Open Stringer Stair Construction
Blind dado the treads in and use a PL Premium Urethane adhesive. Clamp it all together. However - if there is any flex in the tread, you may be asking for stress fractures at the joint later on. Better get them engineered or tested.
From contributor J:
I've run a pair of long, recessed lag bolts into the ends of each tread from the outside of the rough stringer face (with the final facing concealing the bolts). I've also run threaded rods completely through the treads and supporting stringer. I've also seen the treads toe-nailed by carpenters (who managed to hit the nail heads about half the time).
Your question really cannot be answered without knowing all the particulars of the construction. You've mentioned the stringer plan but not the tread construction. Is the 2" thickness net or nominal? (Is that thick enough?) Is it solid or composite? How wide is the tread span?
Keep in mind that whatever carpenters (such as you or me) may suggest or decide, this isn't a decision that we should have to make. This is the architect's responsibility (and liability)? Do you happen to have a structural engineering license?
As far as I'm concerned, open stairs are the most difficult to engineer.
From the original questioner:
Thanks. We have used blind dadoes and PL in the past and it seems to work fine, but this detail is not specified on this job. The architect has specified 2" x 11" x 45" treads. They will be 3 layers of veneer core plywood with solid front and back edges, veneered over top. Total run is about 70" as there are a series of landings.
From contributor V:
Rail bolts plugged from the bottom.
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