Supports need to be added in order to restrain the structure. If no supports were present then the structure would not be static and be unsolvable. It is important to remember that a structure must be restrained in every direction at some point by any of the supports. i.e. every degree of freedom must be accounted for by at least one ‘F’ in the structure.
Similar to the connections between members, the supports are given a 6-digit code to specify which of the node’s degrees of freedom are restrained. Each of the degree of freedom’s must be defined as either Fixed (‘F’), Released (‘R’) or Spring(‘S’). The degrees of freedom are in the following order:
- X Translation
- Y Translation
- Z Translation
- X Rotation
- Y Rotation
- Z Rotation
There are 3 types of letters (or of releases) you can use:
- “F” – Fixed – This means the degree of freedom is fully fixed and the member will transfer this force to the node
- “R” – Released – The force is not being transferred for this degree of freedom
- “S” – Spring – The force is being transferred with some stiffness factor. This will require additional input.
A pin support (which only allows rotation about the z-axis) would be denoted by the code ‘FFFFFR’ whereas a roller (which allows rotation about the z-axis and movement in the x-direction) would be given the code ‘RFFFFR’.
Supports can be selected from the pre-configured support buttons. Along with their fixity codes, these include:
- Fixed Support – ‘FFFFFF’ – Fixed in all translations (x,y,z) and rotations (Mx, My, Mz).
- Pinned (Hinge) Support – ‘FFFFFR’ – Fixed in all translations (x,y,z) and but free to rotate about the z-axis (in this 2D case).
- Roller Support (in x) – ‘RFFFFR’ – Free to ‘roll’ along the x axis and rotate about the z-axis.
- Roller Support (in y) – ‘FRFFFR’ – Free to ‘roll’ along the y axis and rotate about the z-axis.
- Spring Support (in y) – ‘FSFFFR’ – There is a spring support allowing some force to be transferred in the y translation.
Any support outside of these pre-configured ones can be manually typed into the “Restraint Code” field.
When supports are applied, the fixity codes are based on the global axis of your project, not the local axis of the member that is connected to it. For more information on these fixity codes, please visit our blog piece on Fixity codes.
Once the user enters an ‘S” in a particular direction, another input will appear to prompt the user to enter a Stiffness for that particular direction. e.g. if the user enters a Vertical Spring (like the above example) an input for Y Stiffness will appear. The stiffness input is entered as force/distance (kip/ft, kN/m, etc..) which essentially represents how much force is required to compress the spring by a given distance (foot, meter).
The spring stiffness (normally denoted as k) can be calculated in a few different ways (based on Hooke’s Law (F/displacement), or based on the material EA/L) or can be supplied by the manufacturer of the spring support.
If you have a support that is not parallel with the X-Z plane, you can add an inclined support. These supports are not fully fixed or pinned, but are “roller” supports that are free to move in the rotated plane. The reaction results at inclined supports will be orthogonal to the rotation of the support. You must select a node before clicking on the inclined support button as shown.
To model an inclined support, go to the “Supports” menu, then click on the inclined support button. The Inclined Roller Support popup will appear, input the necessary data.