Nodes define locations of elements, provide an FEA link between elements, and function as external supports. Nodal supports are aligned with the global coordinates and have either a free or fixed degree of freedom in each direction. To edit a node, select it in the Model View, and change the properties in the Project Manager | Modify tab. Multiple selected nodes can be edited at the same time. Use a Spring Supports when an elastic support or a support with a skewed orientation is needed.

Global Coordinates

The global coordinate system is shared by all entities in the model. VisualAnalysis has a single global coordinate system and nodal coordinates are specified relative to the global system. Global coordinates are always represented with these upper-case letters (X, Y, Z). VisualAnalysis also uses element local coordinate systems that are distinct to each element. It is important to understand the difference between element local coordinates and the global coordinates. Some data (both input and output) is defined relative to the global system while other data is defined relative to an element's local system. Global Cartesian coordinates (X,Y,Z) are used by default, but polar coordinates (R, Theta) and spherical coordinates (R, Theta, Phi) are also available in the Project Manager | Modify tab.

Degrees of Freedom

Depending on the chosen structure type (Space Frame vs. Plane Frame), nodes may move or rotate with respect to the global coordinate system. Degree of freedom is defined as the ways a node can move or rotate. In Space Frames, nodes can translate in 3 directions (DX, DY, and DZ) and rotate about 3 axes (RX, RY, and RZ). In a Plane Frames, nodes can translate in 2 directions (DX, and DY) and rotate about 1 axes (RZ).

Creating Nodes

In VisualAnalysis, nodes do not need to be explicitly made as they are created automatically when members, plates, and cables are created or when models are imported. There are several ways to create nodes in VisualAnalysis:

• Use the Structure | Create Nodes button (or the Ctrl+Shift+1 hot key) for instant access to the Multiple Node Creation dialog box. Enter the coordinates manually or import the coordinates using the Windows Clipboard (Copy/Paste).
• On the Project Manager | Create tab, double-click on Add Nodes, Singly or Locate Multiple Nodes.
• Create a new grid or edit and existing grid on the Project Manager | Grid tab to define the locations for potential new nodes and draw members, plates, and/or cables between gridlines.
• Use the Structure | Create Members command or the Project Manager | Create | Add Member command to create members with new node new or existing nodes.
• Select an existing node or element and use Structure | Generate Copies or Ctrl+B to generate copies.
• Import a DXF file. When members or plates are imported, the associated nodes are automatically created.
• Use the Structure | Extend Member feature to create a new node and member at a specified distance from the existing member.
• Use the Structure | Split Member/Plate/Cables commands to split elements and create new nodes.

Nodal Tolerance

As models are created and nodes are generated, two nodes will sometimes be created at almost the same exact location. VisualAnalysis provides a feature to control how close together nodes are before they are considered 'identical' and an existing node will be used instead of generating a new node. Use the Project Settings | Nodal Tolerance to define the desired tolerance for the project (1/16th of an inch is used by default). Avoid setting this to a large value (VisualAnalysis limits 36 inches maximum) which might cause tolerance issues. If problems with duplicate nodes arise, adjust the Nodal Tolerance and run the Structure | Consolidate Close Nodes command to find and merge duplicate nodes.

Free or Supported?

Nodal supports are used to restrain the entire structure against rigid body translation or rotations and typically are found at the bases of columns. Common supports are pinned allowing no translation, or fixed allowing no movement at all. First-time users of VisualAnalysis will often confuse nodal supports with "Joints" or "Connections" between members. Member connections are defined by the Structure-Type and End Releases for a member. A support should only be used at a location where load is taken out of the model or where there is a nodal displacement. Do not support nodes at locations where external loads are applied. Also, do not support nodes in directions where spring supports have been placed as this will render the spring support ineffective.

Scissor Joints

Scissor joints are used where all member translate the same amount but parallel members sets will rotate independently of each other. Select the node where members cross, and check the Scissor Joint option in the Project Manager | Modify tab.

Nodal Mass

Additional lumped mass can be applied to nodes in the model. Note: Mass is input as a Force which is convenient when using the English unit system. This mass is only used for dynamic inertial effects and is not used in any static analysis. Self-weight of elements is defined by element properties and other dead loads should be applied as service case loads on nodes or elements. For more information about mass, see Dynamic Analysis.

Nodal Results

Nodal results include displacements or rotations for unsupported degrees of freedom and reaction forces or moments for supported degrees of freedom. The global coordinate sign convention is used for reactions and displacements, where a positive reaction is in the direction of the global axes or the right-hand rule for rotations. Nodal settlements or nodal rotations can only be applied to a fixed degree of freedom and the reaction is calculated for the applied displacement or rotation.

Disconnected Nodes

Use the Tools | Model Check command to find unconnected nodes in the model, as they tend to be graphically small and can be hard to find in a complex project. Use the Tools | Fix Model command to remove unconnected nodes in the model.

Spring Supports

Spring supports are used when the restraint at a node is between free and fixed. Spring supports can be manually created at each node or the Soil-Spring Generator can be used to generate springs under plate elements.

• Use the Structure | Create Spring Supports command to create springs for selected nodes.
• Select one or more nodes graphically, then double-click the Add Spring Supports item ing the Project Manager | Create tab.
• Use the Structure | Generate Soil Springs command to generate soil springs for a group of selected plate elements. Details.

Spring Directions

Translational spring supports can be oriented along lines parallel to the global coordinate system (+X, -X, +Y, -Y, etc.) and are specified by direction cosines. When aligned with the global axes the cosines are either zero or plus or minus one. Complex math expressions such as COS(3/SQRT(3*3 + 12*12)) can be used when entering cosines.

Elastic Supports

Spring supports are often used to model support conditions that are not truly rigid. For example, many soil-based footings have some elastic compression behavior that results in support settlement. When these cases exist, place a spring at the support node and set its stiffness relative to the soil elastic properties. To model a compression-only or tension-only support, the spring will need to be correctly oriented to get the desired behavior. Spring supports take load out of a structure and should be used as external support only. They should not be used to modeling partially rigid connections between elements.

Spring Results

Spring element output is based on the following sign convention. For displacement springs, a positive force is tension and a negative force is compression. For rotational springs, a positive moment indicates the spring is being twisted according to the right-hand-rule.