When there is no need for a spatial analysis of a structural system and the nodes are articulated, an analysis can be made with the calculation hypothesis of the structure without bending rigidity. This option is useful only in the case of designing systems without rigidity in bending, for example, plane trusses and membrane planar elements.
The command can be accessed from the Assumptions - Structure menu. In the dialog box select "Mode: plane" and uncheck the "Bending rigid structure" option.
Consider a plane structural system, articulated at nodes, as in the lower image. The linear elements are declared as "Bar" types and the loads are concentrated at nodes.
Fx - axial force on linear elements
When modeling a truss with articulated nodes, the elements will only be subjected to axial forces.
Unchecking the bending rigid structure option for spatial models (3D) is not recommended. In this case the modeling is very limited as the bar element permits all the rotations and can result in an unstable model.
The best approach for modeling spatial 3D trusses would be to keep the bending rigidity option checked and declare releases at the ends of the beams.
Note: For correctly declaring releases at the bar ends and for general truss modeling refer to the "How is a truss modeled in Advance Design?" FAQ.