Placement sequence of tendons in 2 way post tensioned slabs

[gsmith22]

I posted this on the structural engineering forum but thought it might get a better response here.

I am interested in everyone's opinion on tendon placing options within the banded-distributed layout. Most literature supports a standard scheme of placing distributed tendons+top rebar within the effective width first, followed by all banded tendons+top rebar, finished up with all remaining distributed tendons. Would it not be easier to just place all banded tendons+top bars and then all distributed tendons+top bars? I realize this would result in less drape for the banded tendons. I have found that most designs require top bars to extend more than Ln/6-so doesn't this reduce drape in the uniform tendons in the standard scheme or do the uniform tendons placed after the banded tendons push the top bars in the banded direction down in a concave shape to maintain a larger drape since they are placed after the banded tendons+top bars?
My second question is in regards to bottom bars. I have seen recommendations for placing them uniform in the unifom tendon direction and banded in the banded tendon direction but that would leave large areas of slab without reinforcement in two directions which could be problematic for temp reinf. in the banded tendon direction, non-rectangular slabs, point loads, and other misc detailing problems. Do people use uniform layouts of bottom bars in each direction (albeit at a large spacing relative to a conventional slab)? and if so, do you reduce the drape accordingly since you can't get tendons through the bottom mat since it would be placed before any tendons?
Basically, how varied can your layout be relative to the standard information printed in PT design literature while still mainting the basic prinicples of banded-distributed to reduce tendon weaving to a minimum. Thanks in advance.

Greg

 

[boffintech]

The question: "Would it not be easier to just place all banded tendons+top bars and then all distributed tendons+top bars?"

Easier?! If you want easier try telling the engineer not to specify uniform tendon and banded line tendons at the same profile height over the support.

Worked on a job recently where the parking deck slabs were designed with banded lines+top bars lower than the uniforms tendons+top bars (drop heads at columns). After 5 levels of parking deck it switched over to condominium levels and the layout switched back to normal with banded over uniform(no drop heads).

As an inspector, one of the very first things I identify is the required orientation of the banded vs. uniform tendons. The iron worker has a 50-50 shot at getting it right and 9 times out 10 it's wrong.

 

[gsmith22]

But that is my point. Design literature tells you to specify the same eccentricity, then provide details showing that the uniform tendons within the effective width around the columns are actually tied under the banded tendons (thus not obtaining the true eccentricity noted on the drawings) and the uniform tendons out away from columns can generally obtain the stated eccentricity-assuming the top rebar in the banded direction doesn't extend much furthur than Ln/6 which I find it usually does. So, in your experience, would you find it better if people only placed all banded tendons, then all uniform tendons for several reasons:
1. simplifies placement sequence
2 simplifies column region layering by providing clear non matching eccentricities for the banded and uniform tendons
3. provides an upward reaction (banded tendons) to resist the downward reaction (uniform tendons) in areas away from the columns

The only negative I can see is that your banded tendon total drape will be reduced since they are the inner layer a the column. I feel as though the uniform tendons would have this problem in the "standard scheme" but no one addresses it. Thoughts?