I have based my L structure on the Chicago L prototype. It is designed to be built easily, with minimal cost and sturdy enough to support MTH O Scale subway trains.
When building L structure, you first need to decide on what style you wish to build.
The above graphic shows a couple of different two track styles. Depending on your prototype, additional tracks can be added as needed.
Type A has the columns directly under the centerlines of the tracks. This is a typical style for Chicago's L's that run in alleys as there isn't room for a street underneath.
Type B has the columns spaced at the outer edge of the track. A street can be placed underneath as the columns would sit in the street as on either the Wells or Wabash legs of Chicago's Loop.
Type C has the columns spaced further apart. This would be used for a street, either placing the columns on the sidewalk/curb or in the street.
Whatever you chose, decide on what type you want to build before you start. This way the length of the cross girders can be predetermined.