PT Design - Max and Min Average compression limits

[DEL2000]

In section 9.2 of the sixth edition of PTI's Post-tensioning manual, they have a step number 5B - Calculate Post-tensioning force using "allowable average compression". I realize that this step isn't a code enforced step of the design, but more of a rule of thumb (an unwritten rule, to be exact, but more on that later) to prevent problems from restraint from shrinking. Are there any other reasons why you should limit the average compression (P/A) on your sections?

But as for these "allowable average compression" limits, I could find no where in this manual where it spells what they should be directly. Does anybody have a reference with these values with discussions as to why they are important? How about just simple guidelines used in your respective offices?

I have a pretty good idea . . . 150 psi minimum everywhere (1 way slabs, 2 way slabs, beams), 100 psi for temperature and shrinkage tendons, where used. The minimums are pretty easy.

I guess I am mostly interested in what the maximums are roughly. I believe that slabs (both one way and two way), should be on the order of 250 to 300 psi maximum. What about beams? Can you go up to 500 psi on beams?

Long story short . . . I have a beam that works, a transfer beam supporting four stories of structure above. I am stage stressing it, its overall average precompression (just on the beam web, discounting the flange because I am lazy) is 460 to 530 psi. These beams work pretty comfortably, shear works pretty easily, doesn't require too much flexural steel, deflection is good. So I was looking at the possibility of using a shallower section, since we used a good portion of the ceiling space. The only thing that I am hesitant about is going too much higher on the P/A . . . but I can't find any concrete guidelines on where you should limit yourself too.

Thanks for any help that you can provide.

 

[mrengineer]

I have limited experience w/ PT design, but I have accumulated a stack of stuff in preparation for a PT flat plate design. Looks like the recommended max for beams is generally 350 to 400 psi, with the main concerns being restraint issues and economy.

 

[rapt]

DEL2000,

The maximum limits are all to do with restraint effects. If you calculate the restraint effects (axial prestress, shrinkage, creep and temperature) and include them in your design there is no reason why you cannot use higher axial compressions.

 

[DEL2000]

Rapt,

Thanks for your response. I had been out a few days, and just finally got back to checking on this. Anyways, this may be a dumb question (Note: I am just recently getting into PT Design, and am working under a senior engineer who has a lot of PT Experience but is from the PT Data mold of design and ignores issues such as this). Anyways, "calculating the restraint effects" is something that I don't really know how to do (and can't ask from my Senior Engineer). I haven't ever seen any discussion of that in the text books that I have . . . ?

Really, "restraint effects" seems to be a term that I don't fully comprehend the true meaning behind. Is it speaking, as an example, to the restraining effects of concrete walls and columns tied to the slab, that you take care of with different released joints and additional deformed bar reinforcing?

Sorry for the dumb questions . . . the actual analysis of beams, slabs, and PT seems to be relatively straight forward. Issues about detailing, "restraint effects," and "arbitrary" rules of thumb are where I am having the most trouble in "knowing" if I am doing things right. As always, I am more than happy to research this on my own if you know a good book that you could point me two.

 

[rapt]

DEL2000,

Yes that is what we call restraint and yes, you can temporarily provide releases to allow the PT compression to get into the member but you still have pother shortening to consider also such as creep, shrinkage, temperature change.

I vannot point you to a specific book on this. It is simple engineering logic. The more restraint that you have and the more shorteniung you expect, the more need there is to investigate the effects of it.

The problem with PT members, especially unbonded ones, is that they do not provide good crack control once the concrete cracks so it is important to provide the crack control inj onter ways.

 

[ash060]

Prestressed Concrete Structures
By Collins and Mitchell
One of the best prestressed concrete books out there.

The allowable compression limits are given in 18.4.2 of ACI 318.

 

[rapt]

ash060,

Wrong limits. He is worried about axial prestress not flexural compression.

 

[shin25]

I work for precast and prestress concrete industry, mainly in bridge design and bridge building business.

For your transfer beam, I believe there should not be any prestress limits as long as all the flexural stress limits during service time, resistance during the strength limit state are maintained per ACI-318-02 chapter 18. Also, you have to make sure that you have provided minimum reinforcements.

During design phase, you must need to account for prestress losses due to creep, shrinkage, elastic shortening, steel relaxation and friction. Since this is a staged construction the losses will be dependent on staged prestressing.

Also, the bearing systems that you are going to select for the beams should allow for movements during prestressing and also due to service time creep and shrinkage.

More over, the building members above may need to have ability to account for these movements.

Finally, if your prestressing is too less you may not be able to use some of the code shear strength equation, see ACI-318 chapter-11.