Wood Stair Stringer Design 1

[palves]

Does anybody have a reliable way of designing wood stair stringers? An earlier post referenced a link to the TrusJoist Timberstrand Stringers, but their tables are somewhat limited and my parameters go beyond them in several areas. Now I want a method to check a stringer of any geometry for any load and was wondering what others do since I haven't found much info on the subject.

 

[woodman88]

jporto, 1) Any sawn lumber cut to be stair stringers would under the wood grading rules need to be regraded to determain the design values. 2) I use LVL or PSL lumber and design values then use the minimum depth from the cut to the bottom edge (minus 1/2") to calculate the S and I for the stringer.

Garth Dreger PE
AZ Phoenix area

 

[msquared48]

You can also transpose the vertical load to a load normal to the stringer and use the actual sloped length of the stringer for the beam length.

Mike McCann
MMC Engineering

 

[Ron]

I analyze them as Mike noted. That gives both the beam bending action on the stringer as well as the shear on the connections at top and bottom.

As for the "wood" part...you consider the section at the notch; however, you can get some benefit of the treads and if you close the risers, that helps, but makes the notches deeper, so your stringer has to be a deeper section. I do not "re-rate" the lumber. It is what it is and the notch doesn't change that.

 

[SlideRuleEra]

I agree with msquared48 and Ron on HOW to analyze stair stringers, but woodman88's recommendation to regrade the assumed, reduced member size is good advice. For example, see this summary of grading rules for SYP:

http://www.spib.org/docs/2dimension_eng.pdf

Say that a No.1, 2x12 is notched so that you assume it is now equivalent to a 2x6. From the chart:

No. 1, 2x12 is permitted to have a 3 inch "edge" knot.
If the notching is done so that the 3 inch "edge knot is in the assumed 2x6 section, then the assumed 2x6 section does NOT meet the requirement for even a No. 3, 2x6 (2 3/4 inch "edge" knot, maximum).

With just a little care in how the notching is layed out in a particular board this problem can be avoided, but regrading does have to be considered.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[msquared48]

OK. I see what you are saying.

However, you could still specify in the details the allowable knot, wane, and other physical limitations on stress grades, so that the stress grade reduction does not have to be taken.

Mike McCann
MMC Engineering

 

[racookpe1978]

The not-knot-hole rerating is key - deliberately you're requiring/assuming that a better "piece" (if not grade also) of timber is going to be selected and properly cut.

Doubling the stringers helps protect your assumption: It would be rare to have two 2x12's have maximum-sized knots at the same place in both pieces of wood. Less creaking also, if the two 2x12's are glued and screwed together to make a beam.

i don't make the stairs themselves - as a hobby I make the ornemental iron ballusters and railing above them, rather than than the carpentry itself, but what I've seen work on the more expensive houses is the edge stringers are single timbers (because they are screwed to the walls of the stairs) and the center is doubled up 2x12 or 2x10.

 

[Stillerz]

Also--
When I framed houses I always added a 2x4 ply to bottom of the stringer or even a 2x6 if the notch wasnt too deep. Obviously this is no help for your post, just advice.
You can do all the engineering you want on something like this but it only takes one tiny little oversight for the whole stringer to be compromised when it is being cut.
On more than one occassion I cut step stringers out of No.2 SYP (not my choice) and had them break before I could get them in place. One little defect on the right spot can do this. You have to "crown" your stringers in the feild and depending on length, you dont really have a cjoice as to where the knots will land. This is why i started sandwhiching the bottom with an extra 2x4 or 2x6. Makes the stair significantly stiffer as well.