I see this all too often. A home with a crawl space and dirt or gravel floor and lots of moisture. I know it’s dry rot and how to reduce it but at what point do I recommend a structual engineer be called in for an evaluation?
Never. . .
If/when it gets to the point where it needs replacement, recommend that it be replaced.
That floor design does not require any “engineering.” If you’re unsure, recommend a qualified contractor for evaluation and repairs.