Joist selection

Well, if I did the integrations correctly, the moment of inertia I_x
of a 4x8 missing the upper right quadrant is 22/15 times that of a
2x8. I'm a little surprised that the ratio wasn't smaller. I'm still
not sure if it is worth the trouble, but it is an interesting
exercise.

> just glue, clamp & brad nail and call it good.

Yeah, it's the glue part that gives me pause, the old 2x4s are quite
rough, so I'd need a special glue that can gap fill, which seems like
a lot of trouble.

Cheers, Wayne

Posted by Bobk207 on February 19, 2008, 11:45 pm

> > (but maybe not enough stiffer to warrant to calcs or the

> > construction effort, but you know we both love that incremental

> > added performance, it might just be enough to reduce the deflection

> > to make everyone happy)

> Well, if I did the integrations correctly, the moment of inertia I_x

> of a 4x8 missing the upper right quadrant is 22/15 times that of a

> 2x8. I'm a little surprised that the ratio wasn't smaller. I'm still

> not sure if it is worth the trouble, but it is an interesting

> exercise.

> > just glue, clamp & brad nail and call it good.

> Yeah, it's the glue part that gives me pause, the old 2x4s are quite

> rough, so I'd need a special glue that can gap fill, which seems like

> a lot of trouble.

> Cheers, Wayne

Wayne-

I didn't understand your ratio thing....are you saying that the 2x8
with a 2x4 attached (nailed or glued) is ~1.4 times stiffer than a 2x8
alone? The 22/15 ratio was a little obscure

>>>I'm a little surprised that the ratio wasn't smaller<<<< I don't understand?

a heavy spread of wood glue should easily fill surface of the old
2x4's or use a construction adhesive....adhesives are much better for
shear transfer than mechanical fasteners

by sistering the 2x8 to the 2x4 you'll reduce the deflection to 68%
of the deflection of the 2x8 alone.....so it wll go from 1/2" to ~3/8"

cheers
Bob

Posted by Wayne Whitney on February 20, 2008, 12:05 pm

> I didn't understand your ratio thing....are you saying that the 2x8

> with a 2x4 attached (nailed or glued) is ~1.4 times stiffer than a

> 2x8 alone? The 22/15 ratio was a little obscure

Yes. That's for a 2" x 3.625" 2x4, which is what I have. So the
section ends up being 3.5" x 7.25" (a 4x8) minus one quadrant.

> a heavy spread of wood glue should easily fill surface of the old

> 2x4's or use a construction adhesive....adhesives are much better

> for shear transfer than mechanical fasteners

But wood glue isn't gap filling, is it?

Thanks, Wayne

Posted by RicodJour on February 20, 2008, 2:56 pm

> > I didn't understand your ratio thing....are you saying that the 2x8

> > with a 2x4 attached (nailed or glued) is ~1.4 times stiffer than a

> > 2x8 alone? The 22/15 ratio was a little obscure

> Yes. That's for a 2" x 3.625" 2x4, which is what I have. So the

> section ends up being 3.5" x 7.25" (a 4x8) minus one quadrant.

> > a heavy spread of wood glue should easily fill surface of the old

> > 2x4's or use a construction adhesive....adhesives are much better

> > for shear transfer than mechanical fasteners

> But wood glue isn't gap filling, is it?

Depends on what sort of wood glue you're talking about. If you're
talking about Elmer's PVA, no, but if you're talking about
construction adhesive marketed for specific applications, like Liquid
Nails, or the PL brand of adhesives, they will work fine for you
application.

It seems to me that you're overanalyzing this thing. Two pieces of
wood have been attached with nails in shear for centuries with no
adhesive at all. Balloon framed houses used a ribbon/ledger to
support the floor joists with both being attached to the full height
studs with nails in shear only. They're not falling down. Your house
is from 1908 - prime balloon framing period. Is that how your floor
joists are connected?

Your low height attic load is far less and you're analyzing it far
more. And you'd already determined that the 2x8 on it's own would
take care of the load - it doesn't need the sistered 2x4 to do the
work, so the nails in shear won't be doing a hell of a lot of work.
The glue question is like a guy wearing a belt and suspenders asking
if he needs overalls to keep his pants up.

