That is rather complex problem, I belive it is dificult to program archicad to generate correct geometry in every scenario. In situations as this I would use another tool (perhaps mesh or morph) to create the bottom part of the stairs and only show risers from archicad stair tool in sections and 2D. Also it is rather unfortunte 
that you cannot use stair in boolean operation - if you could, you would be able to easily shedule mass of the stairs.
that you cannot use stair in boolean operation - if you could, you would be able to easily shedule mass of the stairs.
The underside is much better with flat landings.
It is the winder treads that really messes things up.
You can split the structure of the stair - it will remain as one complete stair.
This will give you slightly better control of the underside of the stair - especially on the landings and winders.
But you still won't get a great underside.
The best solution is to model the stair in separate sections for each flight, landing and winder.
So model one stair that is full height - just to get the settings for risers and goings and number of treads.
Using these settings, model the individual parts of the stair and elevate each section to the correct height.
You can set the end connections so all the pieces join correctly.
The down side of this is it is not one complete stair, and you will have to adjust tread numbers, walking line, break line etc., for each part.
It is possible - it just takes longer.
Barry.
Still looking?
