[poor English] about the finite element approach

Question

Hello! 
 I've just used RM bridge for a few days. 
 I thought RM solves problem based on beam theory. 
 However, I found that I can choose some plane elements( and 3D...) during creating cross section in RM modeler. 
 Can anyone briefly explain the principles that RM solve bridge's problem? (I can't understand the FEM application guide) 
 thank you!

Diether · Accepted Answer

Traditionally RM Bridge has been developed as a program based on beam theory. A FEM implementation has been added later on. RM uses a hybrid approach, which allows for using beam and FEM elements in one and the same model. The advantage of this is, that also section-forces, which are required for ULS design code checks, are available, while primary results of standard FEM methods are stresses only.