The base nodes of all the columns are restrained against translation and rotation about all the 3 global axes (i.e., fixed supports at those nodes).

SUPPORTS
1 4 5 FIXED

1. On the Select ribbon tab, select the Node Cursor tool in the Cursors group.
   
   
2. In the View window, hold <Ctrl> and click on nodes 1, 4, and 5 to select them.
3. In the Supports dialog, click Create.

   The Create Support dialog opens.

   
4. Select the Fixed tab (selected by default) and then click Assign.
   

   The structure with the supports assigned

5. Click any open space in the View window to deselect all selected nodes.

   This prevents accidental assignment of unwanted data to those nodes.