the first post doesn't that mean that I actually limited the stress in extreme fiber too ? that was the main question

 

[cooperDBM]

Precompression, as used above, is the average level of prestress in the section, which can simply be taken at the centroid as P/A. In service it may be limited to a minimum and/or maximum value for various reasons, some empirical.

The maximum allowable concrete compressive stress (18.4.1/2) at the serviceability limit state is taken at the extreme compression fibre. This is checked at transfer when the concrete might not be at full strength (f'ci) and in service (f'c). It limits concrete deterioration due to micro-cracking (maybe) and excessive creep deflection.

 

[rapt]

JDG3718

Limiting the P/A to a value will affect the extreme fibre stress, but not place any limit on it. Most codes do not place a limit on it. It f you read TY Lin's book, he suggests 100psi minimum, but that is stupid. Minimum should be about 180-200psi. And no codes place a fixed limit on the maximum. In building structures, we normally try to limit to 300 for slabs to 400 psi beams simply because of restraint effects. But there is no reason not to go higher if needed, especially for transfer beams.
In bridge precast beams this can be much higher, in the order of 1500 - 2500psi.

The real design limit is the extreme fibre stress, for the reasons CooperDBM gave above. This will effectively limit the amount of prestress you can add. Long term/permasnent maximum compression stress should be limited to about .4-.45 fc.

Extreme fibre stress = P/A +- Mp/z +- Mapp/z (+/- and zt/zb depend on the extreme face being checked)

So the P/A limit is not the extreme fibre limit.