PGSuper  3.1
Precast-prestressed Girder Bridges
Maximum Allowable Jacking Force

This section describes how the maximum jacking forces for prestressing strands is computed.

The jacking force is computed as

Pjack.png

where fpj is the jacking stress and Aps is the total area of prestressing strand.

Jacking Stress - 1st Edition

The jacking stress is prescribed in Article 5.9.3 of the 1st Edition of the LRFD specification. The jacking stresses are

fpj_SR_1994.png

for Stress Relieved Strands and

fpj_LR_1994.png

for Low Relaxation Strands.

Jacking Stress - 1st Edition with 1996 and 1997 Interim Provisions to 3rd Edition 2004

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

fpt_LR_1997.png

for Stress Relieved Strands

fpt_SR_1997.png

for Low Relaxation Strands.

The jacking stress is

fpj_1997.png

Substituting, we get

fpj_1997a.png

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,

deltaFpR1.png

Substituting and re-arranging, we get

SolveForFpj.png

This equation is solved for fpj

Jacking Stress - 3rd Edition 2004 with 2005 Interim Provisions and Later

AASHTO removed the computation of relaxation loss prior to prestress transfer with the 2005 interim provisions. The jacking stresses are

fpj_SR_2005.png

for Stress Relieved Strands and

fpj_LR_2005.png

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

deltaFpR0.png

in the equations.