PT Two Way Slab - Distributed Tendon Drape Parallel to Walls 1

[MJC6125]

I'm wondering what the standard practice is for setting the drapes on distributed PT tendons that run parallel to a load bearing wall in a two-way elevated PT slab. See snip below for example. Let's say I have my distributed tendons running east-west on the plan shown. At the tendons associated with the support line at grid B, would you run all distributed tendons in that shaded area as straight along that wall from grids 2-to-3? Would you disregard the wall and treat your distributed tendons more like joists that are spanning to the banded tendons that will occur at grids 2 and 3; in which case you will drape the distributed tendons between grids 2 and 3? Or would you do something in between (using straight tendons for the ones closest to the wall, and using draped tendons for the ones further from the wall and before you get to the edge of the hatched region)?

 

[CURVEB]

I typically make the ones closest to the wall straight and switch to draped tendons some distance away from the wall. Not based on any particular calculation I could see something like 4 ft being a good distance to start your profile. If you wanted to get really detailed about it, I might vary the low point starting at the wall with no drape and incrementing to the maximum drape at some distance away - maybe 1/2 the wall length. The detailer might not love how many different chair heights that results in but it probably follows the slab stresses a bit better than other options.

 

[MJC6125]

Sounds good, thank you. That 4ft distance was the exact sort of thing I was looking for.

 

[KootK]

I keep the cables profiled as they are in the "field". My reasoning:

1) Simplicity of detailing.

2) I don't see that it does any harm. I don't over balance the slab so there should be no uplift at the slab-wall joint.

3) I don't believe that it changes the condition of pre-compression meaningfully. Either way, some of the pre-compression probably bleeds of into the wall due to restraint, even with the fancy slip details.

4) This is what I tend to see in other people's work, past and present.

But, yeah, I've often wondered about this myself so I don't consider my approach definitive at all.

 

[MJC6125]

Definitely like the simplicity of detailing argument if it doesn't cause any harm like you said.

 

[Deker]

I've done it both ways and haven't noticed any difference one way or the other, but I tend to lean toward a flat profile at walls. I wouldn't expect much difference in a slab that is sensibly designed.

Guidelines for the Design of Post-Tensioned Floors

 

[KootK]

There are situations where I'd employ both strategies. Banded tendons over a wallumn? I'm staying high. Distributed up against a long run of wall, I'm usually profiling. It's really this later case that I'd meant to address previously.

 

[OldDawgNewTricks]

25 years ago, I worked for a well-known U.S. structural firm where the company standard was to provide draped tendons in this situation. They also designed the wall boundary zones as columns that could support the entire gravity load without relying on the rest of the wall. The reason for this was so they could claim that they were providing a Building Frame System with "an essentially complete space frame providing support for vertical loads". Therefore, they could use an increased response modification factor, R, for their seismic design.

 

[rapt]

It would depend how you are designing/detailing the direction transverse to the wall. If you have provided top reinforcing into the wall and bottom reinforcing in the slab to allow fully for the 2way bending transverse to the wall, then horizontal.

If not, fully draped as it is detailed as a one way slab.

 

[KootK]

Excepting rapt's fine point about the detailing being generally consistent with the design, what do we see as being the drawback of using draped tendons for this situation? The only one one that I can think of is that you'd incur some curvature induced prestress losses in the tendons extraneously. And I'd not think that a big deal since, obviously, you'd have dealt with that successfully in the neighboring tendons already anyhow.

My gut feel on this is that most people gravitate towards straight tendons for the same reason that it appeals to me: less chance of the load balancing causing weird uplift effects etc at the wall. That said, as I mentioned in my previous response, I don't feel that is much of a concern for a slab that is underbalanced relative to its self weight.

Depending on the design approach / method, draping the tendons might also be more consistent with the design of the banded groups running the other way. Those, presumably, would have been designed in the 2D strip days as receiving shear dumped from all of the crossing distributed tendons. Some of that would be missing with some of the tendons run straight. That said, one would expect this to be of little significance if we're only talking about 4' worth of tendon at the ends of the bands.

 

[Just Some Nerd]

Honestly, it just looks neater to me to run the tendons flat I don't feel it serves much of a purpose being draped, and flat is the "default" in my mind. Gut instinct tells me it makes little to no difference what the tendon is doing when running along a wall of significant length, the presence of the wall itself will be what largely dictates the net outcome.

----------------------------------------------------------------------

Why yes, I do in fact have no idea what I'm talking about