Post tension slab,which direction is prefered to use banded tendons? 1

[RamiHabchi93]

Hi.I have been told that when using banded tendons in a direction and distrubuted in the other it is preffered to put banded tendons in the direction of shorter spans and distributed in the other direction.I want to be sure if this is correct or it there is no general rule to choose the direction.thank you.

 

[cliff234]

Yes. That is the general rule that we use in our office. Also, we run the banded tendons under the uniform tendons at the high point over the columns. Some engineers "weave" tendons over the columns. We find that to be more complicated than necessary.

 

[dik]

Weaving is far too complicated and has little or no benefit.

Dik

 

[hokie66]

Weaving is not complicated if using bonded PT, which has great benefits.

 

[KootK]

In addition to the convenience of non-weaving, the choice of using the short span for the banded direction bears some similarity to what is done with steel framing systems. To an extent, the bands are girders and it pays to have those heavily loaded members span the shorter direction in order to keep moments and deflections under control.

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]

If you look at it logically, the shorter span direction is the stiffer and the same thickness slab in that direction is significantly stiffer than in the longer span direction, so it will tend towards the logical "support direction" with the long span being the "one way" direction. As the short span becomes shorter compared to the long span, the slab tends more and more towards one way action in the long span direction and the short span becomes more and more like a "beam" support.

The longer span direction should have least cover so if the bands are in the short span direction, then the distributed tendons would be above at the supports. If the banded tendons are in the long span direction, they should weave to be at minimum cover at the supports.

So technically, the shorter direction is the banded direction. Interestingly PTI logic is (or used to be) the reverse and it is difficult to understand why!

Even in band beam and slab systems, the band beam in the short direction is still the logical option, even though most designers have not figured this out yet and treat it like a beam and put it in the long span direction. The band beam is a slab thickening, not a beam, and the band in the short direction is more technically correct acting like a drop panel, and no more expensive. It is not until the beam is actually stiff enough to act as a real support for the slab in the other direction that it should be in the longer span direction.

Not that I would recommend anyone ever using the banded distributed logic in a flat slab, being a bonded PT person who along with Hokie prefers column/middle strip logic in both directions. If that method is used, service stresses should be kept very low (much lower than ACI suggests) to avoid unwanted cracking and extra deflection.

 

[Archie264]

Rapt,

I understand the analogous short-beam-long-joist concept but otherwise that's a lot to absorb. I'll take a break and come back to it.

I am not involved in any type of prestressed concrete design though I did take a course in it in school years ago and the topic does interest me. To that point I have followed your and Hokie66's comments over time regarding the bonded vs. non-bonded issue in post-tensioned concrete. I've found that discussion (to the extent I could follow it) very interesting and I appreciate the effort it takes on both of your parts to explain the issue. I'm going to dig out my old textbook to look into it a bit more but I know that in the US in the early days of post-tensioning the ducts were bonded. At least they were on some projects. I wonder what happened?

I could never warm up to the concept otherwise, at least, not in slabs. When people have to worry about a tradesman penetrating a duct at a later date causing the tendon to loose all it's capacity, if not shoot out the edge like a rubber band...well, I don't know if that would actually happen but in any case I'd rather have some type of more convention construction at that point.

Anyway, this is mostly just a note to say thanks to you and Hokie66 for discussing the topic and to say that even if you're not getting responses it doesn't mean you aren't being read.

 

[cliff234]

For what it's worth, we use bonded PT for transfer girders and beams with high PT forces (lots of tendons). We've never used bonded PT thin flat plates because the tendon ducts are bigger than unbonded monostrand tendons and we can't get as much of a drape as we can with unbonded PT. Also, in our neck of the woods, labor costs are high, and threading the tendons through the ducts and grouting them are two labor intensive steps that add cost.

 

[rapt]

Cliff234,

The depth difference is negligible. Flat duct for bonded PT is 19mm deep. With 15.2mm strand, you lose about 3mm.

We compared unbonded costs to bonded many years ago for Australia (we were already using bonded so everything was set up that way) and decided bonded was cheaper overall. But the unbonded strand was very expensive for us at that time.

In terms of lifetime costs for buildings from what I have heard of problems with badly done unbonded, bonded wins hands down.

 

[slickdeals]

In Chicago, they prefer to run the banded tendons in the shorter dimension (not necessarily shorter span) of the building.

 

[rapt]

I had typed a sarcastic reply to this but have deleted it so as not to offend anyone!

Apologies for thinking it in the first place.

 

[Ingenuity]

I had typed a sarcastic reply to this but have deleted it so as not to offend anyone!

Getting 'soft'...man up!

 

[KootK]

In Chicago, they prefer to run the banded tendons in the shorter dimension (not necessarily shorter span) of the building.

Interesting. Do you know the logic behind that preference?

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]

Archie264

Thanks for your comments.

There is more discussion on it in thread507-436063 on unbonded strands in transfer beams.

And yes, bonded tendons are a lot safer when someone accidentally cuts through one. With an unbonded tendon yes, you lose the tendon completely and it will shoot out the end. I have seen the same thing happen with a PT tendon that was not grouted. Ended up with the strand in a stairwell.

I also one saw someone put a 300mm diameter core drill through 3 intersecting bonded properly grouted tendons once in a rib slab. There was not detectable movement of the strand at the surface of the core hole. Which is interesting because development length logic would predict that there would be 1 - 2mm of slip with bond loss away from the cut.

 

[slickdeals]

Wonder what Rapt had typed up now.

@KootK - it has to do with them laying out cables and profiling them as they install deck. The uniforms come after and it allows them to install banded runs with partially installed decks. The only cables they have to worry about is the weaved uniforms (if they have it).

 

[rapt]

OK, So the logic is that we adopt a tendon layout that requires enormous redistribution to work under ultimate design conditions. Essentially doing a yield line design, but basing it on an elastic analysis, not a yield line analysis. But there are 2 options

1 lay the banded tendons in the short span direction and the distributed tendons in the longer span direction which will require a smaller amount of redistribution.

2 lay the banded tendons in the long span direction and the distributed tendons in the shorter span direction which will require a larger amount of redistribution.

Then we ignore the fact that this redistribution is not allowed for service stresses in both cases and will contribute to increased deflections. And also ignore the fact that in cases where the supports are not in a regular arrangement (offset support lines in one or both directions) an elastic FEM analysis is absolutely useless in predicting the way the slab will perform in its yield line solution and for good measure in a lot of cases we will ignore those pesky Mxy moments that make design more difficult, and which are more prominent in irregular column arrangements.

So which do we choose of out 2 possible solutions. Option 2 of course. So that it is even easier to build.

 

[Archie264]

Rapt, thanks for the link, now I've read through that thread as well. It certainly is a interesting topic. I think Kootk may be onto something when he suggested this might something for the code committees to consider. More so, at least, than, as he put it, adding additional load cases.

So these things are out there holding up a whole hotel? Hope the whole thing doesn't become a hole...

 

[rapt]

Archie264 and KootK,

I would not hold my breath waiting for code committees. They tend to reflect the local industry position. And the PTI will give them that position!