4.14 Member Hinges

Member hinges limit the internal forces that are transferred from one member to others. Hinges are only assigned to member ends (nodes); they can never be assigned to other locations such as the middle of the member.

Some member types are already provided with hinges: A truss, for example, does not transfer moments and a cable transfers neither moments nor shear forces. When entering data, the assignment of hinges is therefore blocked for members of such member types.

A member hinge can be related to one of the following axis systems:

• Local member axis system x,y,z
• Global coordinate system X,Y,Z (optionally as scissors hinge)
• User-defined axis system X',Y',Z'

Use the Display navigator (see Figure 4.169) or the member shortcut menu shown on the left to display the local member axes.

For detailed information about the orientation of local member axes in the global coordinate system XYZ, see Chapter 4.17.

Normally, hinges are related to the local axis system x,y,z. Scissors hinges (see Figure 4.144) can only be defined in relation to the global or user-defined coordinate system.

To define an axial or shear force hinge, set the respective displacement free by selecting the relevant check box in the dialog box or table. The check mark therefore indicates that the axial or shear force cannot be transferred at the member end because a hinge has been set. This becomes apparent in the Member Hinge dialog box: In the text box to the right of the check mark, the constant of the translational spring is zero.

You can change the spring constant anytime to represent, for example, a semi-rigid connection. In the table, enter the constant directly into the table column. The stiffnesses of the springs are considered design values.

Hinges for torsion and bending moments must be defined like hinges for forces. The check mark once again indicates that the torsion is free and the internal force is not transferred.

Elastic connections can be modeled through spring constants, which you can enter directly. Pay attention not to use extreme stiffness values, because otherwise numerical problems may arise during the calculation: Instead of very big or small constants, use rigid connections (no check mark) or hinges (check mark).

The option for defining nonlinear hinge properties is described at the end of this chapter.

To graphically assign hinges in the work window, use the menu item

• Insert → Model Data → Member Hinge → Assign to Members Graphically

or

• Edit → Model Data → Member Hinges → Assign Graphically to Members.

First, select a hinge type from the list or create a new one. After clicking [OK], members are divided graphically at one-third division points.

Now you can click the member sides you want to apply the selected hinge to. To assign the hinge to both member ends, click the member in its center area.

With scissors hinges, you can model the crossing of beams. For example: You have four members connected in one node. Each of the two member pairs transfers moments in its "continuous direction", but they do not transfer any moments to the other pair. Only axial and shear forces are transferred in the node.

In this case, the hinge must be assigned either to members 1 and 2 or to members 3 and 4. The other crossing member pair is modeled as bending-resistant without hinge.

Nonlinear properties can be assigned to member hinges. In this way, you can control the transfer of internal forces in detail. The list of nonlinearities provides various options.

In the table, hinge types with nonlinear properties are marked in blue.

With these two options, you can control the hinge activity for each internal force depending on the direction. For example: An axial force hinge defined with the nonlinearity Fixed if positive N has the effect that tensile forces (positive) can be transferred at the end of the member, but compressive forces (negative) cannot. In case of negative axial forces, the hinge is effective.

The internal forces are related to the local member axis system xyz.

The remaining entries of the Nonlinearity list provide detailed modeling options for hinge properties. To access the options, use the [Edit] dialog buttons to the right of the list or in the table (see Figure 4.142).

The activity of the release can be defined separately for the positive and negative zone. In addition to full effectiveness or failure, the release can lose its effect when a certain displacement or rotation is reached; then it begins to act as a fixed or rigid connection. Tearing (no internal force is transferred after exceeding a certain value) and Yielding (internal forces are transferred only up to a certain value, even in case of larger deformations) are also possible in combination with Slippage.

The limit values can be defined in the text boxes below. In the Activity Diagram dialog section, the release properties are shown in a dynamic graphic.

The activity of the release can be defined separately for the Positive and Negative Zone. First, enter the Number of steps (i.e. definition points) represented in the diagram. Then you can enter the abscissa values of the internal forces with the assigned displacements or rotations into the list to the right.

You have several options for the Diagram after last step: Tearing for the failure of the release (no internal force is transferred any longer), Yielding for restricting the transfer to a maximum allowable internal force, Continuous as in the last step, or Stop for restricting to a maximum allowable displacement or rotation followed by a fixed or rigid release activity.

In the Diagram dialog section, the release properties are shown in a dynamic graphic.

With these four options, the forces transferred by the hinge are related to the compression forces that act in a different direction. Depending on your selection, the friction depends on only one normal force or on the total force of two simultaneously acting forces. For example, the friction force for the x-direction can be calculated from just the y-component or the z-component, but also from both or even from the addition of both forces.

The button opens a dialog box where you can define the spring constant C and the friction coefficient μ.

The plastic properties of the hinge can be defined in a separate dialog box.

This nonlinearity type allows for the mechanical simulation of a tube joint with an inner stub between two members. The equivalent model transfers the bending moment via the overpressed outer pipe and after positive locking additionally via the inner stub, depending on the compression state at the member end. Use the button to open a dialog box where you can separately define the parameters for the Inner Tube and the Outer Tube.

A planar system is used. The hinge must be defined as follows:

The hinge type can then be assigned to the members.