Hi there "solidworkers"

i'm new on this solidworks i'm a trainee/rookie, a stud. so i was doing some, more trying then doing, simulation on some parts (assembly), and there some trouble because i have solid parts and some beams (UPN) so its a mix, on the simulation its give some trouble so i mark them all as a solid... so 1st of all things i want to know the differences between a beam and a solid on the simulation. because my structure has beams, metal plates and gussets (metal plates aswell).