WSDOT is now adopting the Refined “Estimate of Time-Dependent Prestress Losses” described below as standard practice for prestress loss calculations. PG Super and PG Splice programs will be using the Refined Estimate of Time-Dependent Prestress Losses as default method for prestress loss calculations.

Time-Dependent Losses
For standard precast, pretensioned members with cast-in-place 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.                     

  Jacking stress

  Yield strength of the strand

             where

Moment caused by diaphragms and other external loads applied to the non-composite girder section

Moment caused by all superimposed dead loads including traffic barriers and overlays

Location of the centroid of the non-composite 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 Time-Dependent 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 time-dependent 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 non-composite 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.

WSDOT Standard Specifications section 6-02.3(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 age-adjusted 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

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 705-7174 or Bijan Khaleghi at 705-7181.

cc:   Mohammad Sheikhizadeh, Bridge Construction - 47354

5.1.4       Prestress Losses

A.       Unchanged

B.       Unchanged

C.       Time-Dependent Losses

For standard precast, pretensioned members with cast-in-place 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.

  Jacking stress

  Yield strength of the strand

(5.1.4.10)

Moment caused by diaphragms and other external loads applied to the non-composite girder section

Moment caused by all superimposed dead loads including traffic barriers and overlays

Location of the centroid of the non-composite 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 time-dependent 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 non-composite 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.