Post-tensioning Restraint

[mrengineer]

I am involved in prelim design of multi-story condo with flat plate construction, and c.i.p. elevator/stair cores for lateral load resistance. We would like to post-tension the slab due to bay sizes as large as 30'x30'. The cores occur near each end of the building, which is a very unfavorable layout with respect to restraint forces due to post-tensioning of the flat plate. The 6th edition PTI Manual Fig 6.4 has suggested temporary release details where the slabs intersect with the walls. However, that does not seem real practical for 4 sided core areas where the slab wraps completely around the core. I would appreciate any input on how others have dealt with this problem.

 

[JAE]

Yikes...sounds like a tough issue to get around.

I'm assuming you are post-tensioning both directions and the long direction stretching from end to end (core to core) would be restrained by the cores and prohibit the natural shortening in the length of the slab.

The first blush thought that popped into my head was separating the building into two halves, each associated with one core, using a temporary "missing" pour strip (could even be one bay wide) and post-tensioning each half separately. Then come in later and tie the two together after the initial shrinkage.

You would still have to deal with some sort of diaphragm chord force element on either side of the floor width, but perhaps this could be done.

The only other idea would be to construct one core with punched out or missing walls in the relevent direction, sort of taking the guts out of its stiffness, and then filling in the walls later to provide the necessary lateral strength/stiffness.

 

[Taro]

Pour strips that wrap completely around the core can be done. Of course you would need to shore the unsupported slab edges which the contractor may not like. But I have done it before.

 

[rapt]

A pour strip across the slab half way between the cores works as long as there a few spans between the cores, at about 1/4 point in one span.

This will get the P/A into the slab but you still have the problem of restraint to shrinkage and temperature shortening as the poour strip will probably be closed after one or two weeks and there will be a lot more shortening to come. To cater for this, you will need significant reinforcement top and bottom in the slabs as well as the tendons.

If you want to do the calculations for this reinforcement, you cannot use the ACI/PTI average moment logic as checking for crack control requires you to check for the actual moments and stresses in each area of slab.