Nodal Supports

The structure transfers its loads through the supports into the foundations. Without any support, all nodes would be free and unrestricted in their displacements and rotations. For a node to act as a support, at least one of the degrees of freedom must be locked or restricted by a spring. In addition, the node must be part of a member.

Imposed deformations of a node are only possible for correspondingly supported nodes.

If you want to assign nonlinear properties to a nodal support, you can define failure criteria for tensile or compressive forces, cracking and yielding, or working and stiffness diagrams.

The name symbol of a user-defined nodal support indicates the restrained degrees of freedom. The following support types are predefined:

• Hinged
• Rigid
• Movable
• Movable in X‘
• Movable in Y'

Basis

The Basic Data tab manages the elementary support parameters.

Coordinate system

Each nodal support has a local coordinate system. By default, it is aligned parallel to the global axes X, Y, and Z. If you have created a user-defined coordinate system or define one using the button , you can also use this reference system.

Support conditions

The support conditions are divided into 'Translational' and 'Rotational' degrees of freedom. The former describe the supports in the direction of the support axes, the latter the restraints about these axes.

To define a support or restraint, tick the check box for the respective axis. The check mark indicates that the degree of freedom is locked and that the displacement or rotation of the node in or about the respective direction is not possible.

If there is no support or restraint, remove the check mark from the corresponding check box. The constant of the translational or rotational spring is then set to zero. You can adjust the 'Spring constant' at any time to model an elastic support of the node. Enter the spring stiffnesses as design values.

In the Nonlinearity column, you can specifically control the transfer of internal forces for each component. Depending on the degree of freedom, suitable entries are available for selection in the list of nonlinearities.

Nonlinearly acting supports are displayed in a different color in the graphic.

Failure if support force/moment is negative or positive

This allows you to easily control whether the support can only absorb positive or negative forces or moments: If a force or moment acts in the prohibited direction, this component of the support fails. The remaining restraints and restraints remain effective.

The directions 'negative' and 'positive' refer to the forces or moments introduced into the nodal support with respect to the respective axes (that is, not the reaction forces from the support). The signs result from the direction of the global axes: If the global Z-axis, for example, points downwards, the load case "self-weight" results in a positive support force P_Z.

Failure of all if support force/moment is negative or positive

Unlike the failure of a single component described above, the support fails completely as soon as the component becomes ineffective.

If you select a different nonlinearity, you can define the parameters in the partial activity, diagram, or friction tabs.

Options

Using the check boxes in this section, you can define further properties of the nodal support. Depending on the selection, the specific direction or stiffness by means of fictitious column tabs are added.

Specific direction

The Specific Direction tab offers the possibility to rotate the support. This means you do not need to create a user-defined coordinate system.

Direction type

Several options are available for the orientation of the support: You can rotate the support about the support axes X', Y', and Z', align it to one or two nodes, or arrange it parallel to a member. You can select the objects graphically using the button .

Stiffness by means of fictitious column

The Stiffness by Means of Fictitious Column tab is particularly recommended for point supports of 2D structures. Here, you can determine the support spring constants from the parameters of a column that is not represented in the model. From the boundary conditions, RSTAB determines the spring stiffnesses of the support. They allow a modeling that represents reality better than the rigid support at the node.

Parameters

An elastic nodal support is used as the 'Model of the support'. The stiffnesses of the translational and rotational springs result from the geometry and material data of the column that you define in this tab.

The 'Column head' geometry can be described as rectangular or circular, optionally with a rotation of the column.

The 'Column height' affects the constants of the translational and rotational springs.

Column cross-section and material

The cross-section and material properties of the column are required to determine the spring stiffnesses. If the column is not 'Identical to the column head' (that is, neither rectangular nor circular), you can select or newly define a suitable column cross-section in the list.

Select the 'Column material' in the list. Using the buttons and , you can create a new material.

