P/S Concrete Bonded vs. Unbonded Tendons

[jibebuoy]

I have a question regarding the use of bonded or unbonded tendon equations for flexural strength (ref: ACI 318-05, section 18.7.2). I am wondering if a structure's rigidity plays into whether or not one can use the bonded equation (18-3) or not.

My structure is a wall that is built just downstream of a dam spillway and is founded on the river bottom which is basalt. The wall is about 40' tall, 10' wide and over 800' long. I am using PT tendon rock anchors to provide stability as well as strength to resist the imposed hydraulic loads that have a head differential of about 35' on either side of the wall (there is spill on one side of the wall but not the other). The defelction at the top of the wall is only about 1/32". This situation is very similar to a PT cantilevered floor slab but on a greater scale.

I am wondering if the bonded tendon equation for flexural strength (18-3) requires a certain amount of delfelction in order for you to realize the greater strength it provides as opposed to the unbonded equation, (18-4/5).

 

[shin25]

The difference between bonded and unbonded tendon is, the strain in the strands and surrounding concrete during the ultimate state is not going to be the same for unbonded condition, whereas, for bonded condition will be the same. This is why the bonded and unbonded conditions have to be considered seperately during the strength calculation.

 

[jibebuoy]

That makes sense and I understand that but my question regards one of rigidity. I have a very rigid structure and my deflection is minimal. Even with the high rigidity, is it still applicable to use the bonded flexural equation, ie: is the structure deflecting enough to transfer load along the length of the tendon?

 

[shin25]

No, that is not going to be the case.

 

[jibebuoy]

So just for clarity, you're suggesting that since there is a very small amount of deflection that the bonded tendon equation for strength is NOT applicable? f that's the case, where does the ACI code point that out? Thanks.

 

[hokie66]

I think if your tendons are bonded, you do your design based on them being bonded. The strain in the tendons is the same as in the concrete, albeit small.