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| Wayne Whitney 2008-02-15, 8:25 pm |
| Hi,
I've got to sister five 17' long ceiling joists, due to removing a
bearing wall. I've done the engineering, and given the existing roof
construction, I've got two options that work:
1) Use double 2x6 select struct at 16" o.c.
2) Use 2x8 select struct at 16" o.c. with a 3.5" long by 1.75" deep
top edge end notch.
The selection is deflection controlled, as 2x6 select struct at 16"
o.c. meets bending and shear.
Which option is better? The material cost for the 2x6s is about $50
more. I'm mostly working alone, so the 2x6s will be somewhat easier
to wrangle into place. Otherwise, it is notching the 2x8s versus the
extra nailing to double the 2x6s.
Thanks, Wayne
| |
| Rima Neas 2008-02-15, 9:26 pm |
| "Wayne Whitney" <whitney@post.harvard.edu> wrote in message
news:slrnfrcbqu.57g.whitney@pizza.private...
> Hi,
>
> I've got to sister five 17' long ceiling joists, due to removing a
> bearing wall. I've done the engineering, and given the existing roof
> construction, I've got two options that work:
>
> 1) Use double 2x6 select struct at 16" o.c.
>
> 2) Use 2x8 select struct at 16" o.c. with a 3.5" long by 1.75" deep
> top edge end notch.
>
I would go with single 2x8 sisters, less wood to buy, haul, and wrangle
with. Besides, 2x8's tend to run straighter than 2x6's. Nailing a single
sister will also involve fewer intrusions into the original rafters, not to
mention hammer strokes. Given that you are sistering existing, presumably
bone dry, rafters, I would also go with KD stuff. Then again, I cannot even
figure out my window sizes...
| |
| Bobk207 2008-02-16, 8:25 pm |
| On Feb 15, 4:33 pm, Wayne Whitney <whit...@post.harvard.edu> wrote:
> Hi,
>
> I've got to sister five 17' long ceiling joists, due to removing a
> bearing wall. I've done the engineering, and given the existing roof
> construction, I've got two options that work:
>
> 1) Use double 2x6 select struct at 16" o.c.
>
> 2) Use 2x8 select struct at 16" o.c. with a 3.5" long by 1.75" deep
> top edge end notch.
>
> Theselectionis deflection controlled, as 2x6 select struct at 16"
> o.c. meets bending and shear.
>
> Which option is better? The material cost for the 2x6s is about $50
> more. I'm mostly working alone, so the 2x6s will be somewhat easier
> to wrangle into place. Otherwise, it is notching the 2x8s versus the
> extra nailing to double the 2x6s.
>
> Thanks,Wayne
Wayne-
Does the notch start at the end of the joist & only run 3.5"?
IMO this isn't really much of a notch ....more of a depth reduction in
the joist shear / bearing zone?
I assume it needs to be there for clearance issues?
You might consider a taper cut rather than a sharp cut out.
Timber is happier if you avoid abrupt section changes but in this
case the local stress state appears to "compress" :notch rather than
wanting to split it open.
If you don't have a copy you might consider getting one
Design of Wood Structures ASD/LRFD
Sixth Edition
by Donald E. Breyer
or a older addition (cheaper) ....always great for timber design
concepts but newer editions conform to newer codes.
& this guy is a real timber expert.
cheers
Bob
| |
| Bobk207 2008-02-16, 8:25 pm |
| On Feb 16, 4:46 pm, Bobk207 <rkaza...@gmail.com> wrote:
> On Feb 15, 4:33 pm,WayneWhitney <whit...@post.harvard.edu> wrote:
>
>
>
>
>
>
>
>
>
>
> Wayne-
>
> Does the notch start at the end of thejoist& only run 3.5"?
>
> IMO this isn't really much of a notch ....more of a depth reduction in
> thejoistshear / bearing zone?
>
> I assume it needs to be there for clearance issues?
>
> You might consider a taper cut rather than a sharp cut out.
>
> Timber is happier if you avoid abrupt section changes but in this
> case the local stress state appears to "compress" :notch rather than
> wanting to split it open.
>
> If you don't have a copy you might consider getting one
>
> Design of Wood Structures ASD/LRFD
> Sixth Edition
> by Donald E. Breyer
>
> or a older addition (cheaper) ....always great for timber design
> concepts but newer editions conform to newer codes.
>
> & this guy is a real timber expert.
>
> cheers
> Bob
Wayne-
I must have missed this thread plus I didn't specifically answer your
question....
I'd go with the 2x8's and taper cut them.
cheers
Bob
| |
| Wayne Whitney 2008-02-16, 8:25 pm |
| On 2008-02-17, Bobk207 <rkazanjy@gmail.com> wrote:
> Does the notch start at the end of the joist & only run 3.5"? IMO
> this isn't really much of a notch ....more of a depth reduction in
> the joist shear / bearing zone?
Yes, everything you say is right, it is a depth reduction needed for
clearance. The reduction from 7.25" to 5.5" is just under 25%, the
code maximum.
An immediate taper cut is not possible. The roof is constructed with
a 1x4 bearing on top of the 2x6 ceiling joists, and the 2x4 rafters
bear on the 1x4. Crazy 1908 construction. So I have to actually
reduce the depth to 5.5" where it is under the 1x4, hence the 3.5"
long end notch. I don't want to modify the roof construction, as that
would open a whole other can of worms.
> Timber is happier if you avoid abrupt section changes
So after the notch clears the 1x4 obstruction, I should do a taper to
increase the depth from 5.5" to 7.25" more gradually? Is a 45 degree
taper OK or should it be shallower?
Thanks,
Wayne
| |
| Bobk207 2008-02-16, 9:25 pm |
| On Feb 16, 5:53 pm, Wayne Whitney <whit...@post.harvard.edu> wrote:
> On 2008-02-17, Bobk207 <rkaza...@gmail.com> wrote:
>
>
> Yes, everything you say is right, it is a depth reduction needed for
> clearance. The reduction from 7.25" to 5.5" is just under 25%, the
> code maximum.
