Using the transfer or development length (depending on whether you are analyzing for service or ultimate conditions), the effective strand area is calculated for each strand. The total effective strand area and location of its centroid are calculated by adding up the effective strand area and location of each strand.

The total effective strand area and location obtained is used for calculating the magnitude and location of the resulting prestress force. Precast/Prestressed Girder assumes that the effect of all the strands is the same as that of an equivalent total strand with the same area and centroid.

The force of each tendon is also considered in computing the c.g. of strands. It is reasonable to consider all of the tensile reinforcement to be lumped at the location defined by d_s, and all of the prestressing steel can be considered to be lumped at the location defined by d_p.