Column (or wall) to PT slab connection

[Uzs1]

Can anyone please tell me what the difference is between traditional RC slab and post tension slab when it comes to the connection design between slab and columns or walls? (I don't know much about PT)

I have this following scenario that I am wondering what to choose and why?
Also, if the answer is dowel bars is the one, then for RC column, how do I select dowel bars in this case considering I have relative small section (350mm dia) to put dowel bars in? Thank you.

 

[centondollar]

Read about PT and you will find out which connection is most commonly used. In short, prestressing of statically indeterminate systems causes secondary (also called parasitic or hyperstatic) bending moments and shear forces, which is usually unwanted. Furthermore, if you connect the PT slab rigidly (with starter bars) to a wall or column and apply post-tensioning (prestressing after concrete has been cast and cured to sufficient strength), the walls will deflect inward.

The easiest analysis and design is given by assuming the slab to be pinned (roller on other end) to the walls or columns and to connect the walls or columns rigidly in the footing to resist lateral (wind, earthquake) loads. Another option is to use longer walls on the edges of the slabs, providing shear wall action to resist lateral loads. In both cases, a minimum amount of dowel bars (enough to resist the shear, but small enough not to prevent rotation) and a construction joint at one end of the slab and a bearing joint (e.g., concrete-concrete direct contact between slab and walls/columns, or a thin friction bearing pad) at the other end of the slab will provide the sought-after simply-supported connection.

 

[rapt]

prestressing of statically indeterminate systems causes secondary (also called parasitic or hyperstatic) bending moments and shear forces, which is usually unwanted.

Not sure why they are usually unwanted. In most cases they are beneficial and wanted.

They are the whole basis of load balancing.

To remove hyperstatic PT effects, you would have to use Concordant tendon profiles which would result in very uneconomical designs as they do not utilize the full available tendon drape for "uplift" effects or effective depth for strength calculations.

 

[rscassar]

You only want to temporarily release the vertical elements that will induce significant restraint to the slabs such as walls. If you have laid out the tendons to achieve 1.4MPa without considering the restraint for walls around the perimeter, you can easily loose some of that prestress into the walls. The 350dia circular column in your original post will not induce significant restraint and should be detailed with the typical top of column detail, bars cogging as close as practical to the top reinforcement.

 

[hokie66]

Don't forget the direction parallel to the wall. Whether prestressed or not, the wall will restrain the slab as it changes volume, so you need to provide bonded reinforcement to control cracking.

 

[Uzs1]

Thank you everyone.

Will the restraint from internal walls an issue and thus require dowel connection as well since you have been mentioning perimeter walls?

Is inwards deflection of columns due to prestressing a concern here for columns?

 

[rapt]

Uzs1,

Shortening from shrinkage and temperature variation will be about 10 times shortening due to PT unless you have extremely high P/A which is not logical in buildings anyway. So it is basically the same in RC slabs.

AS Hokie said, make sure you provide reinforcement for restrained effects for crack control. This is the main difference between RC and PT slabs with shortening. Real bars (at least #4 or #5 at maximum 8" centers instead of the very small amounts normally provided in PT slabs to control the cracking, because it is going to happen no matter what after the connection is completed.

 

[Uzs1]

Thank you.

Sorry this might sound like a stupid question but when prestressing the slab, will there be a corresponding lateral force being applied / transferred to the top of columns or walls (in addition to the volume loss due to shrinkage and temperature) and I should thus provide dowel bars instead of starter bars in my columns and internal walls?