>
> An immediate taper cut is not possible. The roof is constructed with
> a 1x4 bearing on top of the 2x6 ceiling joists, and the 2x4 rafters
> bear on the 1x4. Crazy 1908 construction. So I have to actually
> reduce the depth to 5.5" where it is under the 1x4, hence the 3.5"
> long end notch. I don't want to modify the roof construction, as that
> would open a whole other can of worms.
>
>
> So after the notch clears the 1x4 obstruction, I should do a taper to
> increase the depth from 5.5" to 7.25" more gradually? Is a 45 degree
> taper OK or should it be shallower?
>
> Thanks,Wayne
Wayne-
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?
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. I doubt that a 12" or 16" taper
would reduce the beam stiffness much.
We're really discussing stuff that is pretty much second (or third
order) effects.
If you can't use a taper cut to do the whole job...I'd use a large
paddle bit or hole saw to define the end of the clearance relief &
just jig saw to it.
Cutting a taper out of the relief cut is really overkill.
cheers
Bob
| |
| RicodJour 2008-02-17, 9:25 am |
| On Feb 16, 9:32 pm, Bobk207 <rkaza...@gmail.com> wrote:
> On Feb 16, 5:53 pm, Wayne Whitney <whit...@post.harvard.edu> wrote:
>
>
>
>
>
>
>
>
>
>
> Wayne-
>
> 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?
>
> 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. I doubt that a 12" or 16" taper
> would reduce the beam stiffness much.
>
> We're really discussing stuff that is pretty much second (or third
> order) effects.
>
> If you can't use a taper cut to do the whole job...I'd use a large
> paddle bit or hole saw to define the end of the clearance relief &
> just jig saw to it.
>
> Cutting a taper out of the relief cut is really overkill.
The notch on the upper side isn't nearly as critical as a notch on the
bottom of the joist. There's also the possibility of sistering on a
3/4" plywood scab to help distribute the stress at the reduced cross
section - whether notched or tapered.
R
| |
| Wayne Whitney 2008-02-17, 1:25 pm |
| On 2008-02-17, Bobk207 <rkazanjy@gmail.com> wrote:
> 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
| |
| RicodJour 2008-02-17, 5:25 pm |
| On Feb 17, 1:03 pm, Wayne Whitney <whit...@post.harvard.edu> wrote:
> On 2008-02-17, Bobk207 <rkaza...@gmail.com> wrote:
>
>
> 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'?
I'm still wondering about your sanity! ;) What sort of deflection
are you getting with a 2x8 sistered on?
>
> 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.
Right. And you're doubling the cross section.
R
| |
| Wayne Whitney 2008-02-17, 8:25 pm |
| On 2008-02-17, RicodJour <ricodjour@worldemail.com> wrote:
> What sort of deflection are you getting with a 2x8 sistered on?
From the awc.org calculator, the span rating for a 2x8 SS DF with a
20psf live load and L/360 deflection is 17' 2" and is deflection
controlled. So ignoring any end taper, the deflection at 16' 5" will
be L/360 * (16'5/17'2)^3 = L/412, or 0.47".
I'm ignoring the existing joists when doing this calculation, since
they are 2x4s. Most of the house has 2x6 ceiling joists, so I was
quite surprised to find the 2x4s in the area I'm working on.
Cheers, Wayne
| |
|
| You might be ahead to explain what it is you are doing. I don't
think I would mess around sistering a bunch of joists. If the
existing 2x4 have worked fore years, I would spend the time
designing a beam to carry the load for the wall you are yanking.
It would seem much more simple to me to install one beam and the
carrying ties. By the time you sister the joists in question, you
will have the existing ceiling on the ground.
Just where were you going to special order those 17 foot 2x8/s?
--
______________________________
Keep the whole world singing . . . .
DanG (remove the sevens)
dgriff237@7cox.net
"Wayne Whitney" <whitney@post.harvard.edu> wrote in message
news:slrnfrhp9j.3jc.whitney@pizza.private...
> On 2008-02-17, RicodJour <ricodjour@worldemail.com> wrote:
>
>
> From the awc.org calculator, the span rating for a 2x8 SS DF
> with a
> 20psf live load and L/360 deflection is 17' 2" and is deflection
> controlled. So ignoring any end taper, the deflection at 16' 5"
> will
> be L/360 * (16'5/17'2)^3 = L/412, or 0.47".
>
> I'm ignoring the existing joists when doing this calculation,
> since
> they are 2x4s. Most of the house has 2x6 ceiling joists, so I
> was
> quite surprised to find the 2x4s in the area I'm working on.
>
> Cheers, Wayne
| |
| RicodJour 2008-02-18, 9:25 am |
| On Feb 18, 2:40 am, "DanG" <dgrif...@7cox.net> wrote:
> You might be ahead to explain what it is you are doing. I don't
> think I would mess around sistering a bunch of joists. If the
> existing 2x4 have worked fore years, I would spend the time
> designing a beam to carry the load for the wall you are yanking.
> It would seem much more simple to me to install one beam and the
> carrying ties. By the time you sister the joists in question, you
> will have the existing ceiling on the ground.
Dan's got a great point - it was bound to happen eventually! ;) A
flush beam supporting the joists with joist hangers would be much
simpler and cheaper. There's usually some surgery to create the beam
pocket and you have to plan the assembly (possibly installing beam,
then installing supporting studs), but it's a preferable way to go in
almost all aspects. You mentioned five joists needed sistering, so
that's about a 8' span. You wouldn't need much in the way of a beam
since it's only a ceiling load - you were surprised that the CJs were
2x4s so it's safe to assume that you hadn't been up in any attic
space. A couple of 9.25" LVL's would be more than enough.
> Just where were you going to special order those 17 foot 2x8/s?
They're not special order around here. Generally, depending on the
yard, they stock 18' or 20' 2x stuff.
R
| |
| S. Barker 2008-02-18, 9:25 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
"DanG" <dgriff23@7cox.net> wrote in message
news:NDauj.57696$Ft5.10679@newsfe15.lga...