The only small wrinkle was the 1x4 plate resting on top of the CJs
requiring the notch. You'd already established that shear and bearing
area weren't issues as a single 2x6 was adequate. Since you will have
double the sectional area there's really no question of the notch
being a problem. In balloon framed houses it's not uncommon to have a
flush beam or two supporting the inboard ends of the first floor
joists. The standard way of doing that, before the introduction of
joist hangers, was to nail a 2x3 ledger flat to the side, and flush
with the bottom of the flush beam. The FJs were notched to rest on
the ledger and toe-nailed to the beam. The notch on the bottom is a
_much_ bigger issue than a notch on the top, yet even though you'll
often see splits at the notch when you dissect the house, you rarely
see failures.

My concern with the deflection is due to the fact that I don't
consider 1/360 adequate for a floor. That's code - code is the
minimum acceptable - I don't build that way. People also don't _live_
that way - particularly not with attics. A live load of 30 PSF in a
bedroom is reasonable. People need to move around in a bedroom, and
they don't pile stuff on every square foot of available floor space.
That's not the case for an attic.

It wasn't clear from your earlier posts exactly how much room (height
and area) is in the attic, and it wasn't clear what would be stored
there. A single liquor bottle size box of books can weigh more than
20 PSF, and, if there's room, people stack stuff in an attic. If you
can guarantee that the attic will never see more than the 20 PSF, even
after you sell the house or kick the bucket, then fine - otherwise, go
with a more conservative load and a more stringent deflection
criteria.

As an aside, we could have gotten to the meat of the matter a lot more
quickly if you'd posted a picture of the situation and/or given more
complete information upfront.

Note to BobK: You said nails weren't good in shear. What does that
mean? I had a building inspector say the same thing to me - I didn't
want to argue with him just because he was wrong - he's the building
inspector and the nails in shear wasn't an issue. You I can argue
with! ;) What's your argument against nails in shear?

Okay, that's all for now. I'm hungry and I'm not going to proof read
- let me know if any inanity worked it's way in. ;)

R

Posted by Wayne Whitney on February 20, 2008, 8:59 pm

> It seems to me that you're overanalyzing this thing.

Well, I think I narrowly framed my original question (double 2x6s or
notched 2x8s), and we pretty quickly got to the practical answer (use
tapered 2x8s). However, people were interested in the whole story, so
I've been happy to provide the extra details. That has led to some
interesting side questions, which are perhaps less practical.

> Your house is from 1908 - prime balloon framing period. Is that how

> your floor joists are connected?

No, it's platform framed. The only weird detail is the 1x4 plate
between the ceiling joists and the rafters.

> The only small wrinkle was the 1x4 plate resting on top of the CJs

> requiring the notch. You'd already established that shear and

> bearing area weren't issues as a single 2x6 was adequate. Since you

> will have double the sectional area there's really no question of

> the notch being a problem.

Just to be clear, I won't have double the sectional area. I'll have
the sectional area of a 2x6 after I do the tapering. If I want to
sister the existing 2x4s and consider its sectional area, then I have
to be concerned with the sistering details. But since even the single
2x6 is deflection controlled, there is "reserve" sectional area there,
so I'm not concerned about the possiblity of stress concentration from
the tapering.

> My concern with the deflection is due to the fact that I don't

> consider 1/360 adequate for a floor.

So what do you consider adequate? Will my skim coat plaster ceiling
crack under a live load deflection of L/360?

> It wasn't clear from your earlier posts exactly how much room (height

> and area) is in the attic, and it wasn't clear what would be stored

> there.

The roof is hipped at an 8:12 slope, so over the 8' region, the
clearance at center span varies from 0' to 5'4" or so. I'm not
actually storing anything up there, my real concern is about the
deflection from a person standing (squatting) on a single joist. As
for others storing stuff up there, I guess that's why the building
code requires a 20psf live load where the clearance exceeds 3'6". I
think that's adequate in this situation.

> As an aside, we could have gotten to the meat of the matter a lot

> more quickly if you'd posted a picture of the situation and/or given

> more complete information upfront.

I don't know about that, the meat of the matter from my point of view
was in my original question, that's why I specifically made it as
short and constrained as possible.

Above all, I appreciate the time and attention shown in your
responses, they reflect the practical experience that I lack.

Thanks, Wayne

Page 5 of 9 < 1 2 3 > last >>

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