The geometry of the structure can be modeled in three different ways:
- using 3-D solid elements
- using 3-D shell elements
- using truss elements
If 3-D solid elements are chosen, nodes are defined by providing their coordinates relative to a user-defined coordinate system or the basic coordinate system, and 3-D solid elements are then created using these nodes. A range of 1st and 2nd order solid elements is available in OptiStruct. When using 3-D solid elements, the geometry is fully described by the elements.
If 3-D shell elements are chosen, nodes are defined and the 3-D shell elements are created using these nodes. The 3-D shell elements may be 1st or 2nd order triangular or quadrilateral elements. In addition to the elements, shell thickness information is required to fully describe the geometry of the structure.
If truss elements are chosen, nodes are defined and the truss elements are created using these nodes. A range of truss elements is available in OptiStruct. In addition to the truss elements, information about the cross-section of these elements is required to fully describe the geometry.
The three modeling types described here can be mixed in the same structural model and it is up to the user to select the appropriate modeling type based on the required results. For structural components, such as brackets, solid models are usually selected. For a full car body analysis, a shell model would be suitable. For the analysis of a bridge, a truss model might be appropriate.