# RFEM – Online Manual

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# 4.10 Line Hinges

General description

Surfaces that touch each other on one line are rigidly connected at it. With a line hinge, you are able to exclude particular degrees of freedom from the transfer.

Line hinges can not just be arranged on boundary lines of surfaces. They can also be assigned to lines integrated in a surface, as shown on the left.

A line hinge is an attribute of a surface, not of a line. Thus, the hinge must be assigned to a surface. To assign the line hinge graphically, select the menu option

• Insert → Model Data → Line Hinges → Assign to Lines Graphically.
Line No.

Enter the number of the line where you want to define the hinge. You can also use the list or select the line graphically. When you select the surface before defining the settings in the dialog box, you can import any boundary line of the surface by clicking the [All Boundary Lines] button.

Surface No.

The line hinge is assigned to a surface. As the hinge is a surface property, you can retroactively adjust it in the Edit Surface dialog box.

Side

The options in the dialog box or the column in the table are only accessible if the line is an integrated object of the surface. The arrangement of the hinge determines the way the finite elements on the line sides are taken into account for the stiffness.

To allocate the hinge to the Left or Right of the line, check the direction of the line (▲ in the figure below) and the direction of the local surface axis z.

If the global Z-axis is oriented downward, the following rule applies: "Stand on the line with surface axis z pointing in direction of your feet. Then look in the direction of the line. Left and right are the directions of your arms". If the Z axis is oriented upward, this rule applies with the condition "Look in the opposite direction of the line".

The side option is locked for the boundary line of a surface because the side of the line on which the hinge is effective is clearly defined by the assignment to the surface.

Axial/Shear Hinge or Spring

The text boxes and table columns control the degrees of freedom for axial and shear forces. If a check box is selected, it means that displacement in the relevant direction is possible and the force is thus not transferred. It is also possible to enter the constant of a translational spring.

The degrees of freedom are based on the following definition of the axis system: The x-axis points in direction of the line; the y-axis represents the tangent of the surface plane and the z-axis the normal to the surface.

Moment Hinge or Spring

The degrees of freedom for moments refer to the local axis system of the hinge (x-axis in direction of the line, y-axis as tangent, and z-axis as normal to the surface plane). The check mark means that the rotation is free and the internal force is not transferred. It is also possible to enter the constant of a rotational spring.

The graphic in the dialog box shows the direction of moments. For a "hinge joint" between two surfaces, for example, the release type φx should be selected. Thus, a moment hinge about the longitudinal axis of the line is created.

If you want to apply nonlinear properties to a line hinge, you have to convert the hinge into a Line Release (see Chapter 4.26). For this purpose, you can use a special function in the shortcut menu of the line release, presented in the following article: https://www.dlubal.com/en/support-and-learning/support/knowledge-base/001143.