Traditionally use of subassemblages has been enough to design structures in general, and bridges in particular, part by part. Hence, except the code having contrary mandatory clause (modern codes use to refer to specific kinds of analysis required for the class of building or bridge), you may model separately the subelements you want. It is highly infrequent to produce some structural design just as output of one model, whatever the program we may be using. Hand calculations, spreadsheets, quotes from technical references are recurrent appearances in our work.