Easy solution would be to implicitly associate the structures to the line using Select by Location. Create your relationship class using your primary (LineID on the Transmission Line feature class) and foreign key (LineID on the Structures feature class), then Select by Location from Structures that are within distance X from your transmission line. Field calculate the Transmission line's primary key into the foreign key attribute of your Structures, and then you should be set.
Of course you will always run the risk of associating the line to incorrect structures and vice versa if you have coincident lines, so YMMV.
