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Re: Difference between Initial Prestressing

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Ellen,

Roger gave a good description of what the three "levels" of tendons
stresses as a function of the losses.  I will expand a little.

The jacking force as Roger pointed out will be the force in the tendons
before any losses have occurred.  This force occurs during the
construction phase.  For pretensioned, precast members (double tees,
single tees, hollow core slabs, precast I-shaped bridge girders, etc) this
occurs before any concrete is placed and "lasts" until the jacks are ready
to be removed and the stresses in the steel (the "prestress") is
transfered to the concrete (after placing and curing some).  In
post-tensioning, this is the force in the tendons prior to the releasing
of the jacks that would then initiate the transfer of the stress from the
tendons to the concrete at the anchorages.  Basically, think of it as
there is only force in the steel tendons and none of that force is being
imposed on the concrete yet (i.e. the member is not prestressed yet).

The initial force in the tendons will be the force in the tendons right
after the the initial transfer "begins" into the concrete.  This is the
point right after the jacks are releases and removed.  You are now
imposing the prestress force onto the concrete.  As Roger pointed out,
this force will be lower than the jacking force because you will have some
loses as the tendons slip a little at the anchorages as the jacks are
removed and due to the elastic shrinkage of the concrete due to the
compressive force imposed by the prestress force (the prestress force
creates significant axial compressive stresses in the concrete [which is
the intent] which will mean that the concrete member will get slightly
shorter [P*L/(E*A), where P is the axial force, L is length of member, E
is elasitc modulus, and A is cross section area]...this means the tendon
gets shorter too, which results in the strain in the tendon decreasing and
a corresponding decrease in the stress).  For the purposes of most
prestressed situations, we consider this level to occur when the
self-weight of the member is also acting on it.  In the case of
post-tensioned concrete, this is fairly obvious.  In the case of
pretensioned concrete, it is not so obvious, but the tendons are usually
placed below the neutral axis which means there is an eccentricity to the
prestressing force that will likely cause the member to camber upward
(even in the forms potentially) which then causes the self-weight to
"become effective".  Basically, the idea is that you are checking the
stresses in the concrete at the point at which the maximum prestressing
force will occur and the minimum imposed loading (i.e. dead and live)
which in effect is counteracting against the prestressing force.  In
otherwords, this condition respresents a check to see if enough imposed
loading is present to make sure that the prestressing force does not
overstress the concrete (or it would be necessary to either modifiy the
cross section or reduce the prestressing force).

The "final" prestressing force is usually what is considered when the
member is under its final service conditions.  This essentially means some
signigicant period of time after the construction is completed and the
building is in use.  As Roger pointed out, at this point you will be
including all the loses, which now also include loses due to creep, drying
shrinkage, friction, etc.  At this point you are now checking concrete
stresses due the theoretical minimum prestressing force in combination
with the maximum imposed loading (dead, live, etc) that is counteracting
the prestressing force.  In otherwords, you are now determining if there
is enough prestressing force remaining after loses to keep the imposed
loads from overstressing the concrete.

The forth "condition" is at ultimate.  We consider the prestressing force
to be the same at the "final" condition, but there is still another check.
This would be the ultimate strength check of the prestressed concrete
section, which is similar to what you use when designing/checking
reinforced concrete, but with some significant differences (the main one
is that you don't know the stress in the steel at this point directly
where as we know/define it as fy for convential reinforced concrete).

HTH,

Scott
Ypsilanti, MI


On Thu, 13 Mar 2003, Roger Turk wrote:

> Hi Ellen,
>
> The jacking force is the force placed in the strand before making allowance
> for any losses.  Initial Prestressing force is the force in the prestressing
> strand after making allowance for losses due to anchorage seating and
> immediate elastic shortening, but before accounting for losses due to
> shrinkage, creep, and relaxation.
>
> Final prestressing force is the force in the prestressing strand after
> accounting for all losses, and should be not less than .5 fpy (135 psi for
> 270k strand).
>
> HTH
>
> A. Roger Turk, P.E.(Structural)
> Tucson, Arizona
>
> Ellen wrote:
>
> . > Hi,
>
> . > Just an additional question about prestress...
>
> . > I want to know the difference between Initial Prestressing force and
> . > Jacking force.
>
> . > What's the difference specifically?
>
> . > Thanks in advance,
>
> . > Ellen
>
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