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Nodes & Supports

Nodes serve a number purposes: they define locations of elements and provide an FEA link between elements. They also function as external supports. Nodal supports are aligned with the global coordinates and are either free or rigid in each direction (degree of freedom). If you need an elastic support, or a skewed orientation support, use a Spring Support.


Global Coordinates

A global coordinate system is one that all entities in your structural model share. VisualAnalysis has a single global coordinate system available for this purpose and nodal coordinates are specified relative to the global system. Most graphic views show the direction and location of the global coordinate system, however if the global origin is not visible the global orientation is shown with dashed axes as shown. Global coordinates are always represented with these upper-case letters {X, Y, Z,) or (R, Q; F). The greek characters may be spelled out in places where special fonts are not available: rho, theta, phi.

VisualAnalysis also uses element local coordinate systems that are distinct to each element. It is extremely important that you know the difference between element local coordinates and the global coordinates. Some data (both input and output) is defined relative to the global system and other data is relative to an element's local system.

Nodes are entered and edited in global coordinates. These are Cartesian coordinates (X,Y,Z) by default, but polar coordinates (R, Theta) and spherical coordinates (R,Theta, Phi) are also available in the Modify tab of Project Manager. To edit a node, select it in the Model View.

Degrees of Freedom

Depending on the chosen structure type, nodes may move or rotate with respect to the global coordinate system. The ways a node can move or rotate are called degrees of freedom. In a plane frame for example, nodes may translate in the X and Y directions and they may also rotate about the Z-axis. These are represented as DX, DY, DZ, RX, RY, and RZ for "Dsplacement" and "Rotation" directions, respectively.

Creating Nodes

Generally you never need to explicitly create nodes as they are created automatically when you sketch members, plates, and cables or when you generate or import models. You can precisely position elements after they have been imported and sketch elements to their correct locations in space using a Grid. There are instances where you may wish to manually create one or more nodes. This may be performed through using the Create tab in Project Manager and dragging the node item onto the Model View. See Also: 11 Easy ways to Create Nodes

Nodal Tolerance

As you create models, nodes are generated graphically or numerically. Sometimes due to round-off or grid locations or other issues two nodes will fall "almost" on each other but be slightly off. VisualAnalysis lets you control how close together nodes are before they are considered 'identical' and an existing "close" node will be used in place of generating a new node. Use the Project Settings, Nodal Tolerance to define a tolerance that is reasonable for your project. The default is 1/16th of an inch, which may be appropriate for very small assembly models. A one-foot tolerance may be acceptable for most building models. You should avoid setting this to a very large value--VisualAnalysis limits you to 36 inches--which might cause tolerance issues in strange places in the software.

If you think you have problems with duplicate nodes, you can adjust the Nodal Tolerance and then run the Model | Consolidate Close Nodes command to find and merge duplicate nodes.

Free or Supported?

Most nodes in a model will be free from external supports. Nodal supports are used to restrain the entire structure against rigid body translation or rotations and typically are found at the bases of columns.

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 also End Releases for a member. Supports are strictly "outside" restraint, like your structure sitting on a rock--the rock is a support.

Choose your supports wisely. A support should only be used at a location where load is taken out of the model or where you are specifying a nodal displacement. Do not support nodes at locations where you are applying external loads. Also, do not support nodes in directions where you have placed a spring support as this will render the spring support ineffective.

Scissor Joints

Requires: Advanced Level

Commonly you will have two members that cross each other and are "pinned" together, like the joint in a pair of household scissors. In a real structure this can happen when one beam passes over the top of a girder, and in other situations. Sometimes you do not want to go through the hassles of modeling the full 3D geometry.

A scissor joint is a finite element trick to save you the hassle of defining two offset nodes with a rigid link between them to get the proper behavior. Select the node where Combined Members cross, and check the Scissor Joint option in the Modify tab of Project Manager and VisualAnalysis does the magic for you.

Nodal Loads

You may apply concentrated nodal loads in the form of forces, moments, settlements or rotations. These loads are applied with respect to the global coordinate system, where positive loads are in the direction of the global axes (or follow the right-hand-rule for rotations) and negative magnitude loads reverse the direction. They may act only in a direction where a degree of freedom exists for the structure type.

Nodal settlements and rotations must be applied in a supported direction. The support can come from a nodal support or a spring support.

In order to apply an inclined load (not parallel to one of the global axes), you will need to break it into components and apply the global direction component loads separately.

Nodal Mass

Nodes may also be loaded with additional lumped mass (entered as a Force, convenient for most of using the English system of units). This mass is like self weight, but is ONLY used for dyanmic inertial effects, it 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 and rotations for unsupported degrees of freedom. In supported directions, reaction forces and moments are produced following the global coordinate sign convention, where a positive reaction is in the direction of the global axes (or the right-hand rule for rotations). If you apply a nodal settlement or rotation load to a node, that becomes the displacement results for that direction. Regardless, force reactions are still calculated.

Disconnected Nodes

Sometimes nodes can become disconnected to member elements, even though they lie at a coordinate that coincides with the member element.  This can occur when opening models created in different VisualAnalysis versions or other supported file formats.  It can become tedious to manually associate these nodes with the members that cross them.  VisualAnalysis  can automatically resolve these issues using the Fix Disconnected Nodes option in the Split Member dialog.

If nodes are created "loose" or orphaned and are not connected to anything at all you can find them using the Analyze | Check Model for Errors command, as they tend to be graphically small amid the clutter of a complex project.

Spring Supports

Spring Directions

Translational spring supports may oriented along lines parallel to the global coordinate system and are specified by direction cosines. When aligned with the global axes the cosines are either zero or plus or minus one.

Inclined Supports

VisualAnalysis offers pre-defined spring orientations for common directions like +X, -X, +Y, -Y, etc.. When entering cosines you may use complex math expressions such as COS(3/SQRT(3*3 + 12*12)). You can use negative cosines, which is most useful for using compression-only soil-spring supports, or tension-only supports.

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, you may place a spring at the support node and set its stiffness relative to the soil elastic properties. More information on calculating the stiffness is given in a section below. If you wish to model a compression-only support, you will need to orient the spring in the correct direction to get the correct behavior.

Spring supports "take load out" of a structure and should be used as external support only. They are not used for modeling partially rigid connections between elements. To model a settlement at an elastic support, you should use a Nodal Settlement Load.

Spring Results

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

Determine Soil-Spring Stiffness

When spring elements are used to model soil stiffness, the following procedure should be followed. VisualAnalysis offers a soil-spring generator for this purpose

1. Acquire a measure of the elasticity of the soil supporting the structure. The most common measurement for this is the subgrade modulus. Many references give values of this parameter based on general classification (dense gravel for example) or based on blow count measurements. The subgrade modulus has units for force per length cubed.

2. Convert from subgrade modulus to a spring constant, simply multiply by the area of soil covered by the footing or plate.

For example if you had a 3 foot by 3 foot square footing resting on a soil with a subgrade modulus of 300 lb/ft3 you would use a spring whose stiffness was 2.7 kips/foot (assuming just one spring for the whole footing).