>
> Just where were you going to special order those 17 foot 2x8/s?
>
> --
> ______________________________
| |
| Bobk207 2008-02-18, 1:25 pm |
| On Feb 18, 4:56 am, RicodJour <ricodj...@worldemail.com> wrote:
> On Feb 18, 2:40 am, "DanG" <dgrif...@7cox.net> wrote:
>
>
> Dan's got a great point - it was bound to happen eventually! ;) A
> flush beam supporting the joists with joist hangers would be much
> simpler and cheaper. There's usually some surgery to create the beam
> pocket and you have to plan the assembly (possibly installing beam,
> then installing supporting studs), but it's a preferable way to go in
> almost all aspects. You mentioned five joists needed sistering, so
> that's about a 8' span. You wouldn't need much in the way of a beam
> since it's only a ceiling load - you were surprised that the CJs were
> 2x4s so it's safe to assume that you hadn't been up in any attic
> space. A couple of 9.25" LVL's would be more than enough.
>
>
> They're not special order around here. Generally, depending on the
> yard, they stock 18' or 20' 2x stuff.
>
> R
Rico-
If he's got the ceiling plaster down, wouldn't throwing five long
2x8's up there & nailing them in place be quicker than creating beam
pockets, cutting the joists, installing a beam with hangers?
cheers
Bob
| |
| Bobk207 2008-02-18, 1:25 pm |
| On Feb 17, 10:03 am, Wayne Whitney <whit...@post.harvard.edu> wrote:
> On 2008-02-17, Bobk207 <rkaza...@gmail.com> wrote:
>
>
> 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'?
>
>
> 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
| |
| RicodJour 2008-02-18, 1:25 pm |
| On Feb 18, 10:37 am, Bobk207 <rkaza...@gmail.com> wrote:
> On Feb 18, 4:56 am, RicodJour <ricodj...@worldemail.com> wrote:
>
>
> If he's got the ceiling plaster down, wouldn't throwing five long
> 2x8's up there & nailing them in place be quicker than creating beam
> pockets, cutting the joists, installing a beam with hangers?
| |
| RicodJour 2008-02-18, 1:25 pm |
| On Feb 18, 10:37 am, Bobk207 <rkaza...@gmail.com> wrote:
> On Feb 18, 4:56 am, RicodJour <ricodj...@worldemail.com> wrote:
>
>
>
> If he's got the ceiling plaster down, wouldn't throwing five long
> 2x8's up there & nailing them in place be quicker than creating beam
> pockets, cutting the joists, installing a beam with hangers?
Damn send button! Sorry about that.
Without pictures it's tough to tell what's the quickest solution.
There's room above the CJs, and the OP mentioned having to notch, but
he didn't say whether it was one or both ends. There might be room to
put the beam above the existing CJs without cutting them and hang 'em.
The sistering option seems to be throwing a lot of extra expense and
weight at the problem and getting a fairly large deflection as a
result. Sistering may be a bit faster, but I don't think a 1/2"
deflection is acceptable. It's definitely not necessary. Doesn't
seem like a great solution.
R
| |
| Wayne Whitney 2008-02-18, 1:25 pm |
| On 2008-02-18, Bobk207 <rkazanjy@gmail.com> wrote:
> 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
| |
| Wayne Whitney 2008-02-18, 1:25 pm |
| On 2008-02-18, DanG <dgriff23@7cox.net> wrote:
> 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
| |
| Wayne Whitney 2008-02-18, 1:25 pm |
| On 2008-02-18, RicodJour <ricodjour@worldemail.com> wrote:
> You wouldn't need much in the way of a beam since it's only a
> ceiling load - you were surprised that the CJs were 2x4s so it's
> safe to assume that you hadn't been up in any attic space.
Well, I was surprised because the rest of the ceiling joists are 2x6s,
just the five where I'm working are 2x4s. C'est la vie.
As to the load, according to the 2006 IBC, if the clear space between
the joist and the rafter is under 42", I can call the space "attic
without storage" and use a 10psf live load. Otherwise it is "attic
with limited storage" and I need to use a 20psf live load. That's not
much less than the 30psf for bedrooms.
Now the outermost 3 of the 5 joists do have the restricted clearance
because of the hip roof, so I could use only 10psf live load there.
However, for a 16.5' span with joists 16" o.c., a 200lb point load in
the middle of a single joist gives the same deflection as 14.5 psf
does. So I'm going to stick with 20psf live load everywhere.
> but I don't think a 1/2" deflection is acceptable.
I'm curious as to why you say that. This is the deflection due to
live load only, so it is just while someone is walking around up
there. The IBC requirement for ceiling joists supporting a plaster
ceiling is L/360, and 1/2" is L/412. So do you believe L/360 is
inadequate?
Thanks, Wayne
| |
| Bobk207 2008-02-18, 8:25 pm |
| On Feb 18, 7:46 am, RicodJour <ricodj...@worldemail.com> wrote:
> On Feb 18, 10:37 am, Bobk207 <rkaza...@gmail.com> wrote:
>
>
>
>
>
>
> Damn send button! Sorry about that.
>
> Without pictures it's tough to tell what's the quickest solution.
> There's room above the CJs, and the OP mentioned having to notch, but
> he didn't say whether it was one or both ends. There might be room to
> put the beam above the existing CJs without cutting them and hang 'em.
>
> The sistering option seems to be throwing a lot of extra expense and
> weight at the problem and getting a fairly large deflection as a
> result. Sistering may be a bit faster, but I don't think a 1/2"
> deflection is acceptable. It's definitely not necessary. Doesn't
> seem like a great solution.
>
> R
Wayne, I really couldn't figure out the ASCII dwg. 
Are you're really sistering the exisitng 2x4 ceiling joists?
Are the 2x4's staying or going?
If the 2x4's are staying & 2x8's are going in next to them, I'd glue
& staple (16ga) or brad nail the 2x8 to the 2x4. It would boost the
overall stiffness of the new joists even more.
