Modeling structures composed of beams can be more complicated than modeling shell, plate, or solid structures. First, it is necessary to define bending, extensional, and torsional stiffness that may be complex functions of the beam cross sectional dimensions. Then it is necessary to define the orientation of this cross section in space. Finally, if the centroid of the cross section is offset from the two finite element nodes defining the beam element, these offsets must be explicitly defined. Fortunately, Patran provides a number of tools to simplify these aspects of modeling.

The cross section properties are defined on the element property forms shown on pages General Section Beam (CBAR), 94 and Tapered Beam (CBEAM), 112. The properties can be entered directly into the data boxes labeled Area, Inertia i,j, Torsional Constant, etc. or by pushing the large I-beam icon on these forms to access the Beam Library form. The Beam Library forms are a much more convenient way of defining properties for standard cross sections and are shown below.

The first step in using the beam library is to select the section icon for the particular cross section desired (e.g. I-section).Then the dimensions for each of the components of the beam section must be entered.

Finally, a section name must be entered and the Apply button pushed. The other options available with the beam library are documented in the Patran Reference Manual, see Beam Library (p. 483) in the Patran Reference Manual. Once one or more beam sections have been defined, these can be selected in the section data box on the element properties form.

 

The Bar Orientation data box on the Input Properties form is used to define how the y-axis of the beam cross section is oriented in space. By default the Value Type is Vector. This tells MSC⁄Nastran that the cross section y-axis lies in the plane defined by the beam’s x-axis (the line connecting the two node points) and this vector. The Value Type pop up menu may be changed to Node ID. In this case the y-axis lies in the plane defined by the x-axis and the selected node.

When the Value Type is Vector and the Bar Orientation data box is selected the following select box appears on the screen.

After the orientation has been defined, there are two ways to verify its correctness in Patran. The first option is in the Element Properties application. By selecting the Show Action, the Definition of X Y Plane property, and Display Method Vector Plot, the vectors defining the orientation will be shown on the model. A second option can be used when the Beam Library has been used to define the beam cross section. There is an option on the Display form Display>LBC/Element Property Attributes (p. 393) in the Patran Reference Manual called Beam Display. The menu allows different display options for displaying an outline of the defined cross section on the model in the correct location and orientation.

Users should be aware of one difference between the Patran and MD Nastran definitions for cross section orientation. In Patran the orientation is completely independent of the analysis coordinate system at the beam nodes. In MD Nastran, the orientation vector is assumed to be defined in the same system as the analysis system at the first node of the beam. In Patran it is perfectly permissible to define the orientation in a different coordinate system from that analysis system. When the NASTRAN input file is generated, the necessary transformation of this vector to the analysis system at node 1 will be performed.

Two data boxes are provided on the Element Properties, Input Properties form to optionally define an offset from either node 1 to the cross section centroid (Offset @ Node 1) or from node 2 to the cross section centroid (Offset @ Node 2). The same select menu tools are available for defining these vectors. One difference between the orientation definition and the offset definitions, however, is that for the offset the magnitude of the vector is important. Because of this, the select menu tools are usually not very convenient. Typically, offsets are defined by typing the definition (e.g <x, y, z> or <x, y, z> coord n>) into the appropriate data box.

Two options are available for verifying the definitions of offsets; these options are very similar to those for orientations. The Element Properties, Show Action will allow the end offsets to be displayed as vectors on the model. This option is not especially useful because the vector plot shows only the direction of the offset, not the magnitude of the offset. It is usually much more useful to view the Beam Display menu on the Display form Display>LBC/Element Property Attributes (p. 393) in the Patran Reference Manual to select the display option with offsets. The viewport will then show the beam displayed in both the offset and non-offset positions.

Figure 2‑1 shows a simple example of a circular cylinder stiffened with Z-stiffeners. The cross section was defined by selecting the Beam Library icon on the Element Properties/Input Properties form. The Z cross section was selected on the Beam Library form, the cross section dimensions input, a section name input, and the Apply button pushed. On the Input Properties form, the Use Beam Section toggle is set to ON. The defined section name is selected in the [Section Name] data box. The string <-1.0 0. 0.> coord 1 is typed into the Bar Orientation data box to align the cross section orientation with the radial direction of the global, cylindrical system. Similarly, the strings <-2.0 0.0 0.0> coord 1 and <-2.0 0.0 0.0> coord 1 typed into the Offset @ Node 1 and Offset @ Node 2 data boxes define the end offsets to be radially inward.