Composite structures loads considering the erection sequence in the simulation with partially completed elements has a complex structural system that changes in time and requires a non-linear stress distribution as shown in the picture below.

 

This requires the conversion of accumulated section forces into equivalent for each individual component, split:

It also needs the individual parts to be converted into full Cross-Section, join:

More information is found at the RM Bridge installation at File\Examples\Demo Examples & FAQ\ Composite Structures - Join Split where the description document can be found and the dataset can be loaded.