Flexural Post-Tensioned Beam Capacity while under Tension 1

[azogr]

I am looking for suggestions on the best way to determine the remaining flexural capacity of a post-tensioned beam under various tension loads. Would you just assign certain strands to resisting the tension load and others to resisting the flexural bending? Maybe reduce the Aps*fps component by the amount needed to resist the tension when calculating Mn? Just curious how others are going about this. I can't seem to find any information on post-tensioned beams simultaneously in flexure and tension. Thanks in advance.

 

[KootK]

Would you just assign certain strands to resisting the tension load and others to resisting the flexural bending? Maybe reduce the Aps*fps component by the amount needed to resist the tension when calculating Mn?

While I don't think that either of these methods is 100% correct, I suspect that they would generally be conservative and sufficiently accurate for typical cases where the tension force would be small relative to the precompression.

Without knowing the particulars of your case, I'd recommend something like the method illustrated below where a greater degree of accuracy is required. And, of course, you might as well nail it if it's something that you'll be automating anyhow.

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.

 

[rapt]

azogr

As the strands are at a varying d from the surface and the tension force would normally be applied at the plastic centroid in tension or at the neutral axis, it is not logical to simply remove strands to allow for it.

A strain compatibility analysis as Kootk has shown would be the best solution, but the prestress stress will not necessarily be fpy as he has shown. It will depend on the strain at the depth of the prestress if bonded or whatever unbonded stress relationship you are using for unbonded.

 

[KootK]

Agreed regarding fps vs fpy

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.

 

[azogr]

Gents, thank you for taking the time to respond. This makes sense and should work well for automating my existing spreadsheet. The members I am working with will typically be resisting an equal share of tension and flexure. They will have unbonded straight tendons placed symmetrically both above and below the neutral axis. KootK, are you utilizing an unfactored tension load when using this approach?

 

[KootK]

I was thinking to factor the tension load.

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.

 

[azogr]

I would agree.

http://www.eng-tips.com/

 

[azogr]

oops, fat fingered something.

 

[IDS]

The procedure is the same for axial tension, zero axial load, or axial compression.

With the compression face at the specified ultimate axial strain, find the NA depth where the resultant axial force is equal to the applied axial force, then take moments about the centroid (or any other reference axis).

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[azogr]

So the tension load should be considered when calculating the area of the compression stress block as well? For instance the equilibrium equation would be modified to 0.85f'c*b*a = Aps*fps + As*fy - As'*fy' - Tu when under a tension load (Tu).

 

[KootK]

Yep.

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.