Joist selection

Page 3 of 3  


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.

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.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
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

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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).

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.

As others suggested, in my neck of the woods 20' is a stock length.
Cheers, Wayne
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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

Right, that was the original question.
Thanks, Wayne
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Related Threads

    HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.