I don’t think that the sections like solid rods or plate strips, that you have defined from the Dutch section database, can be designed as per the BS 5950 code in STAAD.Pro and so I am surprised that the software is even attempting the design. I would let my colleagues who are more familiar with the BS 5950 implementation, confirm my observation or correct me if I am wrong.
On a side note, please do not post STAAD/RAM related questions to the Files section of the Bentley Communities. These should be posted in the STAAD|RAM forum .