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		I think your only direct question that has not been answered
is, "How might the length be determined?"
		I determine the length by putting trial lengths into the
canned, prestressed beam design program that I use for the rest of the beam
design. It always works - it never makes a mistake - I trust it implicitly
:-). Actually, I have used it for about 15 years through a number of
versions as have the rest of the engineers here. It makes mistakes a lot
less frequently than I would. There are more of these programs available
than I would have expected.
		There is one thing under your, "Anything else?" category:
When the prestress force is released, unless the strand is  relaxed
relatively slowly, it snaps back at its supports (stressing abutment or
concrete) like a rubber band will snap back at your hand if it breaks. The
longer this length, the harder it snaps. When beams are poured in a long
line and there is a foot or two (at most, precasters like to fill their
production capacity) between beams, this force is small. If the strand is
debonded a long way, it can break its bond to the concrete. ACI addresses
this in 12.9.3, where it requires the unbonded strand development length to
be doubled. If only a small length of the casting bed is filled with beams,
the energy released by detensioning 50' - 100' or more of strand
__abruptly__ can break the concrete. Detensioning slowly solves this
problem. Likewise, sawing through a slab with _unbonded_ strand is
dangerous. The saw operator will know when the saw encounters a _bonded_
strand as he will know when the saw encounters mild steel reinforcement, but
any energy released by breaking the bond and strand relaxation is contained
within the concrete.
	Jim Getaz
	Precast Engineer
	V: 540-665-3204
	F: 540-665-3250

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