wood stair stringer design

huangyhg

wood stair stringer design
does anyone have a design reference for wood stair stringers which are notched for the stair treads?  this is for an exterior wooden stairs with landings for a 3 story apartment.  the live load for these stairs is 100psf and one of the spans is over 14'.  the architect wants the stringers to be notched for the treads.  according to breyer, he cautions against notching beams in bending and says it is not fully understood.  according to nds, sect 4.4.3, it says notches are not permitted in the middle 1/3 span.  before i go see the architect, am i missing something?

many people use an "extra" stringer (non-notched) as a sister stringer.  btw - does this meet fire code??
in my office, we design stair stringers as a beam of depth equal to the depth of stringer remaining perp to the notch at the intersection of the tread and riser.  if needed, the stringers could be doubled up if overall depth of the stringer is of concern.
can the outside stringers be nailed or screwed to the side walls?
one thing to watch out for is that if a board is notched, and say you leave 6 inches uncut, it's not necessarily as strong as a 6-inch board, and i believe boards need to be re-rated after they are cut because of knots and such.  in this case, for this reason, if you sister on a 6-inch board you would more than double the strength.  
the minimum for load rating doesn't necessarily give a nice firm staircase anyway.  if you go well above that minimum, the stairs will feel nice and rigid.  
vertical support posts in the middle would help a lot too (if that's an option.)
because of a stair program i wrote, i've talked to hundreds of stair builders and i've come across a couple that used engineered 3x12's but glued a 2x3 on the top (where the treads were going to be cut out.  this left more uncut stringer, while most of the 2x3 is cut away anyway.  an excellent glue-job would be necessary though.
yet another option is to not cut the notches all the way in, and partially support the treads with brackets.  for example, you are using 11.5 inch treads.  only cut the notchces (run) in, say, 6 inches.  now you have 5.5 inches of overhang, and this is supported by brackets or boards.  this is sort of a hybrid between box and notched, and leaves more of the stringer for span support, but you have to spend more time attaching and supporting the treads because they are no longer just happily sitting on the notch.
consider using lsl stringers ilevel has literature for this application.
can you add stainless steel straps to the underside of the stringers and design them as compsite   
i missed the exterior part.  lvl and lsl must be weather resistant.  consider using a "boxed" condition where you route the stringers 1/2" for the treads and risers.  even then, a pt 2x12 syp fails for a 14' span.  you did not give a width, so it would be hard to say what really works.  i would be very cautious about notching treated dimensional lumber for this condition.  as crouton said above, the material would have to be regraded after notching to verify what you have left.