With the Divide-Combine All Intersections command, the purlins are split in main beams, columns and/or trusses or split purlins are combine.
Location of Divide-Combine All Intersections Command
You can access it under the Ribbon menu Steel tab, Steel heading.
Click the divide-combine all intersections icon.
Select the purlins you want to split.
The ideCAD will automatically split all your purlins.
You can selectively combine previously divided purlins.
You can set the distance between the purlins between the dividing process in the division on intersections section in the extensions tab of the Purlin Settings dialog.
Purlins before divide-combine all intersections command
Choosing the purlins to divide
Purlins after divide-combine all intersections command