Design Memorandum
TO: All Design Section Staff
FROM: Bijan Khaleghi
DATE: March 4, 2007
SUBJECT: Revisions to Prestress Losses
WSDOT is now adopting the Refined “Estimate of TimeDependent Prestress Losses” described below as standard practice for prestress loss calculations. PG Super and PG Splice programs will be using the Refined Estimate of TimeDependent Prestress Losses as default method for prestress loss calculations.
where
Duration of time between strand stressing and prestress transfer, typically 1 day.
Jacking stress
Yield strength of the strand
where
Moment caused by diaphragms and other external loads applied to the noncomposite girder section
Moment caused by all superimposed dead loads including traffic barriers and overlays
Location of the centroid of the noncomposite girder measured from the bottom of the girder
Location of the centroid of the composite girder measured from the bottom of the girder
The Approximate Estimate of TimeDependent Prestress Losses as described below may be employed for calculations of temporary prestress losses. PG Super and PG Splice programs will be using the Approximate Method for temporary Prestress Loss calculations.
Temporary Losses
For checking stresses during release, lifting, transportation, and erection of prestressed girders, the elastic and timedependent losses may be computed based on the following assumptions.
Lifting of girders from casting beds 
For normal construction, forms are stripped and girders are lifted from the casting bed within one day.
Transportation 
Girders are most difficult to transport at a young age. The hauling configuration causes reduced moments in the girder creating the potential for overstress between the harping points. Overstress may also occur at the support points depending on the prestressing and the trucking configuration. This is compounded by the magnitude of the prestress force not having been reduced by losses. For an aggressive construction schedule girders are typically transported to the job site around day 10.
When losses are estimated by LRFD 5.9.5.3, Approximate Method, the losses at the time of hauling may be estimated by
Erection
During construction the noncomposite girders must carry the full weight of the cast slab and interior diaphragms. This loading typically occurs around 120 days for a normal construction schedule.
Final Configuration 
The composite slab and girder section must carry all conceivable loads including superimposed dead loads such as traffic barriers and overlays and live loads. It is assumed that superimposed dead loads are placed at 120 days and final losses occur at 2000 days.
The attached file includes the necessary revisions to WSDOT Bridge Design Manual section 5.1.4 for prestress losses.
WSDOT Standard Specifications section 602.3(25)M will be modified to reflect the new practice.
Background
The approximate estimate of prestress losses described in LRFD Article 5.9.5.3 and BDM Section 5.1.4 are suitable for specific cross sections, loadings and environmental conditions. The approximate method simplifies the prestress loss calculations but with the extensive use of computer software by the bridge designers this simplification is irrelevant.
The approximate method could still be used for temporary losses due to lifting, shipping and erection of precast prestressed girders. Derivation of approximate method for temporary losses is described below.
Development of Approximate Loss Equations for Shipping:
The time dependent loss at the time of hauling is:
The first term relates to the effects of shrinkage, the second to the effects of creep, and the third to relaxation.
The loss due to shrinkage at time of hauling is:
For standard precast girders the transformed section ageadjusted effective modulus of
Most prestressing is transferred within the first day of concrete placement and the
Generally, the concrete stress at the bottom fibers at service is kept close to zero. Thus, the individual stresses
due to effective prestress, girder weight, SIDL, and live load add up to zero at time infinity. In beams of
common spans, the stress due to external loads is about equally divided between girder weight, deck weight,
and live load. Also, the total stress due to external loads is equal and opposite to the stress due to effective
prestress. Therefore, if the effective prestress is assumed to be 80% of the initial prestress, the
The approximate shrinkage and creep losses are roughly one forth of the final loss. The total relaxation loss is 2.5ksi so it is reasonable to approximate relaxation at time of hauling as 0.6ksi.
Using an average value for the relative humidity correction factor and changing notation to avoid confusion with the refined method, the time dependent loss at time of hauling can be approximated by
If you have any questions regarding this design issue, please contact Richard Brice at 7057174 or Bijan Khaleghi at 7057181.
cc: Mohammad Sheikhizadeh, Bridge Construction  47354
F. Posner, Bridge and Structures – 47340
5.1.4 Prestress Losses
A. Unchanged
B. Unchanged
C. TimeDependent Losses
For standard precast, pretensioned members with castinplace slab subject to normal loading and environmental conditions and pretensioned with low relaxation strands, the total prestress loss may be estimated as
The first term relates to initial relaxation that occurs between the time of strand stressing and prestress transfer.
where
Duration of time between strand stressing and prestress transfer, typically 1 day.
Jacking stress
Yield strength of the strand
(5.1.4.10)
where
Moment caused by diaphragms and other external loads applied to the noncomposite girder section
Moment caused by all superimposed dead loads including traffic barriers and overlays
Location of the centroid of the noncomposite girder measured from the bottom of the girder
Location of the centroid of the composite girder measured from the bottom of the girder
D. Temporary Losses
For checking stresses during release, lifting, transportation, and erection of prestressed girders, the elastic and timedependent losses may be computed based on the following assumptions.
Lifting of girders from casting beds 
For normal construction, forms are stripped and girders are lifted from the casting bed within one day.
Transportation 
Girders are most difficult to transport at a young age. The hauling configuration causes reduced moments in the girder creating the potential for overstress between the harping points. Overstress may also occur at the support points depending on the prestressing and the trucking configuration. This is compounded by the magnitude of the prestress force not having been reduced by losses. For an aggressive construction schedule girders are typically transported to the job site around day 10.
When losses are estimated by LRFD 5.9.5.3, Approximate Method, the losses at the time of hauling may be estimated by
Erection
During construction the noncomposite girders must carry the full weight of the cast slab and interior diaphragms. This loading typically occurs around 120 days for a normal construction schedule.
Final Configuration 
The composite slab and girder section must carry all conceivable loads including superimposed dead loads such as traffic barriers and overlays and live loads. It is assumed that superimposed dead loads are placed at 120 days and final losses occur at 2000 days.
