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POST-TENSIONED: Calculating Friction Losses

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I have never had occasion to calculate/estimate prestress losses due to
Friction/Wobble/Anchor Seating, all of which are typically concerns for
Post-Tensioned Concrete only.

The governing equations are defined in ACI 318 Section 18.6.2. However,
I'm not absolutely sure I "get" how to apply this.

For example, my typical detail for PT slab design shows a tendon
trajectory in the slab beams that is an "S-curve" specified to occur for
a distance of two times the beam depth. Now, it seems to me that the
"alpha" term in ACI 318 Eqs. 18-1 & 18-2 would "always" be Ninety
Degrees in such a case. But that's a qualitative assessment on my part,
because I'm not sure if I completely understand the definition of "alpha."

So, I'm asking for some guidance from anyone out there who does--or has
done--post-tensioned concrete design: How have you typically gone about
computing the loss due to friction as defined in ACI 318 Sec. 18.6.2?

[N.B. Sorry for the rambling missive. I very often find that I can't
really phrase my question until I've stumbled around a bit, gathering my
thoughts...]

Happy Thanksgiving, Everyone.


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