Prestress Losses
Precast/Prestressed Girder provides three different methods for computing losses in the LRFD method.
- Approximate Method: This method computes losses using the approximate estimate of time dependent losses as specified in LRFD Art. 5.9.5.3, without considering the true time dependent values of creep and shrinkage (from Art. 5.4.2.3.2 and Art. 5.4.2.3.3). In this method, Precast/Prestressed Girder computes the different components of equation 5.9.5.3-1 and treats them as an estimate of long term prestress loss due to creep, shrinkage and relaxation of steel. Precast/Prestressed Girder computes the elastic shortening component and adds both to obtain final losses as shown in equation 5.9.5.1-1. When using gross section properties (non-transformed option), Precast/Prestressed Girder computes elastic gains (due to strand elongation when subjected to external loads) and also the elastic loss (shortening) and adds them to the remaining components (long term) to obtain total final losses. When using transformed section properties analysis, even though Precast/Prestressed Girder displays the different loss components, it does not include them in the stress computation, as the effects are implicit. An additional reference where clarifications can be obtained on the methodology is the NCHRP Report 496: Prestress Losses in Pretensioned High-Strength Concrete Bridge Girders.
- Refined Method: This method computes losses using the refined estimate of time dependent losses as specified in LRFD Art. 5.9.5.4, considering the true time dependent values of creep and shrinkage (from Art. 5.4.2.3.2 and Art. 5.4.2.3.3). In this method, Precast/Prestressed Girder computes the different components of equation 5.9.5.4.1-1, i.e. the long-term prestress loss due to creep, shrinkage and relaxation of steel. Precast/Prestressed Girder computes the elastic shortening component and adds both to obtain final losses as shown in elastic gains (due to strand elongation when subjected to external loads) and also the elastic loss (shortening) and adds them to the remaining components (Iong term) to obtain total final losses. When using transformed section properties analysis, even though Precast/Prestressed Girder displays the different loss components, it does not include them in the stress computation as the effects are implicit. An additional reference where clarifications can be obtained on the methodology is the NCHRP Report 496: Prestress Losses in Pretensioned High-Strength Concrete Bridge Girders.
- Pre-2005 Interims: This option allows the users to use methodology commonly used by the bridge community prior to the introduction of the new loss equations/provisions in the 2005 Interims. This option is primarily intended to allow for proper transition of older projects to the new version of Precast/Prestressed Girder.
Prestress losses are computed at release (at transfer) and at Final (after transfer) at midspan section only. According to LRFD Art. 5.9.5, the prestress losses in pretensioned member fpT at transfer is:
Prestress losses are computed at release (at transfer) and at Final (after transfer) at midspan section only. According to LRFD Art. 5.9.5, the prestress losses in pretensioned member fpT at transfer is:
Δfp=ΔfpES+ΔfpR1
Where:
ΔfpES=Loss due to elastic shortening.
ΔfpR1=Loss due to relaxation of steel transfer
At final( after losses) calculated at midspan only:
ΔfpT=ΔfpES+ΔfpSR-ΔfpCR+ΔfpR2
ΔfpT=Total prestress loss
ΔfpES=Loss due to elastic shortening
ΔfpSR=Loss due to shrinkage
ΔfpCR=Loss due to creep of cconcrete
ΔfpR2=Loss due to relaxation of steel after transfer