I'm understanding this all correctly?
cheers
Bob
| |
| Wayne Whitney 2008-02-19, 3:25 am |
| On 2008-02-19, Bobk207 <rkazanjy@gmail.com> wrote:
> Wayne, I really couldn't figure out the ASCII dwg.
Sorry, it didn't work for you. Are you displaying it with a
fixed-point font? If not it won't make any sense.
In words, the house is 26' by 51' overall but has a reentrant corner.
The full width section is 26' wide and 43' long; then the width goes
down to 17' wide for the last 8' of length. This 8' x 17' area is
where I'm working and where the existing ceiling joists change from
2x6s to 2x4s. There had been an 8' long bearing wall dividing the 17'
span, which I removed.
I'll respond to the rest of you post shortly.
TThanks, Wayne
| |
| Wayne Whitney 2008-02-19, 3:25 am |
| On 2008-02-19, Bobk207 <rkazanjy@gmail.com> wrote:
> Are you're really sistering the exisitng 2x4 ceiling joists? Are
> the 2x4's staying or going?
Well, now the ceiling is down and the 2x4s joists are still there.
Since the rafters are attached to the 1x4 cleat that is attached to
the top of the ceiling joists (via a 2" thick block in the case of the
2x4 joists), I thought it would be simpler to leave them in place.
Otherwise I'll have to tie the 1x4 cleat into the new joists.
> If the 2x4's are staying & 2x8's are going in next to them, I'd glue
> & staple (16ga) or brad nail the 2x8 to the 2x4. It would boost the
> overall stiffness of the new joists even more.
I was just planning to use 0.148" x 3.25" nails at 16" o.c. (two
staggered rows of 32" o.c. each). Does that seem inadequate?
Cheers, Wayne
| |
| Bobk207 2008-02-19, 3:25 am |
| On Feb 18, 9:12 pm, Wayne Whitney <whit...@post.harvard.edu> wrote:
> On 2008-02-19, Bobk207 <rkaza...@gmail.com> wrote:
>
>
> Well, now the ceiling is down and the 2x4s joists are still there.
> Since the rafters are attached to the 1x4 cleat that is attached to
> the top of the ceiling joists (via a 2" thick block in the case of the
> 2x4 joists), I thought it would be simpler to leave them in place.
> Otherwise I'll have to tie the 1x4 cleat into the new joists.
>
>
> I was just planning to use 0.148" x 3.25" nails at 16" o.c. (two
> staggered rows of 32" o.c. each). Does that seem inadequate?
>
> Cheers, Wayne
IMO .148" nails into old timbers is asking for trouble...they're
just too big. I prefer smaller nails (like .113, .120 or .131) but
more of them. I also like 16 gauge staples & 16 gauge brads. Of
course I'd use glue & setup clamp them together before I shot them.
Since I know you like the details.... 
to get the 2x4 & the 2x8 to work together as a composite beam you need
enough shear transfer between.
think of a 2x6 with two 2x4's laid flat (top & bottom) on it to create
an I beam.....to get the assembly to act like an I beam you need to
transfer shear from the top & bottom faces to the 2x6 into the 2x4
"flanges".
The calc involves determining the shear flow required to "activate"
the 2x4 flanges and a fastener schedule that will supply that shear
transfer.
In your case the 2x4 "sistered" to side of the 2x8 makes an offset T-
section which if properly glued / nailed will be stiffer than the sum
of the 2x4 & the 2x8
(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)
You could do a quick & dirty moment of inertia calc to see if the
composite section is enough stiffer to warrant doing the fastener
schedule calc or if the stiffness boost is good......just glue, clamp
& brad nail and call it good.
I tried to find a link to a page that did the shear flow / fastener
calc but I could find one to show the shear flow calc .... q=
VQ/I
cheers
Bob
| |
| Wayne Whitney 2008-02-19, 1:25 pm |
| On 2008-02-19, Bobk207 <rkazanjy@gmail.com> wrote:
> (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
| |
| Bobk207 2008-02-20, 3:25 am |
| On Feb 19, 8:34 am, Wayne Whitney <whit...@post.harvard.edu> wrote:
> On 2008-02-19, Bobk207 <rkaza...@gmail.com> wrote:
>
>
> 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.
>
>
> 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
[color=darkred]
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
| |
| Wayne Whitney 2008-02-20, 1:25 pm |
| On 2008-02-20, Bobk207 <rkazanjy@gmail.com> wrote:
> 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
| |
| RicodJour 2008-02-20, 5:25 pm |
| On Feb 20, 12:05 pm, Wayne Whitney <whit...@post.harvard.edu> wrote:
> On 2008-02-20, Bobk207 <rkaza...@gmail.com> wrote:
>
>
> 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.
>
>
> 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
| |
| Wayne Whitney 2008-02-20, 8:25 pm |
| On 2008-02-20, Wayne Whitney <whitney@post.harvard.edu> wrote:
> 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.
Whoops, that's not quite right. The section is 1.5" x 7.25" plus 2" x
3.625". That section has a moment of inertia that is 22/15 times the
moment of inertia of the simple 1.5" x 7.25" rectangle.
Wayne
| |
| Wayne Whitney 2008-02-20, 8:25 pm |
| On 2008-02-20, RicodJour <ricodjour@worldemail.com> wrote:
> 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
| |
| Wayne Whitney 2008-02-20, 8:25 pm |
| On 2008-02-20, RicodJour <ricodjour@worldemail.com> wrote:
> 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?
I'm not BobK, but I took this comment to mean that nails are not as
good as adhesive when you want continuous shear transfer. And that
you need continuous shear transfer to make the pieces of a built up
beam act as a composite section. That's a different story than
nailing a ledger to a stud wall, where the members are only
overlapping in a small area.
Cheers, Wayne
| |
| RicodJour 2008-02-20, 9:25 pm |
| On Feb 20, 8:59 pm, Wayne Whitney <whit...@post.harvard.edu> wrote:
> On 2008-02-20, RicodJour <ricodj...@worldemail.com> wrote:
>
>
> 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.
