Unbonded Post-Tensioning

[abusementpark]

Here's a dumb question:

In any unbonded post-tensioned system, what is the actual physical mechanism that engages the tendons for the purposes an ultimate strength check?

In the ultimate strength check, we assume the tendons are stressed to yield, but what physical mechanism engages the tendons to increase their stress beyond the initial prestess if there is no strain compatibility between the concrete and the tendons?

 

[abusementpark]

Is it the curvature from the deflection?

 

[paddingtongreen]

If they are unbonded, they are simply an external, eccentric, compression force applied to the ends. I have no idea what load factor they get. Some of the initial tension will be lost due to the concrete strain, but will be regained when the concrete is loaded and is strained in the opposite curvature.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[lutein]

in a typical post tensioned flexural members (beam or slab), the tendons are at lower portion of beam for positive moment and at top portion of beam for negative moment. in other words, the tendons are at similar location of mild reinforcing. the unbonded tendons are stressed and engaged with the flexural members (beam or slab) from the end anchorage; the end anchorage is made out of steel wedges which keep the tendons in permanent tension. Therefore, in order for the unbonded tendons to be ineffective, the end anchorage will have to break and skip, then the tendons cannot be used for moment resistant.
otherwise, when the flexural member is experiencing moment, the tendons will contribute as Atendon*fpe.

 

[letrab]

Yes it is only the curvature from deflection

 

[doka1]

What is the reason for unbonding only certain strands in prestressed beams? More specifically, if I remember correctly, they only unbonded the final 8' of each side, 75' span...

 

[TXStructural]

Unbonding parts of tendons in precast, prestressed beams reduces the prestress in certain regions. It is essentially the same as not having those tendons in the unbonded regions - the tendons have to be placed and stressed over the entire length during casting.