To try to answer your question of what is considered structural steel, in my experience, when AWS D1.1 is the specified code in the contract documents, the AISC Steel Construction Manual, which includes the Code of Standard Practice, is also specified. The COSP is available at this link:
https://www.aisc.org/WorkArea/showcontent.aspx?id=25256In Section 2, the COSP provides a definition and a detailed list of items which are structural steel, as well as other steel, iron, or metal items, though the COSP provisions are not intended to apply to the latter.
Edit:
What code is the applicable code in your contract specs? I've heard the same "since it's not structural it's not technically D1.1 structural steel code to go by" but in my review of the contract specs, D1.1 was the applicable code, and the material specs and thicknesses used were within D1.1 parameters.... two things that would also need to be considered with regard to the applicable code, but when in doubt, an RFI should clear up any confusion.