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Subject Author Date
Joist selection Wayne Whitney 02-15-2008
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Posted by Wayne Whitney on February 21, 2008, 11:08 am

> 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

Posted by Bobk207 on February 21, 2008, 12:09 pm
>
> > 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

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

Posted by Art on February 21, 2008, 5:14 pm
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

Posted by Bobk207 on February 21, 2008, 12:25 pm
>
>
> > > > 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.
>
> 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.
>
> > 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. <
>
> 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

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

Posted by Wayne Whitney on February 20, 2008, 8:21 pm

> 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


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