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notched stair stringer question...

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ampersand

Structural
Sep 2, 2005
30
Suppose you are calcing some exterior wood stairs, built the traditional way- (2) 2x12 stringers, notched, resulting in a remaining depth of 5-1/2".

I've never been sure what adjustment factors to use, and NDS literature seems to indicate that we are not allowed to notch a bending member more than 1/6 the depth, which would mean we simply cannot build stairs this way. Also, I have spoken to someone at AWC about ripping members, and he informed me that once a member is ripped, it must be regraded before we can know what allowable stress to use. Our notched stringer is quite similar to a 2x12 ripped down to 2x6 dimensions, but with some extra bits above which hold our treads where we want them...

But contractors always build stairs this way, and I would like to accomodate them... so any advice would be appreciated.

Cheers!
 
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When we are hampered by a *determined* (polite word) Plans Checker, the only way to resolve this is to provide full depth engineered wood (LVL, Microllam) 2x12 nominal, unnotched, as the outside stringers. The treads are supported by plywood cleats. We still provide a notched center stringer, but it isn't included for capacity.
Another trick, if possible, is to provide a mid-span 2x4 stud cripple wall to cut the span in half.
 
The use of notched 2x12's as stair stringers to the best of my knowledge is based on traditional practice. The person you spoke with at AWC was correct that once solid sawn lumber is ripped it must be regraded to establish design values.

What may happen is that the 3" to 4" knot or 3" hole, that is allowed in your 2x12 may end up in the unotched area. So instead of having a section equivalent to a 2x6, you might have a section equivalent to a 2x4 or even a smaller section.

I believe one of the reasons, the use of notched 2x12 worked so well in the past was because of the higher level of craftsmanship. In the past I think builders culled out more material. When building a stair stringer they probably selected 2 x 12's which would have mainly clear wood in the unotched section. I am sure some of the framers today if given a notched 2x12 with a 2" hole, would use it. In the past I think most builders would have tossed such a piece aside.
 
Thanks for the replies. I think I have no choice but to use full-depth lumber on this job...
 
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