post tension question 1

[bpstruct]

I don't do much post-tension design. I was thinking that the prestress force would be indicated on the PT slab plan in terms of a unit force, but I am seeing force per unit length. Is that force per pt spacing? Not sure I understand this.

 

[JAE]

Usually each cable is given a single force that is required to be provided.

 

[bpstruct]

That is what I would've expected, but I'm looking at someone else's plans and they have it listed as klf....makes no sense to me.

 

[JAE]

Do they provide a klf value along the slab edge - leaving the spacing of the cables up to the prestresser? Thus the prestressing force, F = X(klf) x spacing

 

[bpstruct]

No. It just seems wrong to me. They have it listed at each cable.

 

[BAretired]

Seems wrong to me too.

BA

 

[SethGuthrie]

Could be that they are specifying an amount of lift along the length of the tendon, to balance for example 80% of the dead load, but also not standard notation.

 

[ron9876]

That sounds like the required post tensioning force per foot in the uniform direction.

 

[hokie66]

I haven't seen it done that way, but expect that SethGuthrie is correct. The load balancing required is specified, and it is left up to the PT designer to determine everything else.

 

[cliff234]

For one-way slabs we specify how many kips/ft of post tensioning we need. For beams we specify the total force. For two-way slabs we specify kips/ft in the uniform direction and we specify the total force in the banded direction. Let the p.t. supplier figure out how many tendons are required. The number of tendons will depend on the final effective force per tendon and that value varies depending on the length of the pull, whether or not the tendons will be pulled at one or both ends, the amount of drape, shrinkage of the concrete and several other variables that the p.t. supplier will account for when figuring out the final effective force. Slab tendons are usually grouped in bundles of 2 to 4 tendons. If you specify a tendon force of 25k/ft and the effective force of the tendons is 25k then you will not get one tendon per foot. You will most likely get four tendons grouped together every four feet. If you don't do too much p.t. design then you should have someone who knows p.t. design the floor for you or supervise and train you.

 

[bpstruct]

Thanks to all. I am not designing the slab, and it is a two-way slab. I am reviewing drawings another engineer produced for a building that has some structural problems. I don't think those problems are due to the slab, but I didn't understand that method of calling out prestress.