The OP's next response said this, which introduced the topic of light to her original question: "The reason I wanted to go with the open staircase is that it is U-shaped and located in the corner of the structure. It has landings that are bumped out from the structure with windows. I was hoping the open risers would help bing light in to the basement..." I did not say the forum should be a place to congratulate each other on home design and avoid critical design issues. I have no problem with your statements about the codes applicable to winder stairs and your suggestion for the grab bar, which we already have and is a good suggestion. But my stairs are already built and not going to be changed and no one responding to the thread even suggested that they were thinking about building winder stairs, so it served no constructive purpose for you and Sue to go beyond the code issues and give your personal opinions on my stairs other than to make me, the home owner who was criticized for building the winder stairs, to regret posting a picture on this forum, and doing things that discourage homeowners from posting picgtures of their already built/can't be changed homes is not good for the forum, IMO. I did measure the depth of our treads at the wind, and the minimum depth at the wind (at the wall) is 4-1/2", so our code is probably like the code in your area.

I would look at doing 1/4" steel plate similar to what brick masons use to re-inforce them along the length of the step. If you have to go more than few inches, you might as well go the entire way. You will probably have to go thicker than the standard tread thickness eventhough ipe is quite hard. Keep in mind the additional weight of the steel plate as well as the ipe tread as ipe and steel weight a lot when its that thick. Which leads me to the next thing - I'd consider a lot more than just 'blocking' to hold a freeform tread. My guess is that the weight of each treat could be 150lbs but thats not the big issue - its the dynamic load as people are bounding up the stairs. . . You may want to look at 2x12 continuous along the run of stairs or something sturdier than that. Ipe is available in larger pieces as the tree is quite large. You'll just have to find a good specialty lumber supplier that can get it for you. I've seen curved handrails cut out of ipe, so its definitely available. It's just very hard and very heavy.

We have 5" walnut floors and dark-stained oak treads on our stairs. I don't think they have to match.

OK, finally took some pics of the stringer and transition to baseboard in hall at top of stairs - think this will work? Lovely gap - over 1" from wall to edge of stringer Colonial "stop" molding (not thick enough so have to lay flat instead of laying it against wall - my cousin was going to finish off the end with a mitred return, just tape holding it right now) But that still leaves the problem of transitioning to the thinner, lower baseboard, and a gap b/t the stringer and the wall So I'm thinking of running the stop molding vertically as well - he'll just have to figure out the angle to join the 2 pieces of stop (and of course cut the baseboard so it fits snugly against it). Either that, or just do the mitred return and then put a block in between like crazyhouse6 did? Which do you think will look better? I want to run the stop vertically at the bottom of the stringer too, to hide the end grain and cover the awkward chipped corner bead on the drywall where it meets the stringer just to the side of the newels (and not do the molding where DH is holding it here, just run the oak board to "box" in the bottom of the newel higher than the stringer), so would it be best to do the same treatment top and bottom? Sorry, I can get DH to hold the newel and take some better pics of the bottom tonight while I hold the stop over the end grain.