The 'extra' details are usually the things that determine the best
solution. There seems to be a bell curve to newsgroup questions.
Newbies leave out all the pertinent information, then have to fill it
in when people point the necessity of the omitted information.
Posters with a bit more posting experience, but maybe not more
technical information/experience, post more information up front.
Posters with more technical experience often try to simplify the
question to the one issue that concerns them. This leads back to the
'insufficient information for meaningful response' replies.
>
> No, it's platform framed. The only weird detail is the 1x4 plate
> between the ceiling joists and the rafters.
>
>
> 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.
That was the low blood sugar talking. Sorry about that. Should have
said _almost_ double the cross section. I don't any practical
advantage to the tapering in your particular situation. This is what
BobK wrote in his first reply:
"You might consider a taper cut rather than a sharp cut out.
Timber is happier if you avoid abrupt section changes but in this
case the local stress state appears to "compress" :notch rather than
wanting to split it open."
I agree with the second part of the last sentence, and the
_generality_ of the first part. Your situation won't benefit from
tapering. It won't change any stress state. The same way doctors are
supposed to "first, do no harm", carpenters are supposed to "not
remove any wood without a reason." In your specific situation there
is not reason to do it, so why do you want to do it? You should
listen to BobK. ;)
>
> So what do you consider adequate? Will my skim coat plaster ceiling
> crack under a live load deflection of L/360?
Eventually. They all do. I don't think you're taking a big risk, and
frankly if you're not storing anything up there you're not taking any
risk at all. You could do me a favor though. After the supplemental
CJs are in place, take an elevation of the bottom of a central joist
at midspan. After you have rocked and plastered, do it again. I'm
curious to see what sort of deflection you get from just the dead
load.
>
> 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.
For you, yes, I agree. Do not doubt that someone will do something
stupid in that attic in years to come.
>
> 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.
And therein lies the rub. You were able to inspect the situation
firsthand and have way more information to help make a decision.
Playing detective requires me to ask questions - some of which will be
dead ends. I still have to ask them. The more experience I get the
more finely tuned my analytical skills become. These newsgroups help
me develop those skills and hopefully save somebody a 'learning'
experience.
> Above all, I appreciate the time and attention shown in your
> responses, they reflect the practical experience that I lack.
Hey, don't mention it. It's fun batting these things back and forth.
R
| |
| Bobk207 2008-02-21, 3:27 am |
| On Feb 20, 6:08 pm, Wayne Whitney <whit...@post.harvard.edu> wrote:
> On 2008-02-20, RicodJour <ricodj...@worldemail.com> wrote:
>
>
> I'm not BobK, but I took this comment to mean that nails are not as
> good as adhesive when you want continuous shear transfer. And that
> you need continuous shear transfer to make the pieces of a built up
> beam act as a composite section. That's a different story than
> nailing a ledger to a stud wall, where the members are only
> overlapping in a small area.
>
> Cheers, Wayne
Wayne-
You're not me, you should thank goodness for that
but your answer
>I'm not BobK, but I took this comment to mean that nails are not as
good as adhesive when you want continuous shear transfer. And that
you need continuous shear transfer to make the pieces of a built up
beam act as a composite section. That's a different story than
nailing a ledger to a stud wall, where the members are only
overlapping in a small area. <
was spot on to what I would have written
R-
I checked my posts & didn't find a direct reference to nails in shear
being "not good"
but if I did, I was / am wrong.
Nails are fine in shear, I have no argument with nails in shear,
that's what they're meant for (just not for withdrawal)
I have no idea what that building inspector was thinking... ?????
but where one needs continuous shear transfer to develop composite
action (built up box beam, I beam, etc) adhesives are much better
than discrete mechanical fasteners because the stress is well
distributed and the shear connection is stiffer than nails.........but
since I often do "belt, suspenders, staples & duct tape"....... I
prefer adhesive AND fasteners.
Also.guilty........Wayne & I tend to over analyze things
(him more than me, because he has less experience but he CAN do
thecalcs & ask lots of questions...me, because I'm always looking for
"better / optimum".
cheers
Bob
| |
| Bobk207 2008-02-21, 3:27 am |
| On Feb 20, 5:21 pm, Wayne Whitney <whit...@post.harvard.edu> wrote:
> On 2008-02-20, Wayne Whitney <whit...@post.harvard.edu> wrote:
>
>
> Whoops, that's not quite right. The section is 1.5" x 7.25" plus 2" x
> 3.625". That section has a moment of inertia that is 22/15 times the
> moment of inertia of the simple 1.5" x 7.25" rectangle.
>
> Wayne
Wayne-
Your 22/15 ratio is making this old engineer crazier than I normally
am.
A simple "factor of 1.5x stiffer" would be be easier for us practical
types to parse.
That said, yeah, the sistering is a little more work
but with caulk tube dispensed adhesive, some set up clamps, shoot a
bunch of staples or brads, remove the clamps & move to the next
joist. You've only got five joists to do.....
"cost of quality vs cost of non-conformance"
how upset will you be when the ceiling finish cracks?
your 2x8 is deflection controlled, you've got lots of strength but the
plaster finish wants stiffness........ you'll get L/520 instead of L/
360
If you really want to "geek out" ....check the joist slope as a
function of span for your assumed loading condition & compare to
suggested limits for brittle finishes.
it's not really deflection that matters but changes in deflections
cheers
Bob
| |
| Wayne Whitney 2008-02-21, 3:27 am |
| On 2008-02-21, Bobk207 <rkazanjy@gmail.com> wrote:
> Your 22/15 ratio is making this old engineer crazier than I normally
> am.
OK, I'm a mathematician, but when I'm doing engineering, you are
right, I should say 1.5 because that easier to understand and 1.5 =
22/15 within the accuracy of our modeling of reality. :-)
> but with caulk tube dispensed adhesive, some set up clamps, shoot a
> bunch of staples or brads, remove the clamps & move to the next
> joist. You've only got five joists to do.....
