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. This assumption may cause the ultimate capacity calculation under positive moments to be somewhat conservative when a large number of top strands are used if the option to use AASHTO equations is selected. When the option to use strain compatibility is select, this is not an issue.

For example, consider the situation of a pattern that has several strands at the top. When calculating the total strand area and location of strands, Precast/Prestressed Girder will add up all the strands, including the top strands. But, since the top strands are in compression, we can disregard the effect of these strands. The result would be a smaller total strand area (which would decrease the value of Mu-prvd) and a larger "d" (which would increase the value of Mu-prvd. The net effect would be a larger Mu-prvd value than the one calculated by the program.