Joist selection

[Bobk207]

Creating a composite beam out of the 2x8 & 2x4 combo (with glue & nails) increases his joist stiffness by 50% which goes in the direction that you suggested....L/360 may not be stiff enough, L/520 is better.

Since his ceiling is unloaded....if he creates this composite asymmetric T beam he will get the added stiffness benefit. ....and his plaster may have a better chance of survival.

IMO a 50% increase in stiffness is worth the extra work (if you really think that L/360 is too limp then you should also agree that a 50% stiffness boost is worth it)

BTW tapers are elegant,

notches, esp when the kerfs intersect or they're made with saw & rigging axe

are ugly...... :)

cheers Bob

[Wayne Whitney]

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

[Bobk207]

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

[Wayne Whitney]

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.

[Art]

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.