Column conditions

The type of support at the column head and column base is included in the determination of the translational and rotational springs. The following options are available for selection in the list:

• Hinged
• Partially restrained
• Rigid

With the 'Partially restrained' option, you can specify the degree of restraint at the column base as a percentage.

The 'Shear stiffness' of the column is taken into account by default when determining the stiffnesses.

Support springs due to fictitious column

This section lists the constants of the support springs that result from the geometric and material properties of the column. The values are transferred to the 'Basic Data' tab.

Partial activity

The Partial Activity of a support component is available as a nonlinear property of the support (see image selecting support nonlinearity).

Define the activity of the support for the 'Negative range' as well as for the 'Positive range'. The sign convention is explained in the failure section. In the 'Type' list, various criteria for the effectiveness of the support are available for selection.

• Complete: The component of the support is fully effective.
• Fixed from support displacement/support rotation: The stiffness of the translational or rotational spring is only effective up to a certain displacement or rotation. If this is exceeded, a rigid support or a restraint becomes effective.
• Cracking from support force/support moment: The support is only effective up to a certain force or moment. If this is exceeded, the support fails.
• Yielding from support force/support moment: The support is only effective up to a certain force or moment. If this is exceeded, the strains increase, but the stresses no longer do.
• Failure: The component of the support is not effective.

Most support types can be combined with a 'slippage', whereby the support only becomes effective after a certain displacement or rotation.

Diagram

The Diagram of a support component is available as a nonlinear property of the support (see image selecting support nonlinearity).

In the 'Displacement' or 'Rotation' column, define the number of definition points of the working diagram with the corresponding values. In the 'Force' or 'Moment' column, you can then assign the abscissa values of the displacements or rotations to the support forces or moments.

The following criteria are available for selection for the 'diagram start' and the 'diagram end':

• Cracking: The support is only effective up to the maximum value of the force or moment. If this is exceeded, the support fails.
• Yielding: The support is only effective up to the maximum value of the force or moment. If this is exceeded, the strains increase, but the stresses no longer do.
• Continuous: Beyond the definition range, the spring constant of the last step is applied.
• Stop: The permissible deformation is limited to the maximum value of the displacement or rotation. If this is exceeded, a rigid support or a restraint becomes effective.

Stiffness diagram

The Stiffness Diagram of a support component is available as a nonlinear property of a rotational support.

First, in the 'Stiffness dependent on' list (at the bottom of the tab), define the support force component on which the spring stiffness depends. The option |P| represents the resulting support force.

Then, in the 'Force' column, define the number of definition points of the working diagram with the corresponding properties. In the 'Spring' column, you can then assign the respective spring constants.

The following criteria are available for selection for the 'diagram start' and the 'diagram end':

• Cracking: The support is only effective up to the maximum value of the force. If this is exceeded, the support fails.
• Yielding: The support is only effective up to the maximum value of the force. If this is exceeded, the strains increase, but the stresses no longer do.
• Continuous: Beyond the definition range, the spring constant of the last step is applied.

Friction

In the 'Nonlinearity' list, four options are available for defining the Friction of the translational support depending on another support component (see image selecting support nonlinearity).

The transferred support forces are related to the compressive forces acting in another direction. Depending on the selection in the 'Basic Data' tab, the friction depends on only one support force or on the total force of two support forces acting simultaneously. The following relationship exists between the support force and the friction force:

The FAQ 003537 explains how friction can be taken into account at a nodal support.

The following column model shows a support in which horizontal forces are transferred by friction. However, the horizontal forces may be at most 10 % of the vertical force. In LC 1, this condition is met. In LC 2, the model becomes unstable because the horizontal load is too large.

Scaffolding hinge

The Scaffolding Hinge is available as a nonlinear property of the support for the rotational degrees of freedom φ_X and φ_Y. This allows you to define scaffolding supports for temporary structures such as working scaffolds or props.

In the 'Scaffolding Hinge' tab, you can define the M-φ working diagram. The parameters are described in more detail in the product feature scaffolding support.