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Re: Post-tensioned Construction ...

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Just to complete Gail's post for throughness (as Gail is likely more
than aware of this information but omitted it for "time" reasons...but
as I am anal and like to be through, I just had to complete)...

The 0.8*fpu is one of the three limits for the stress in prestressed
tendons due to the jacking force.  The other two limits are 0.94*fpy
(where fpy is the yield stress) or a maximum value recommended by the
manufacturer of the tendons or anchorage devices.

And the permitted stress limits are lower for once the prestress transfer
has occured (i.e. the jacks are released/removed).  Typically, those lower
limits are the lessor of 0.82*fpy or 0.74fpu.  For post-tensioning
tendons at the anchorage devices and couplers, the limit immediately after
force transfer is 0.70fpu.

Thus, in a "built" case (i.e. finished building not during the
construction phase when the jacking is occuring), the maximum force in the
tendons would more on the order of 0.153 * 270 * 0.74 = 30.5 kips or lower
if the yield stress is less than 244 ksi...for those who aren't aware the
yield stress of prestressing steel is not nearly as obvious as it is for
standard rebar or structural steel for steel shapes and is determined by
the stress occuring at a particular strain value [1% strain for grade
270 tendons if memory serves me correctly]).

This puts the 30,000 number from Gail's favorite quote a little better
into its (incorrect) context that I believe that Gail was trying to
convey.

HTH,

Scott
Ypsilanti, MI


On Wed, 12 Mar 2003 GSKWY(--nospam--at)aol.com wrote:

> Here are some more of my favorite quotes from published articles. Although,  some people might get confused by the fact they are being told that a 270 ksi strand "carries" 30,000 psi.  Would you really want to hire someone who can't even do the math?
>
> Hint:  strand area = 0.153 * 270 ksi * 0.8 = 33 kips (the force that a 0.5-inch strand is stressed to,  ACI 318 limits the stressing force to 80% of the ultimate strength of the strand.
>
> Gail Kelley
>
> ******
> Dick Bonin, a marketing representative for The Western Group, a company that performs post-tension tendon and concrete repairs, said when one of them snaps it's like a .44 Magnum going off. "These tendons carry over 30,000 psi. You are taking a 100-foot, half-inch wire rope and stretching it close to 10 inches like a rubber band. These things can cut a hole in a car. They can come out of the concrete and blow concrete for two blocks if you don't watch it."
>
> Bonin has seen the destruction in vivid detail. "I've looked at some pictures that would scare the hell out of you," he says, "where the wires have gone through the concrete and gone through walls and doors. They [the cables] are under tremendous pressures and can be very dangerous."
>
> Bonin's comments are echoed by Arnie Rodio, owner of Pacesetter Plumbing in Lancaster, Calif. "There's a story about the cables coming out of slabs and cutting a guy's foot off."
>
> ******
>
> Gail Kelley
>
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