Good question bluefish, unfortunately I can only hint a few points, because I am not really good at this
I will use a girder bridge for an example
Suppose you have 4 girders and 3 lanes.
Then each lane is supported by one or two girders.
YOu must define the lanes. And associate each girder with whatever lanes it takes load from specifying distance fro the centrelines. This can be very messy and you need to exercise patience here. So girder 2 for example probably takes load from lane 1 and 2, and it may be that girder 2 is 6ft(2m) from lane 1 but only 3ft(1m) from lane 2.
Then for each lane you must define the vehicle loading. This is easy because vehicle loads are essentially lane loads.
I used this once and I got great results, clean influence lines and total moments and shears.
To start with, re-solve the simple example given in the tutorials. The rest will be easier to follow
respects
ijr