Defining the Boundary Conditions
One very important aspect of generating a model that will behave in a realistic fashion is the correct application of boundary conditions (the ability of nodes to translate and rotate). By opening the "Properties | Boundary Conditions" dialog box and clicking on any node, you will see that they are all free to move in any direction and to rotate any way they choose. Obviously, this would not be the case in real life and we must selectively prevent certain nodes from moving in particular ways:
WARNING: The thermal and secondary geometric expansions/contractions of the slab and columns must not be artificially restrained by unnecessary boundary node fixing. These are the most common cause of wrong and over-optimistic results!
Firstly, we will tackle the axis of symmetry at the right-hand side of the structure. On any axis of symmetry it is usual to restrain any horizontal node movement normal to the axis, rotation about the axis and also rotation about vertical axis. However, two parallel axes of symmetry like this would be over-restrictive and would cause thermal buckling at low temperatures.
To do this:
1. Open the "Properties | boundary Conditions" dialog box.
2. Select the 'Attach' button.
3. Fix the translation in the X-direction and rotation about the Y- and Z-axes by clicking the relevant buttons.
4. Select the nodes along the axis of symmetry using the mouse.
5. Click the 'Apply' button. The nodes will change colour.