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Re: Difference between Initial Prestress

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Gail,

It is interesting that you mention the Zia equations for losses.  Admittedly, 
I have not done a lot of P/S (either post- or pre-) lately, but the Zia 
equations have been a problem in this high temperature, low humidity (average 
annual RH < 25%) country.  When P/S concrete was first permitted (Bureau of 
Public Roads Guide to Prestressed Concrete), lump sum losses were listed, 
IIRC, as 30,000 psi plus friction losses for post-tensioned concrete, 35,000 
psi for pre-tensioned concrete.  Using Zia's equations, it is not unusual for 
me to get losses in the 40,000+ psi range, yet, I have not seen any 
indications that the old lump sum loss values have created any deficiencies 
in prestressed concrete performance.

I recall in 1960-61 an overpass was constructed over a mine haul road for a 
new open pit copper mine.  The bridge used an inverted channel section for 
what is now termed a multi-beam type superstructure.  While the haul trucks 
were supposed to have an interlock to prevent them from traveling with the 
bed raised, it was not unusual for the interlock to be bypassed.  As a 
result, some drivers forgot to lower the bed after they dumped their load, 
and the raised bed would take out at least one of the legs of the inverted 
channels.  When this first happened, the P/S contractor decided to do some 
[unscientific] tests on the damaged concrete beam that he had removed to see 
how tough P/S concrete was.  He had a crane pick up and drop the 60' long 
damaged beam on the desert floor.  After doing that several times, no further 
damage was observed, so he had the beam turned over and repeated the 
experiment.  It was only after the beam had been turned over and dropped did 
further damage occur.

A. Roger Turk, P.E.(Structural)
Tucson, Arizona

Gail Kelley wrote:

. > Using the equations developed by Zia et al,  you will always end up with 
. > 2 or 3 kips as the long term prestress losses.  The major factor in long 
. > term losses is the loss due to shrinkage.  The age at stressing, i.e. 2 
. > days versus 4 days has only a minor effect in post-tensioned 
. > construction.  It is more significant in precast members where you are 
. > dealing with a discrete amount of concrete.  There was a recent NCHRP 
. > study that revisited the issue of long term losses but the focus was more 
. > on the losses in preast members using higher strength concretes.  The Zia 
. > study didn't look at anything greater than 6000 psi concrete,  precast 
. > currently is moving to 8000 and 10,000 psi concrete, sometimes higher.

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