By default, a member or frame element is a beam. However, if you choose the model type to be either “2D Truss” or “3D Truss”, then the frame element becomes a truss element. The program assigns full moment releases automatically to the ends of all members and suppresses all three rotational DOFs Dox, Doy, Doz for each and every node. For the model type “2D Truss”, the program also suppresses translational DOF Dz. Generally speaking, if a model contains only 2D or 3D truss elements, you should choose the model type as “2D Truss” or “3D Truss”. If a model contains both trusses and beams, you should choose the model type“2D Frame” or “3D Frame & Shell”, and assign appropriate moment releases to individual beams. It may also be necessary to assign appropriate restraints to nodes to ensure stability of the model. You may choose the appropriate model type by running the command Analysis > Analysis Options.
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