Analyzing Existing Post Tension Beam and Slab

[RFreund]

I am trying to analyze the capacity of an existing post tensioned one way slab and beams. I have the existing structural drawings but not the post tensioned shop drawings. The structural drawings specify a required tension strength per foot for the strands (call this value Fp). Should this force be used when solving for the moment capacity as opposed to finding the strand stress using approximate methods or strain compatibility to find fp (the strand stress then the strand force would be Fp=fp*Ap)?

Therefore using Fp the moment capacity would be:

a=Fp+Asfy/(0.85*f'c*b)

Mn=Fp*(dp-a/2)+Asfy*(dt-a/2)

Where As,fy and dt all relate to the nonstressed tension steel.

Thanks.

EIT

 

[ash060]

If it is post-tensioned slab, the tendons are probably unbonded so the fp will be different than if it is bonded. The tension on the drawings is the effective force after all the losses that number would be used to determine fse and from that the moment capacity of the section

 

[RFreund]

ash -

The drawings specify the type of strand (7 wire ASTM 416)and list the ultimate, final, transfer and over stresses. However I'm not sure how to get from a post tension force (given on the drawings) to an effective stress if I don't know how many or what size strands were used (I'm assuming they are half inch but this still does not help). I'm I missing something?

Also another side question - for unbonded tendons would the stress/strain be constant when checking strain compatibly.


EIT

 

[hokie66]

You can't calculate the capacity without knowing how much reinforcement you have.

 

[Lion06]

For the P/T, I would use the effective force listed on the drawings. The supplier would provide calcs showing that the effect force, after all losses, is not less than this.

As hokie mentioned, you'll also need the mild-steel, but that should also be on the drawings. I haven't done a ton of P/T jobs, but it my limited experience, it's usually the service level stress that controls the design, not the ultimate moment strength.

 

[RFreund]

I have attached two calculations for completeness of the thread. One is for an unbonded tendon assuming a constant tension force. The other calculation uses approximate method that ACI presents and also uses strain compatibility. For this particular project I derived the area of prestressed steel by using the limit of 0.6fpu listed on the existing drawings and the effective force required that was listed on the drawings. Feel free to comment.

Lion - I will have to look into the serviceability checks.

Thanks

EIT

 

[RFreund]

Second Calc

EIT