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 the wall (there is spill on one side of the wall but not the other). The deflection 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 deflection in order for you to realize the greater strength it provides as opposed to the unbonded equation, (18-4/5).

 

[rapt]

jibebuoy,

If it is bonded, do it from 1st principles by strain comaptibility. Then yoiu do not need to worry about applicability of the formula. The formula is provided for those who do not know what they are doing or are too lazy to do it properly and, as you suggest, may have limitations that are not defined as do many code formulae.

Strain compatibility calculation has none of these limitations.

The unbonded formula assumes a certain amount of lengthening ofd the tendon under deflection and is also really dependent on the amount of deflkection/curvature occuring over the whole length of the tendon. It could never be used in your cae of minimal curvature.

 

[jibebuoy]

rapt

Did you mean to write 'unbonded' as your second word in the third paragraph? Just checking. Thanks.

 

[rapt]

Yes,
I did mean unbonded in the 3rd paragraph. For unboned, if there is no curvature, there is no increase. The code formula is very misleading and really only apples for normal building members where there is a lot of curvature at ultimate though the code gives no limitations.

The bonded formula (I did mean bonded here) on the other hand, assumes full curvature at ultimate with the concrete strain at .003 and determines the steel stress from this so there is at least some logic to it, though it is limited to cases where the concrete strain does reach .003.