Mitred corners using the Split function in the Structure Tool (formerly Objective)
To create the correct mitred junction at Point A:
1. Select the two Steel beams or Joinery elements to be intersected,
2. Choose Ci Tools> Objective*> Split, (*Structural in Archicad 28)
3. Click on the internal intersection of the two beams to define the start of the Split axis,
4. Click on the external intersection of the two beams to define the end of the Split axis,
5. Carefully select the small offcuts that are no longer needed, and delete.
Now you have a clean intersection as pictured. Before:
…and after: