pour strip in pt flat plate

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pour strip in pt flat plate
i am working on design of 11 story condo with 8" pt flat plate.  shape/column layout is wacky, but lets say for talking purposes that building is a 240' long trapezoid, with building width 136' on one end, 72' on the other end.  the area where i would like to place a pour strip has max column spacing approx 28'.  would you use a narrow pour strip in the middle of the bay (most likely requiring shoring to be left in place for 30 days minimum), or use a wide pour strip and cantilever from the columns at either side to the pour strip?
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what is the purpose of the pour strip? why 30 days?
dik
each half of the building will have a c.i.p. core area for stairs and elevator.  need pour strip so that elastic shortening due to post-tensioning is not restrained by the core walls.  also need to allow for shrinkage of the slab.
i'm not sure about shrinkage, but if the cores are the problem with post-tensioning, is it possible to isolate the cores, stress the slab and then do a pour strip at the cores; this might bring the casting of the pour strip to about day 14 or less.  what is the approximate total elastic shortening of the slab between the cores?
we are in the very early stages of this project, so i haven't done much number crunching.  the core areas are about 112' apart, so i would imagine that there would be about 1" of elastic shortening due to pt.  i am aware that temporarily isolating the core areas from the slab is a possibility.  my concern with that idea is doesn't that make it more difficult for the contractor with regards to stability of the structure until the core area is hard connected to the slab?  at any rate, since the configuration of the building does not readily lend itself to a building expansion joint, my gut feeling is that a pour strip somewhere near the building is a necessity.   
approx shrinkage? = (.2/4000)*112*12 approx 0.1"? assuming that the slab is stressed to approx 200 psi... and ec approx 4000 ksi?
can you sleeve/wrap your wall steel through the slab with with urethane foam? and not bother to use pour strip?
dik
oops... shortening, not shrinkage
the urethane foam idea has merit-i have also seen details using a splice sleeve that is grouted later.  so i gather that even though there is temporarily no hard connection between the core and the rest of the structure, the amount of potential movement of the structure is obviously small compared to what you would have with a pour strip, and is of little or no concern?  then all i have to do is convince myself that i don't need an e.j. due to 240' length.  (notice l775's thoughts on this in thread 507-146604).
one of the first post-tensioned slabs i did was about 250' long with no ej; with pt, i never thought it would be needed.  what i learned from that project was that i'd spec'd that the strands be tensioned for each end... first time on site, when i learned that the second stressing with poly sheaths had no elongation... and quickly allowed stressing from one end only... friction losses were nada!
dik
i am more concerned with restraint cracking due to concrete shrinkage and pt induced elastic shortening   than i am due to friction losses.  but it sounds like the 240' length is within your comfort zone based on experience.  i appreciate your thoughts on the matter.
mrengineer,
normally i would provide the pouir strip at about .2 - .25 * span from one end of the central span, where the moments are smallest. this means propping the longer slab until they are joined.
placing it at midspan will have serious effects on your deflections, as the pour strip length will be plain rc and will crack much more than the pt slab. the wide pour strip will be even worse for this. i seriously doubt that the 8" slab would work in this span with a central pour strip. then, i think 8" is probably too thin for these spans anyway, especially if the end span is 28', but the average moment design approach in aci will allow you to justify it (too bad about statics).
sleveing the bars in cores is too much trouble. pour strips around coreds are a possibility but a lot of top  steel would be needed as well as propping.
the amount of shrinkage restraint you will save will be about 20% after about 1 month. but you will still get 80% of shrinkage + 100% of temperature change + creep. depending on the temperature at the time of pouring a slab and the temperature in the building after it is finished, this could be more than the shrinkage restraint.
without the cores this shortening would not be a problem except at the bottom couple of floors, but with the cores it will be significant throughout the height of the building.
240' is longer than i would allow without an expansion joint with flexible restraints such as small columns. i would definitely have an expansion joint with stiff cores towards each end, but then i do not like unsightly cracks. you could solve it with extra untensioned reinforcement instead of the joint.