Joist selection

Wayne Whitney · 2008-02-16T00:33:34+00:00

Hi,I've got to sister five 17' long ceiling joists, due to removing abearing wall. I've done the engineering, and given the existing roofconstruction, I've got two options that work:1) Use double 2x6 select struct at 16" o.c. 2) Use 2x8 select struct at 16" o.c. with a 3.5" long by 1.75" deep top edge end notch.The selection is deflection controlled, as 2x6 select struct at 16"o.c. meets bending and shear.Which option is better? The material cost for the 2x6s is about $50more. I'm mostly working alone, so the 2x6s will be somewhat easierto wrangle into place. Otherwise, it is notching the 2x8s versus theextra nailing to double the 2x6s.Thanks, Wayne

Feb 16, 2008 88 Replies

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Wayne Whitney 18 years ago

Rico needs it, as he says L/360 isn't stiff enough. :-) Based on the discussion I see it as a small definite improvement, when the time comes next week I'll figure out if it is worth the small definite extra work.

Yes, that's a good thing, as the old growth DF is #2 or #3, while the new members will be SS, so we can call it a wash. :-) Actually it's not really a wash, as a few of the old joists have large knots on the bottom half near midspan, that's really bad.

Assuming the old and new have the same E = 1.9 Mpsi and a 20psf live load and 10psf dead load, the old 2x6s spanned 13'9" (good for 14'4" at L/360 deflection) and the old 2x4s spanned 9' (good for 9'1" at L/360). The new 2x8s will span 16'5" (good for 17'2" at L/360). Everything is deflection controlled. So the new 2x8s will be just about as stiff as the old construction.

Good point.

Cheers, Wayne

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Bobk207 18 years ago

Wayne-

Is the E for new growth DF really 1.9 Mpsi?

btw correct me if the NDS says (or you if know) but SS is about flaws not a high E.

Old growth DF has very closely spaced growth rings & I was told that wood density drives E not knots / flaws.

I'd bet on the E of the old timber being higher but the new SS having few / no flaws.

Have you considered engineered timber? It can have some pretty high E's.

cheers Bob

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Wayne Whitney 18 years ago

I don't have the NDS, but the American Wood Council's span calculator at says the E of douglas fir depends on the grade--1.6 for #2, 1.7 for #1, 1.8 for #1 & better, and 1.9 for SS. Since the American Wood Council publishs the NDS, I'm assuming they are using NDS values! I should buy a copy sometime.

Well, since our calculations use the E value ignoring "strength reducing features" (timber people don't like the term flaws), and these flaws will increase the deflection, I assume the tabulated E value is penalized based on grade.

That's got to be overkill for 6 ceiling joists. If I ever add a 500 square foot second story, I'll consider engineered timber for the floor joists that will interleave the existing ceiling joists.

BTW, the fact that higher E is achieved in engineered timber using the same fibers as solid sawn lumber suggests to me that the flaws must negatively impact E--I don't think it is all attributable to the glue. But I don't really know anything about glues.

Cheers, Wayne

PS. Most the NDS tabulated values are something like the 5th percentile values, as wood specimens vary in strength alot depending on the locations of flaws. Thus the overstrength in practice can be quite high, up to 4 times or more. The exception is the tabulated E value, which really is an average. There is a separate published E_min value, which is comparable to the other tabulated values as a

5th percentile (or something like that). For douglas fir the E_min is 1.2 or 1.3, I think. As I recall, the main time E_min arises in calculations is just when considering buckling.

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Art 18 years ago

You might want to think about your plasterboard layout and where you will have seems it'd be a good idea to glue and nail the sisters just to make sure there is no movement.

The rest I would just nail in place. Glue is a waste of time, IMO.

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