Joist selection

I don't have the NDS, but the American Wood Council's span calculator
at <http://www.awc.org/calculators/span/calc> 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.

> but SS is about flaws not a high E.

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.

> Have you considered engineered timber? It can have some pretty high

> E's.

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.

Posted by S. Barker on February 18, 2008, 9:41 am

Yes, nothing special about the long 2x8's where i live either. my local
lumber yard has them up to 24' in 2' increments.

s

> Just where were you going to special order those 17 foot 2x8/s?

> --

> ______________________________

Posted by Wayne Whitney on February 18, 2008, 11:59 am

> You might be ahead to explain what it is you are doing.

OK, here comes. The house is one story over crawlspace/basement, with
a hip roof. Here is some ASCII art that shows the plan dimensions and
the original location of bearing walls (B):

43'
!---------------------------
! ! 8'
! ------
26' !BBBBBBBBBBBBBBBBBBBBBBBBBBN !
! NBBBB! 17'
! N !
!--------------------------------
51'

The 26' wide by 43' long part of the house has continuous full
thickness 2x6 ceiling joists. The rear 8' of the house has 2x4
ceiling joists. In this rear 8' area, I have removed the ceiling and
the bearing wall for the ceiling joists. I have also removed the
non-bearing wall (N).

> If the existing 2x4 have worked for years, I would spend the time

> designing a beam to carry the load for the wall you are yanking.

Well, since the wall N is also gone, I'd have to design and install a
17' long beam as well as the 8' long beam over the former bearing
wall. That seems like a lot more work than installing 5 new ceiling
joists. I'm pretty happy with the solution of using 2x8s with end
tapers.

> Just where were you going to special order those 17 foot 2x8/s?

As others suggested, in my neck of the woods 20' is a stock length.

Cheers, Wayne

Posted by Bobk207 on February 18, 2008, 10:38 am

> > I was thinking of cutting a taper that did the job of the notch but

> > sounds like the taper at the joist end would result in a rather

> > shallow joist end? . . . I doubt that a 12" or 16" taper would

> > reduce the beam stiffness much.

> If I do the minimum taper possible over a length of about 17", I have

> an end depth of 5". Does that seem sane with a 2 x 8 x 17'?

> > But as you mention you are deflection critical , shear or bending

> > stress not an issue. So I think that the taper would work....check

> > the shear for the reduced depth.

> Well, I assume the shear should be checked at the face of the support,

> since going into the support the shear stresses are decreasing as the

> bearing comes in. In that case, if I maintain the 5.5" section depth

> at the face of the support, there is nothing to check, as I already

> know a 2x6 passes bending and shear.

> Thanks,

> Wayne

Wayne-

As Rico mentioned, he does sometimes question our sanity on these
design issues.

The taper or the end relief would work (IMO) with or without a plywood
side plate.

Does a joist edge depth of 5" vs 5.5" really make that much different?

I'd cut the taper & be done.

Rico-

I think Wayne's thinking about using single (notched) 2x8's OR
doubled 2x6's.

cheers
Bob

Posted by Wayne Whitney on February 18, 2008, 11:40 am

> The taper or the end relief would work (IMO) with or without a

> plywood side plate. Does a joist edge depth of 5" vs 5.5" really

> make that much different? I'd cut the taper & be done.

Thanks, that's what I've settled on with the group's feedback. :-)

> I think Wayne's thinking about using single (notched) 2x8's OR

> doubled 2x6's.

Right, that was the original question.

Thanks,
Wayne

Page 9 of 9 << first < 1 2 3

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