Two-Way Unbonded PT Slabs - Rebar Placement Constructability 1

[Seppe]

Hi all. Question regarding the field interplay of PT tendons and accompanying mild reinforcement.

For two-way unbonded flat plate construction in the vicinity of isolated columns/supports, my understanding has always been that banded tendons maintain maximum drape, and that with the exception of the line of "integrity" distributed tendons aligned directly through the column and right under the banded tendons, all other distributed tendons achieve maximum drape as well. This is conceptually and spatially very clean and easy to achieve.

However, when mild reinforcement is introduced, it seems to me that something's got to give, and in my opinion it really shouldn't be the PT whose positioning is adjusted given the comparative sensitivity of active reinforcement vs. passive reinforcement.

To illustrate, the top rebar running in the banded direction should ideally maintain the same clear cover as that associated with / established by the maximum drape of the banded PT directly over the column. However, for this to happen, the distributed tendons at "max drape" (just on the outer boundary of the column location) would need to tuck under the straightline rebar.

How do you folks account for this aspect of constructability in your design, if at all?

Do you maintain maximum drape of distributed PT in your calculations, and then assume a slight "fall" in the banded rebar by increasing your rebar clear cover dimension by an an extra 0.5" increment?
Do you maintain banded rebar positioning and in turn reduce drape of the affected distributed PT?
Or do you disregard altogether the spatial collision of distributed PT and banded direction rebar, discounting the effect of the rebar dive-down based on the knowledge that negative moment magnitude is significantly reduced at that location?

Perhaps this is a trivial consideration. Nonetheless, I would like to hear your feedback. Thanks, folks.

 

[KootK]

something's got to give, and in my opinion it really shouldn't be the PT

I think that it should be the PT that gives. Constructabiity, sort of by definition, trumps all. Always and forever.

How do you folks account for this aspect of constructability in your design, if at all?

In design and detailing, I consider the lane/plenum in which the PT can exist to be the space between the top of the highest bottom bars and the bottom of the lowest top bars.

Do you maintain maximum drape of distributed PT in your calculations, and then assume a slight "fall" in the banded rebar by increasing your rebar clear cover dimension by an an extra 0.5" increment?

I will avoid, like the plague, anything that will result in PT cable having to be woven through other stuff, be it rebar or other cable. Sometimes, with wide PT bands, the distributed cables do get woven through the band. I've seen it result in tight radiused kinks in the tendons that are surely much more of a detriment than the loss of a little drape. I wish I could find some of my old photos of some weaves. They would make your bile rise.

Do you maintain banded rebar positioning and in turn reduce drape of the affected distributed PT?

Yup.

Or do you disregard altogether the spatial collision of distributed PT and banded direction rebar, discounting the effect of the rebar dive-down based on the knowledge that negative moment magnitude is significantly reduced at that location?

Nope. What makes you say that the negative moment demand is reduced at this location?


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]

I agree with Koot that the distributed all go to minimum cover with parallel bars in the same 1st layer and the banded tendons are immediately below with the parallel bars in the second layer.

But then I would always make the shorter span direction the banded direction and the longer span the distributed direction so this is the logical placement anyway. I think you do this in reverse in the USA!

But then I am a bonded PT man so would never do it anyway and would actually use a two way slab layout with column and middle strips in both directions, which is essentially banded in both directions with equal spaced (distributed) tendons between in both directions. And the tendons do end up being weaved but this is possible with bonded PT as the individual duct lengths are 6m and are joined together once in place. And you do not end up with Koots tight radiused kinks which are definitely a no no.

 

[hokie66]

Amen to rapt. Unbonded should be illegal, as it is in Australia.

 

[Ingenuity]

This from the world-authority on PT:

 

[rapt]

Ingenuity,

You are supposed to indicate when using sarcasm for those who are slow to pick up on it! That is why emoticons were invented!

 

[Ingenuity]

You are supposed to indicate when using sarcasm for those who are slow to pick up on it!

Edited the text to add bold pink font to the sarcastic bit...

 

[Seppe]

Helpful information, folks. Thanks.

To KootK's inquiry about reduced moment, I was simply referring to the fact that negative moment magnitude at a distance away from the column is less than that which occurs at face of column. As such, a downward variance in the clear cover of the top banded direction rebar - at the ends of the rebar that are most remote from the column - could be justified or somewhat palatable.

I've seen the "distributed tendon placement governs over top banded rebar placement" approach endorsed by Bondy and Allred for the aforementioned remote rebar ends. Yet, there are numerous other publications that explicitly state the opposite, as Ingenuity has referenced above.

I think capturing the effects of either of the above is easy enough to do on the design side of things. But, as alluded to above by Koot, it's constructability that matters at the end of the day. If contractors are going to be prone to dropping the distributed tendons wherever they occur in the extents of the top banded direction rebar, then it behooves the EOR to acknowledge this practice as part of their contract documents.

Thanks again for everyone's thoughts.