PGSuper
3.1
Precast-prestressed Girder Bridges
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This section describes how the maximum jacking forces for prestressing strands is computed.
The jacking force is computed as
where fpj is the jacking stress and Aps is the total area of prestressing strand.
The jacking stress is prescribed in Article 5.9.3 of the 1st Edition of the LRFD specification. The jacking stresses are
for Stress Relieved Strands and
for Low Relaxation Strands.
The 1st Edition with 1996 and 1997 interim provisions change the method by which the allowable jacking stress is to be computed. Article 5.9.3 specifics the strand stress immediately prior to prestress transfer as
for Stress Relieved Strands
for Low Relaxation Strands.
The jacking stress is
Substituting, we get
NOTE: From now on, we will only deal with the stress relieved strands. The formulation for low relaxation strands is similar.
From LRFD Equation 5.9.5.4.4b-1,
Substituting and re-arranging, we get
This equation is solved for fpj
AASHTO removed the computation of relaxation loss prior to prestress transfer with the 2005 interim provisions. The jacking stresses are
for Stress Relieved Strands and
for Low Relaxation Strands.
WSDOT retained the computation of relaxation loss prior to prestress transfer. The method for compute the maximum jacking stress is the same as described above however WSDOT uses the term
in the equations.