Doubling up floor joists
Ron Harriman
Planning to span a 19-foot unsupported section in one leap
here. Span tables say that 2x12 #2 dougfir joists will do it
if placed 16" OC.
Clearance mandates something less tall than 2x12. Hmmmm,
can I double up on smaller joists? Check my math here:
Bending strength increases linearly with width and as the
square of depth. A 2x6 is half the depth of a 2x12 and thus
1/4 its strength. Would take FOUR 2x6s in place of one 2x12.
A bit silly. Okay, so if I can come up with the equivalent
of four 2x6s in larger joists but less than 2x12, I'm OK.
Try 2x8. It's 1.33333 times the depth of a 2x6 and hence is
(1.33333)^2 = 1.7777 times as strong in bending strength.
Two 2x8s would give 2x(1.7777) = 3.55 times as strong as a
2x6. But the span table dictates 4x as strong. Not sufficient.
Try 2x10. It's 1.6666 times the depth of a 2x6 and hence is
(1.66666)^2 = 2.7777 times as strong in bending strength.
Two 2x10s would give 2x(2.7777) = 5.55 times as strong as a
2x6. The span table dictated 2x12 is only 4x as strong, so
two 2x10s are considerably stronger than a 2x12.
Same type of math says that a 3x10 would be a reasonable
substitute for a 2x12.
Obviously all this depends on my local inspector's take, but
I am interested in if I've constructed my logic properly.
Anyone know if the national code has a hard and fast rule
for joist doubling or tripling or whatever? I am always leery
of subjective inspector determinations.
Ron H
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Shannon Pate
bond notching 2 x 12's...
ASP
"Ron Harriman" <dr...@my-deja.com> wrote in message
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the seal
will give you the answers in a jiffy and you will have confidence that its
based on valid calculations.
-seal
Ron Harriman wrote in message <908obn$hh9$1...@nnrp1.deja.com>...
ca...@my-deja.com
Ron Harriman <dr...@my-deja.com> wrote:
>
>
> Planning to span a 19-foot unsupported section in one leap
> here. Span tables say that 2x12 #2 dougfir joists will do it
> if placed 16" OC.
>
> Clearance mandates something less tall than 2x12. Hmmmm,
>
We use a program called BeamCheck made by Advanced Relational
Technology to solve these problems. The results indicate that
(2) 2x10 @ 16" o.c D.F. North #2
2x10 @ 8" o.c.
(2) 2x8 @ 12" o.c.
or 2x8 @ 6" o.c.
will all work, assuming your floor load at 30#/SF live and 10#/SF dead
and all lumber #2 Douglas fir north.
If you would like more information contact Tom Easton at:
Cad Northwest Custom Home Design
ca...@zzz.com
(503) 245-8974
http:/cadnw.com
Jake Watson
that. There is more to sizing joists than just strength. Deflection
and floor vibration are also concerns that must be dealt with. The long
span you are talking about will take special consideration. If you
insist on using dimensional lumber, please provide us with more
information and maybe I can help you. What kind of floor covering do
you have? Is it a storage area or just a living room? Are there any
perpendicular partition walls?
As for hard and fast rules:
You are right about strength being squared for bending. It is also
linear for shear, and a fourth order for deflection. If you can
maintain ratios greater than 1 for all those elements, you should be
good to go. One other note, as you increase the total cross sectional
area ( 2-2x8s instead of 1 2x12) the overall weight of the floor system
increases. This means you need more strength just to hold up the floor
joists.
Hope this helps.
Jake Watson, E.I.T.
Salt Lake City
Ron Harriman wrote:
>
> Planning to span a 19-foot unsupported section in one leap
> here. Span tables say that 2x12 #2 dougfir joists will do it
> if placed 16" OC.
>
> Clearance mandates something less tall than 2x12. Hmmmm,
> can I double up on smaller joists? Check my math here:
>
--
Jake Watson, E.I.T.
Salt Lake City, UT