Trouble assigning frame section to line.

[billyboy48]

I have a curved girder bridge with 7 girder lines and consists of 8 spans. Girder cross sections vary in depth and both flanges vary in thickness and width. I have my centerline of girders broken up into segments which correspond to changes in girder cross section. My problem is that when I select a line segment and then assign the required cross section to that line the section is assigned to all of the girder lines. My question is, What am I doing wrong? What can I do to correct this problem?

Thanks,
billyboy48