Post Tension Slab (P/A)

[DDRJ]

Hi,
I am relatively new in designing post tension member and as a start I am trying to study how RAPT do the design of Post tension slab.
I came across the P/A value @ transfer and at service.I was told that the limit for service is P/A = 1.2 - 1.8 MPa to control the cracking of concrete.

Where can I find this limit and How can I manually get the P/A value result of rapt.

Thanks.
(Im so grateful for your response)

 

[rowingengineer]

DDRJ,
I am worried that you are embarking on a learning excise without considering the path, a design program is not a place to start, it is a second or third stop. 1'st you need to understand what PT is all about and how it works, there are a lots of good information sources just to get you understanding what P/A is all about. Then once we understand the inputs we can start to look to software, I know rapt has a good help menu, but really it isn't there for teaching PT from the basics.

As for the easy answer, the P/A is report near the end. Read 7.3.7.3 Frame Definition and Design: Flexural Design - Service page 6 of the current downloadable help menu.

http://www.raptsoftware.com/RaptHelp.pdf

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

 

[rapt]

DDRJ,

In the Service and Transfer output results, it would be the item headed P/A!

There are no codified limits on P/A limits (I think ACI says .7MPa minimum but this is meaningless in partial prestress design which we do in Australia). As long as you design for it, you can technically use any P/A level. The values you have mentioned are recommended limits from experience.

 

[Ingenuity]

The Post Tensioning Institute of Australia have a few documents that may assist you at:

http://www.ptia.org.au/Downloads.aspx

And specifically this document may be of assistance to get your started:

http://www.ptia.org.au/Documents/Post-tensioning in Building Structures.pdf

I would purchase a good local textbook on the subject and play around with RAPT and you will soon get an appreciation of the typical basis of design, along with hopefully a good mentor you can bounce ideas off.

I am curious- Is fair to say that current undergraduate BEng programs in Australia do not teach a comprehensive subject of prestressed concrete? When I was an undergraduate I did two courses on prestresed concrete - one was a core subject and the second was elective that was somewhat advanced.

 

[DDRJ]

Thanks guys for your willingness to educate.
By the way is this P/A the compressive stress at balance condition (where A is the gross area)?
If this P/A is at service meaning I now have a different value of Area?

 

[Ingenuity]

When engineers state "P/A" as it pertains to prestressed concrete it is based upon gross (uncracked) concrete sectional area.

It is simply the effective prestress force (ie after losses) divided by sectional area.

You technically have P/A at transfer, ie after immediate losses, where the effective prestress force is at its maximum, and then in-service P/A, where the effective prestress force accounts for the long-term losses.

P/A has nothing to do with "balance condition", other than under a purely "balanced condition" where the applied loadings (eg self weight) equals the applied prestress (eg uplift due to cable curvature) such that under these service loadings you have no bending moments, and only axial compression (ie P/A).

 

[asixth]

The AS3600 values are

Minor crack control - 0.7MPa or 0.175% deformed bar.
Moderate crack control - 1.4MPa or 0.35% deformed bar.
High crack control - 3.0MPa or 0.6% deformed bar.

I try to stick to the moderate as a minimum and high level where you need to control cracking like roof slabs. Roof slabs will normally have a sheet of mesh of the whole slab to help keep the cracks tight.

 

[rapt]

Asixth,

Those rules are for the secondary direction in restrained slabs! And no one should ever use the Minor Crack Control option because it actually provides no crack control whatsoever. Even the other 2 options will provide limited crack control if only PT is provided to provide the full crack control in a restrained slab. Reinforcement at a maximum spacing of 300mm or preferably 200mm would be required for good crack control.

For the primary direction, the code recommends 75% of those values for shrinkage and temperature, but also requires strength checks and the strength minimum requirements would be significantly higher than ,75 * .7Mpa or even .7MPa. And that would be even higher for beams