You will get the following error message if you try to change to the spacing on a girder truss:
You can change the spacing of a girder by re-adding it to layout or by following these steps in iDesign.
1). Save the truss new and click OK.
2). Click on Loading from the vertical toolbar and select the Clear Loads icon.
3). Now select the Modify Load Case icon and select the spacing tab. Make the change needed to the spacing.
4). Now engineer, plate and save the truss to layout. After the truss is selected on layout and you are back in iDesign it should prompt to reapply point loads. Click Yes.
Now take a look at your component drawing and the spacing should reflect the change you just made.