Sir, I deal with degree engineers on a daily basis. They will pull out a book answer on me all the time. In practical application it doesn’t work. Here is why...You are relying on that 2x12 and tread to carry the load. However, the design is wrong because the single point of failure is the 2x4 scrap piece holding the tread. Therefore, when they dry-rot or split they will fail because you do not have a second point of failure. Which would be the middle stringer. They will be solid for a yr maybe two, but after that I would not trust those stairs. I hope none of your family weights over 200lbs. I’m sorry to point this out. I been doing construction for over twenty years. I just want your family to be safe. That’s all, I’m not trying to be over critical of your work. Here is an example I fixed last week.
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