(a)Stage 1-Initial conditions at transfer
At this stage the prestress force has its highest value, Pi, the concrete has a relatively low strength, f’ci and the external moment is low, being caused only by the self-weight of the girder.
The stress on the top face of the girder is
Ft = - Pi/Ag + Pieg/Stg - Mdg/Stg (6-25)
The stress on the bottom fiber of the girder is
Fb = - Pi/Ag - Pieg/Stg + Mdg/Stg (6-26)
Where Ag, eg, Stg, and Sbg refer tosection properties of the girder alone and Mdg is the moment due to self-weight of the girder.
These calculated stresses must satisfy the initial stress limits for the girder concrete.