I feel like I'll have to shoot at least 0.131" nails. I mean, I don't
think the prescriptive building code recognizes construction adhesive
as a substitute for nails?
BTW, is it fair to say that the adhesive connection is stiffer than
the nail connection because the nail connection has an initial
slip/crushing before it gets fully engaged?
Cheers, Wayne
| |
| Wayne Whitney 2008-02-21, 3:27 am |
| On 2008-02-21, RicodJour <ricodjour@worldemail.com> wrote:
> Posters with more technical experience often try to simplify the
> question to the one issue that concerns them.
OK, that is what I was trying to do, sorry if I wasn't successful in
simplifying away the extraneous details while still providing the
necessary details.
> Your situation won't benefit from tapering.
After thinking about it, I believe there will be a benefit as far as
installing the new joists. Since they will have to fit between the
wall top plate and the rafter 1x4 plate on both ends, I believe I'll
have to put them in place flat, one end at a time, and then turn them
upright. That would be hard to do if they are notched to fit tightly
between the two plates. With a taper, the interference will be
confined to a fraction of the thickness of the wall. Of course, that
still may make it difficult. :-)
> After the supplemental CJs are in place, take an elevation of the
> bottom of a central joist at midspan. After you have rocked and
> plastered, do it again. I'm curious to see what sort of deflection
> you get from just the dead load.
The weight of the 1/2" gypsum board is 2 psf, and I believe 1 psf
would be generous for the skim coat plaster. So at 3 psf, the
calculated deflection is 3/20 * 0.5" = 0.075", or 0.05" if I
succesfully sister the existing joists. As to measuring it, that
might take me six months, so we'll both have probably forgotten by
then. :-)
> Do not doubt that someone will do something stupid in that attic in
> years to come.
Well, there's a limit to what I can do. It seems like it would be
fair to match the stiffness of the ceiling joists before I removed the
wall, and the stiffness of the ceiling joists in the rest of the
attic. A quick check shows that both of these are/were at about L/360
deflection for 20psf live load, which is what I'll be providing.
Cheers, Wayne
| |
| RicodJour 2008-02-21, 9:26 am |
| On Feb 20, 10:51 pm, Bobk207 <rkaza...@gmail.com> wrote:
> On Feb 20, 6:08 pm, Wayne Whitney <whit...@post.harvard.edu> wrote:
>
>
And why exactly do you need the shear transfer in your situation? You
don't. You've already established that the new framing members can
take the load on their own. There's little reason to tie the new CJs
to the old so you don't really need adhesive, or nails for that
matter.
[color=darkred]
> You're not me, you should thank goodness for that
>
> but your answer
>
>
> good as adhesive when you want continuous shear transfer. And that
> you need continuous shear transfer to make the pieces of a built up
> beam act as a composite section. That's a different story than
> nailing a ledger to a stud wall, where the members are only
> overlapping in a small area. <
But you don't need the existing 2x4s, so there is no reason to be
concerned about shear transfer, right? Stiffness attracts load, and
the 2x6s or 2x8 will be far stiffer than the existing 2x4. The 2x4,
if sistered, won't be called on to do any work until the 2x8 has
deflected enough that the 2x4 becomes involved, and that won't happen
with a 1/2" deflection over a 17' span.
> was spot on to what I would have written
This is where theory and reality vary. I have no argument that two
structural members that are attached continuously and not at isolated
points, will do a better job in shear transfer. But there have been
bolted and nailed composite beams in use for a long time - they're not
failing.
So what does the adhesive add in this situation? Expense and time
wasted. If the beam was in a more critical location, I'd agree with
the belt and suspenders approach. In this specific situation there is
absolutely nothing gained.
> I checked my posts & didn't find a direct reference to nails in shear
> being "not good"
>
> but if I did, I was / am wrong.
>
> Nails are fine in shear, I have no argument with nails in shear,
> that's what they're meant for (just not for withdrawal)
> I have no idea what that building inspector was thinking... ?????
Me neither! He's a nice guy, though, so I let him slide. Interesting
thing about his building department though - they had a big scandal a
little while back. Four separate guys got busted for shady dealings -
one of them was the commissioner. None of the guys knew what the
other ones were doing. One guy, and you'll love this, refused to
inspect/approve a homeowner's drywell installation until the homeowner
bought four tickets to some firemen's benefit dinner! WTF?! So, for
$900 or whatever it was - money he never saw, this guy threw away his
career and is facing potential jail time.
> but where one needs continuous shear transfer to develop composite
> action (built up box beam, I beam, etc) adhesives are much better
> than discrete mechanical fasteners because the stress is well
> distributed and the shear connection is stiffer than nails.........but
> since I often do "belt, suspenders, staples & duct tape"....... I
> prefer adhesive AND fasteners.
>
> Also.guilty........Wayne & I tend to over analyze things
>
> (him more than me, because he has less experience but he CAN do
> thecalcs & ask lots of questions...me, because I'm always looking for
> "better / optimum".
Better/optimum also takes into account expense, effort and added
benefit. I fail to see how construction adhesive affects any of those
criteria in a positive way in Wayne's situation. If you just want to
say, 'But it makes me _feel_ better!', then I wouldn't argue.
R
| |
| Wayne Whitney 2008-02-21, 1:26 pm |
| On 2008-02-21, RicodJour <ricodjour@worldemail.com> wrote:
> But you don't need the existing 2x4s, so there is no reason to be
> concerned about shear transfer, right? Stiffness attracts load, and
> the 2x6s or 2x8 will be far stiffer than the existing 2x4. The 2x4,
> if sistered, won't be called on to do any work until the 2x8 has
> deflected enough that the 2x4 becomes involved, and that won't
> happen with a 1/2" deflection over a 17' span.
