Loads and Restraints are the boundary conditions for the truss problem we are solving. For Truss Solver to find a solution, there must be at least three Restraints applied to the truss. Restraints are specified displacements, typically with value zero. Additionally, there must also be some form of non-zero boundary conditions applied, either a load or non-zero displacements at some other nodes. See the Discussion of proper Boundary Conditions for more on determining the required number of BC's.
Both loads and restraints need to have two quantities defined:
You can only apply one load or restraint in a given direction on a node. That is, you can't have both an X-direction force and X-direction displacement specified at the same node.
Loads and Restraints are specified from the Load/Restraint item on the Model menu, which opens the Edit BCs dialog:
This dialog lists the boundary conditions on the truss. Click the Add button to add a load/restraint. This displays the Add BC Dialog:
Select a node, Force or Displacement, direction and value for the desired BC and click Accept. We will first add an X-Direction displacement restraint with a value of 0 (fixed displacement) on node 1:
After we click Accept, the new BC shows up in the Edit BC's dialog list:
Once a boundary condition has been defined at a node, the node changes to blue in the main window:
repeating this process for two more restraints and a load (with a value of 1 for simplicity) gives:
and nodes 2 and 3 change to blue as well:
To change a BC you first Delete it and then Add a new one.
You will always need one displacement value specified in each of the X and Y directions as well as a third displacement specified to stop the truss from rotating.
Once you have defined the correct number of boundary conditions, the problem can be solved!