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4.8 Line Supports
Line supports describe the boundary conditions of all FE nodes available along a line: Displacements and rotations on these internal nodes can be prevented or limited by translational and rotational springs.
You can assign nonlinear properties to displacements of line supports so that supports are ineffective in case of tension or compression.
Use the menu option Insert → Model Data → Line Supports → Graphically or the toolbar button shown on the left to open the following dialog box:
The support types Hinged (YYY NNN) and Rigid (YYY YYY) are predefined and can be selected from the list. After clicking [OK], you can graphically assign the selected support type to the lines.
Use the [New] button to create another type of support. The dialog box shown in Figure 4.99 is displayed.
Line supports can only be defined on lines that belong to a surface or a solid. Enter the line number into the table column or text box of the dialog box, or select it graphically.
The support parameters can be related Locally to the line axes x,y,z or Globally to the axes X,Y,Z. Indexes in the Support Conditions dialog section as well as headlines of table columns E to J change depending on the selected setting.
The display of the local axis system of lines including numbering can be set in the Display navigator. You can also use the shortcut menu of a line support.
It is possible to rotate the axis system of a local line support. The Rotation about a positive angle β rotates the support clockwise around the positive line axis x.
The entered support rotation is shown in the dynamic dialog graphic.
When the calculation is complete, it is possible to evaluate support reactions of a rotated line support in relation to the global and the local axis system.
When a surface is supported by a wall, it is considered an elastic support that depends on the stiffness of the wall. A fixed line support would not represent the flexibility correctly. For such a support type, you can define a Wall: RFEM calculates the constants of translational and rotational springs from the wall's material and geometry. This option is especially useful for 2D plates in order to avoid singularities, which would occur for a rigidly supported line.
Define the geometry in the Wall Size dialog section. In addition to the Width t, the Height h affects the constants of the translational and rotational springs.
In the Parameters dialog section, select the Material of wall from the list of already defined materials or create a [New] wall material (see Chapter 4.3).
To determine the spring stiffnesses, specifications for Support conditions at wall head and wall base are required. If you consider the shear stiffness of the wall, it is also considered for the constants of the Support Springs Due to Wall.
The spring constants determined from the parameters are displayed on the right in the dialog box.
The wall widths on the supported line are displayed in the RFEM graphic.
To define a support, select the corresponding option in the dialog box or table. The check mark indicates that the relevant degree of freedom is blocked and the line displacement in the respective direction is not possible.
If you do not want to define supports, clear the corresponding check box. RFEM sets the constant of the translational spring to zero in the Line Support dialog box. The spring constant can be modified anytime, in order to represent an elastic support of the line. In the table, enter the constant directly into the table column.
The spring stiffnesses are considered design values.
Assigning a failure criterion is described below.
Restraints are defined analogously to supports. Again, the check mark indicates that the relevant degree of freedom is blocked and the line displacement in the respective direction is not possible. In the same way, the constants for rotational springs can be defined once the check boxes are cleared. In the table, enter the spring constant directly into the corresponding table column.
The New Line Support dialog box (see Figure 4.99) provides buttons for different support types, making the definition of degrees of freedom easier.
The buttons have the following functions for support properties:
Hinged with restraint about Z'
Sliding in X' and Y' with restraint about Z'
Sliding in X' with restraint about Z'
Sliding in Y' with restraint about Z'
Sliding in Z' and Y' with restraint about Z'
You can assign the following nonlinear properties to the supports or translational springs of a line support:
- Failure if support force is positive
- Failure if support force is negative
The nonlinear properties can be accessed in the dialog box and table by using the list (see Figure 4.99 and Figure 4.100). For each element of the support, you can define whether only positive or negative forces are transferred on the supported line using these settings.
Positive or negative refers to the forces that are introduced to the support in direction of the respective axes (they do not refer to the reaction forces of the line support). The algebraic signs therefore result from the direction of the local or global axes. For example, if the local z-axis of a line is directed downwards, the load case "Self-weight" results in a positive support force pZ'.
Nonlinear effective supports of lines are displayed with a different color in the graphic. In the table, you can recognize support elements with a failure criterion by a blue check box.