OK, we have that the new 2x8 joist alone is deflection critical but
adequate for the 16'5" span. Some have suggested that the 1/2"
deflection this gives under live load is too much for that span. :-)
So the question was raised of how to use the existing 2x4 joist to
stiffen the new 2x8 joist. A few comments:
First, you statement "stiffness attracts load" is true when members
are acting compatibly (same deflection) but separately. This would
apply if the 2x4s are left in place, unattached to the 2x8s, and we
considered that the gypsum board was sufficient to give equal
deflections.
In this case, the moment of inertia (b*h^3/12) of the 2x8 (at 1.5" by
7.25") is six times that of the 2x4 (at 2" by 3.625"). So the overall
system stiffness is 1 + 1/6 = 7/6 = 1.16 times as much as the 2x8
alone.
However, if we adequately tie the 2x4 to the side of the 2x8 (bottoms
flush), such as by using frequent smaller nails, or a good adhesive,
then the two members will act together as a composite section. This
composite section, from the previous discussion, will be 22/15 = 1.47
times as stiff as the 2x8 alone.
This difference arises because the two members are different heights,
and we are attaching them off center. In the more typical situation
of both members being the same height, (a 2x8 sistered to a 2x8), the
two calculations would give the same answer (twice as stiff). This is
behind your experience that sistering without adhesive works fine.
Similarly, if the 2x4 were attached to the 2x8 so their centerlines
were at the same height, then again both calculations would give the
same answer (1.16 times as stiff).
Lastly, as to the question of adhesive versus many small nails, I have
the recollection that under load the nails will have an initial nail
slip before being full engaged, while the adhesive won't have this
initial slip. So in a deflection critical situation, the adhesive
will work better. If the sistering is to increase strength, and the
fastening is just to share the load, the difference is not important.
I'm not 100% sure my recollection here is correct, perhaps BobK can
confirm this.
Cheers, Wayne
| |
| Bobk207 2008-02-21, 1:26 pm |
| On Feb 21, 8:08 am, Wayne Whitney <whit...@post.harvard.edu> wrote:
> On 2008-02-21, RicodJour <ricodj...@worldemail.com> wrote:
>
>
> OK, we have that the new 2x8 joist alone is deflection critical but
> adequate for the 16'5" span. Some have suggested that the 1/2"
> deflection this gives under live load is too much for that span. :-)
> So the question was raised of how to use the existing 2x4 joist to
> stiffen the new 2x8 joist. A few comments:
>
> First, you statement "stiffness attracts load" is true when members
> are acting compatibly (same deflection) but separately. This would
> apply if the 2x4s are left in place, unattached to the 2x8s, and we
> considered that the gypsum board was sufficient to give equal
> deflections.
>
> In this case, the moment of inertia (b*h^3/12) of the 2x8 (at 1.5" by
> 7.25") is six times that of the 2x4 (at 2" by 3.625"). So the overall
> system stiffness is 1 + 1/6 = 7/6 = 1.16 times as much as the 2x8
> alone.
>
> However, if we adequately tie the 2x4 to the side of the 2x8 (bottoms
> flush), such as by using frequent smaller nails, or a good adhesive,
> then the two members will act together as a composite section. This
> composite section, from the previous discussion, will be 22/15 = 1.47
> times as stiff as the 2x8 alone.
>
> This difference arises because the two members are different heights,
> and we are attaching them off center. In the more typical situation
> of both members being the same height, (a 2x8 sistered to a 2x8), the
> two calculations would give the same answer (twice as stiff). This is
> behind your experience that sistering without adhesive works fine.
> Similarly, if the 2x4 were attached to the 2x8 so their centerlines
> were at the same height, then again both calculations would give the
> same answer (1.16 times as stiff).
>
> Lastly, as to the question of adhesive versus many small nails, I have
> the recollection that under load the nails will have an initial nail
> slip before being full engaged, while the adhesive won't have this
> initial slip. So in a deflection critical situation, the adhesive
> will work better. If the sistering is to increase strength, and the
> fastening is just to share the load, the difference is not important.
> I'm not 100% sure my recollection here is correct, perhaps BobK can
> confirm this.
>
> Cheers, Wayne
Wayne-
Thanks for posting this...my PC is acting up & I've lost a couple post
before I sent them.
I hope you're not making Rico's head hurt with those numbers, mine is
hurting
Be careful your post are starting to look a little like Nick's from
Villanova.
You're spot on about sistering vs composite beam construction....we're
talking about making an asymmetric T - beam.
In a true sistering situation a handful of nails & you're good to
go.
In your situation we really want the 2x8 & the 2x4 to act as if
they're the same piece of timber. If my memory serves me you need
something like 100 psi or more in wood to get that behavior...pretty
hard to do with nails.
btw when I retrofitted my attic joists (2x4's) I glued (epoxy) & osb
cleated members onto the top edges of the existing 2x4's to create
~2x8's
I did this crazy thing because I had used redwood 2x6's leftover from
a job from 10 years ago. The glued "top sistered" joists were stiffer
than the 2x6's & I saved 2" of attic headroom that was critical.
I glued (brushed clean, no clamping, no cleats) test members together
& tested them to failure. Glue stress at failure 173 psi shear.
Based on my calcs & testing the attic joists are ~6x stiffer now &
can easily handle an attic storage load.
cheers
Bob
| |
| Bobk207 2008-02-21, 1:26 pm |
| On Feb 21, 5:35 am, RicodJour <ricodj...@worldemail.com> wrote:
> On Feb 20, 10:51 pm, Bobk207 <rkaza...@gmail.com> wrote:
>
>
>
>
> And why exactly do you need the shear transfer in your situation? You
> don't. You've already established that the new framing members can
> take the load on their own. There's little reason to tie the new CJs
> to the old so you don't really need adhesive, or nails for that
> matter.
>
>
>
>
>
> But you don't need the existing 2x4s, so there is no reason to be
> concerned about shear transfer, right? Stiffness attracts load, and
> the 2x6s or 2x8 will be far stiffer than the existing 2x4. The 2x4,
> if sistered, won't be called on to do any work until the 2x8 has
> deflected enough that the 2x4 becomes involved, and that won't happen
> with a 1/2" deflection over a 17' span.
