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QUERY: How do you determine fpy?

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Hello, all.

I am extremely rusty when it comes to prestressed concrete design.  In 
fact, it's been about ten years since I did anything with it.  Now I find 
I have some evaluation of existing prestressed concrete structure to do, 
and I'm trying to get back up to speed.

The ACI 318 code, Chapter 18, makes reference to the value fpy, which is 
the specified minimum YIELD strength for prestressing steel.  This is 
analogous to fy, for mild steel reinforcing.

However, I can't find anything in the smattering of references that I have
that tells you how to estimate this value, even though it is used in a few
"checks" in the design equations.

Most of the design equations use fpu (Ultimate Strength), fse (residual 
stress after losses), and fps (stress at service load).  Nothing about 

Anyone have a clue?

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