Post tension slab & RC Wall

[dougantholz]

Here is my condition:
2 way plate pt parking structure. I have "I" shaped shear walls in the middle of the building with only 45' of slab beyond the walls on either end. My columns align with the wall where I would want to string my banded tendons thru the wall. Do I terminate the tendons at the face of the wall or do I string them thru the wall at the center of gravity of the slab. By the way - stringing thru doesn't necessarily mean running down the center of the wall it will mean puting cables on either side and some down the middle.

 

[dougantholz]

Bump!
Still no input?

 

[haynewp]

I am not going to be much help because I have only came across this situation one time and my practical PT experience is limited, but I will try. I had a similar situation with a one way PT garage where there was a large beam framing into the end of the wall (beam longitudinal axis was parallel to the longitudinal axis of the wall) and I decided not to stop the tendons at the end of the wall because I was afraid of the shrinkage and creep that could bust the slab around the interface to the wall. In fact, I ended up using a reinforced concrete beam instead of a PT beam at this location with connections specified to be installed later in the project phase.

Your idea of running the tendons along either face of the wall at the center of gravity sounds better than stopping all that force at the end of a large shearwall. I would think the movement of the slab in time could cause an issue at this location if you were to try and restrain the force resulting from stopping the tendons. Maybe JAE can help more when he comes back from his hiatus.


The deplorable mania of doubt exhausts me. I doubt about everything, even my doubts.
-Gustave Flaubert

 

[JAE]

Not sure of the value of my input here - but I would think you'd want to lap the tendons past the wall some distance to ensure a "soft" tie-in between the slab and walls.

Terminating your tendons, and the resulting prestress, at the same place there is an abrupt geometric change doesn't sound very good to me.

 

[rapt]

dougantholz,

Probably because more info is required to answer it properly.

Presumably the tendons need to resist a maximum negative moment at the wall so they will be at a high point there. They need to extend well past this point to transfer the force to the wall reinforcement. If the slab is extending along with the wall then I would make make the tendons continuous. They could drop down to the centre of the slab a few metres past the start of the wall.

I have seen people detail beams joining to a longitudinal wall cog the bars down at the back face of the beam. This is useless. The tension force in the bars needs to be transmitted to the wall concrete and reinforcement. Otherwise no negative moment capacity will be generated. There will simply be a big crack immediately after end of the bars. Its is not just a shrinkage/creep problem, the force in the reinforcement from flexure needs to go somewhere.

 

[FloridaPE]

Your concept or continuing the banded strands along the axis of the wall is correct. Think of it as a long column and keept the strand pattern as if it were passing through a column.

I recommend keeping the strand pattern at the same height until you are clear the support. When checking the strand before placing concrete, it is important that the strand not have a dip in the flat profile. Otherwise the upward force could pop off a piece of slab. This sometimes happens on slab bottoms when there is a hump in the flat area of the strand. When the force is applied it will blow out the bottom of the slab. It's scary, but you can grout around the strand, patch the slab, and keep going.

If at all possible, the shear wall should be near the center of the stessing pattern to avoid cracking in the slab. This can be accomplished with a 36" wide pour strip to separate the different slab pours.

 

[dougantholz]

Thanks Florida! That was exactly what I was looking for.