>
>
> This is where theory and reality vary. I have no argument that two
> structural members that are attached continuously and not at isolated
> points, will do a better job in shear transfer. But there have been
> bolted and nailed composite beams in use for a long time - they're not
> failing.
>
> So what does the adhesive add in this situation? Expense and time
> wasted. If the beam was in a more critical location, I'd agree with
> the belt and suspenders approach. In this specific situation there is
> absolutely nothing gained.
>
>
>
>
> Me neither! He's a nice guy, though, so I let him slide. Interesting
> thing about his building department though - they had a big scandal a
> little while back. Four separate guys got busted for shady dealings -
> one of them was the commissioner. None of the guys knew what the
> other ones were doing. One guy, and you'll love this, refused to
> inspect/approve a homeowner's drywell installation until the homeowner
> bought four tickets to some firemen's benefit dinner! WTF?! So, for
> $900 or whatever it was - money he never saw, this guy threw away his
> career and is facing potential jail time.
>
>
>
>
> Better/optimum also takes into account expense, effort and added
> benefit. I fail to see how construction adhesive affects any of those
> criteria in a positive way in Wayne's situation. If you just want to
> say, 'But it makes me _feel_ better!', then I wouldn't argue.
>
> R
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 2008-02-21, 1:26 pm |
| On 2008-02-21, Bobk207 <rkazanjy@gmail.com> wrote:
> Rico's hammering us (& rightfully so) ..........are we really adding
> any real performance benefit if you sister the old 2x4's to the new
> 2x8's? Yeah, the composite will be 50% stiffer, but do you need it?
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.
> Another thing to consider, old growth DF has a higher elastic
> modulus than currently available new timbers.
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.
> you'll be fine as long as your new installation in reality is at
> least as stiff as the old one.
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.
> Additionally since this composite joist exercise is really above &
> beyond the code (you've already exceeded code min with the 2x8,
> right?) you can do whatever you want.
Good point.
Cheers, Wayne
| |
| Bobk207 2008-02-21, 1:26 pm |
| On Feb 20, 10:23 pm, Wayne Whitney <whit...@post.harvard.edu> wrote:
> On 2008-02-21, Bobk207 <rkaza...@gmail.com> wrote:
>
>
> OK, I'm a mathematician, but when I'm doing engineering, you are
> right, I should say 1.5 because that easier to understand and 1.5 =
> 22/15 within the accuracy of our modeling of reality. :-)
>
>
> I feel like I'll have to shoot at least 0.131" nails. I mean, I don't
> think the prescriptive building code recognizes construction adhesive
> as a substitute for nails?
>
> BTW, is it fair to say that the adhesive connection is stiffer than
> the nail connection because the nail connection has an initial
> slip/crushing before it gets fully engaged?
>
> Cheers, Wayne
Wayne-
I was just kidding you....I know your the mathematician.
Those ratios make my head hurt...I think in factors & %'s.
Rico's hammering us (& rightfully so) ..........are we really adding
any real performance benefit if you sister the old 2x4's to the new
2x8's? Yeah, the composite will be 50% stiffer, but do you need it?
In another post you state that the new 2x8's are stiffer than the old
in place unmodified 2x6's. If that's so then you really don't need to
add the extra stiffness beyond the new 2x8's
But
if the old 2x6's are stiffer than L/360 & the new 2x8's are stiffer
than L/360
BUT if the new 2x8's are less stiff than the old 2x6's ........you'd
be reducing the framing stiffness & the plaster MIGHT be unhappy.
In another post you state....
[color=darkred]
deflection for 20psf live load, which is what I'll be providing. <<<
you'll be fine as long as your new installation in reality is at least
as stiff as the old one.
Another thing to consider, old growth DF has a higher elastic modulus
than currently available new timbers. I sat on a committee with a
REALLY old engineer (old enough to be my dad) & he'd sometimes spec
the number of growth rings per inch on bending members!
Your comment about nail size.....I like smaller nails, they damage
the wood less than larger nails. In fact .148 is about the largest
nail I use unless I'm using VERY large timbers. Based on my research .
148 is about the largest nail that 2x's & 4's will accept without
damaging trauma to the wood. I've tested lots of timber connections &
large nails too close together (code allowed spacing is often too
close) will create a connection that is weaker than smaller nails.
Additionally since this composite joist exercise is really above &
beyond the code (you've already exceeded code min with the 2x8,
right?) you can do whatever you want. Yeah, .113's are a good
choice for "stitching" but I prefer a LOT of smaller
fasteners.......an infinite number of infinitesimally small fasteners
would approach the behavior of adhesive
My fastener of choice is a 16ga staple or 16ga brad but the .113 is ok
too.
[color=darkred]
Yup, that's why I prefer smaller fasteners & more of them. Smaller
fasteners have smaller localized loads on the timber. 16 gage staples
are good for ~250 lbs (per staple, ultimate shear failure load...lots
of slip. They have effectively zero slip at 50 lbs per staple)
Nails have slip, glue doesn't; glue can be brittle & can fail, nailed
connections are ductile & typically do not fail....that's why I
combine them.
cheers
Bob
| |
| Bobk207 2008-02-21, 5:25 pm |
| On Feb 21, 9:42 am, Wayne Whitney <whit...@post.harvard.edu> wrote:
> On 2008-02-21, Bobk207 <rkaza...@gmail.com> wrote:
>
>
> 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
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 2008-02-21, 5:25 pm |
| On 2008-02-21, Bobk207 <rkazanjy@gmail.com> wrote:
> Is the E for new growth DF really 1.9 Mpsi?
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.
| |
|
| Wayne Whitney wrote:
>
> However, if we adequately tie the 2x4 to the side of the 2x8 (bottoms
> flush), such as by using frequent smaller nails, or a good adhesive,
> then the two members will act together as a composite section. This
> composite section, from the previous discussion, will be 22/15 = 1.47
> times as stiff as the 2x8 alone.
>
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.
--
Art
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