Can I use luan for the inside core of a curve stair stringer? My inside radius is 36 inch.
From contributor M:
Are you talking plywood or solid lumber?
The truth is, any construction grade C or D plywood or strand board (OSB) sheet is a structural panel but their faces are rough and have voids. It's also true that I know of no uniform building code designating the type of material for laminated stair stringers. Consequently, I have seen laminated stair stringers made from many different kinds of materials, including various composites and luan plywood. Be careful here, some local building departments require free-standing stair structures to be fully engineered.
The person to answer this question (with any degree of competence) is a person with a degree in structural engineering and/or the architect. I can tell you that a supporting stair stringer is just a load-bearing beam spanning a horizontal distance. The fact that it is inclined and cut in a saw-toothed pattern are usually ignored in the calculations. The remaining vertical depth of the stringer (after the cut) constitutes the depth or width of the beam and the span is measured horizontally (and not along the pitched beam itself). The fact that the stringer is curved, laminated, and part of a torsion box construction might also be ignored (or might not). Simple wood beam “span allowance tables” are then applied to the stair structure. Both "live and static loads" are part of the engineering with live loads normally @ about 100 psf. The individual, technical structural properties of the wood specie itself are also derived from regular reference sources.
After all of this information has been taken into account as well as a few other things like fire code considerations, your question may (or may not) be answered. The use of fiberglass and/or steel incorporated into the wood can also help stiffen the structure but is usually more trouble than it's worth (you'll get an argument here). Any "open tread" (open riser) stairs are the greatest challenge to keep from springing since the stringers themselves can twist and flex individually without the support of a good torsion box design.
FYI: A few of my own rules-of-thumb (which may have no engineering validity at all). I normally make the remaining vertical depth of a cut inside stringer at least equal to the height of the riser. The vertical depth of the outside stringer is then determined by a horizontal, radial line drawn from the bottom of the inside stringer to the bottom of the outside stringer. The thickness of a supporting stringer is normally a net 2 1/2" to 3 1/2" which translates into a nominal 3" or 4" beam.
If you really want to understand this stuff, you have to study it because it's not as simple as you might think. If you get yourself into larger, commercial projects, the structural engineering will most often be mandated and will need to be included in your cost estimate.