Please help clarify those Post Tension concepts 1

[T2ioTD]

Please help clarify those Post Tension concepts

I have a couple of questions regarding post tension slabs, and though I'm reading and playing with FEM software, they elude me.

1. Balancing x percent of the dead load means cancelling x percent of the deflection due to the dead load? But deflection is different in each span, so which span is the target to reach?

2. Balancing x percent of the dead load means balancing the whole dead load in the x direction, then again in the y direction? Or is it balancing the part normally supported in the x direction independently of the part supported in the y direction (ie, to each his x percent)?

3. Why don't we settle for a target compression stress by the [post tension cables + DL] instead of the x percent of dead load carried? This seams to me much easier to work out 'semi manually' while using FEM software.

4. The codes specify a max compressive stress and a maximum tensile stress of the concrete when using PT. Sorry for the dumb question, but can't we always achieve those targets by adding passive rebars? I'm asking this because some FEM software don't seem to wait for the user to introduce the passive rebars, and they shout the "failed" word too quickly.

That's it. Hope I'm not asking too much... Thank you in advance.

 

[SWComposites]

Sigh. The codes are not set up to make it easy to run FEM analysis. Nor should they be. FEM is just one of the tools that can be used for design, not something one just dumps in things and presses a button and hopes to get pretty plots with no red blobs or "failed" messages.

 

[rapt]

I hope you are not using FEM software to actually teach you how to design PT!

1 The concept of "cancelling" the dead load is a dangerous one. The DL is still there and is factored for things like ultimate strength and load reversals.
And in deflection design, it depends on how you are allowing for long term deflection. If you are using a multiplier method, the concept of cancelling the deflection does not work as shrinkage deflection is not really load related (except the effect from neutral axis depth). It is a fudge method used for long term deflection calculation for RC members and should not be used with PT, though some software does use it.

2 The load balance should be consistent in both directions.

3 you can use any method you want.

4 It depends on where you are design and the code you are using and the PT approach you are using. If you are using the ACI/PTI banded/distributed tendon layout, you cannot design for Partial Prestress. So the extra reinforcement solution does not work.

if you are not allowing for column/middle strips and instead use average across the whole panel, you cannot design for Partial Prestress. So the extra reinforcement solution does not work.

If you design it properly allowing for the stress concentrations etc that you really get, then you can design using Partial Prestress.