Post-tensioning effect on lateral system

[fredPE]

Just a general question . . . I heard a contractor one time talk about doing the cladding on a 20+ story tower that had P/T Slab floors. He talked about how it was hard to get the cladding to line up from floor to floor as they went up the building, because the building would move back and forth as they applied the post-tensioning on each successive upper floor. It wasn't moving a lot, on the order of 1/4" I believe.

As I am getting into concrete p/t slab design using Ram Concept, I notice a hyperstatic lateral force in the reaction reports on shearwalls that can be very significant (depending on the layout of shearwalls, if shearwalls are not placed at the center of zero movement, for instance), and I am not quite sure what I should do with that force. Are you always supposed to design your shearwalls for the lateral force that the Post tensioning imparts to your structure? Does the post-tensioning impart a lateral force to your shearwalls, or does that lateral force get disipated somehow through restraint cracking.

 

[pappyirl]

fredPE

A lot of tall buildings are post-tensioned and the slabs do shrink once the tensioning stress has been applied. The amount of movement depends on the size of the pour, concrete shrinkage conditions etc. It seems hard to imagine a 1/4" causing problems fitting the cladding, what are the construction tolerances? The slab would not move back and forth, it would move towards the stiffest element of the building, normally a shear core.

If there is a force in your shear walls you have a restraint issue. The PT slab should be free to shrink towards one stiff location i.e. a shear core. If you have 2 shear cores at either end of the slab, one of these needs to be released via a pour strip or other release mechanism. This removes the lateral force due to the prestress, you will also have to pay attention to long term shrinkage. The alternative is to provide reinforcement to control the cracking, but this does not usually realise the full material saving benefits of PT.

 

[haynewp]

You want to avoid putting the force into your shearwalls. Look at using pour strips where the PT is applied adjacent to shear wall locations. Also need to consider creep effects on stiff lateral elements.

 

[haynewp]

I mean slip joints next to the shear walls.

 

[rapt]

fredPE,

I hope you are an experienced PT designer if you are starting to use RAM Concept or any other FEM PT program to design PT.

You should really know how to design PT before starting to use software like that.