Joist selection

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

Posted by Wayne Whitney on February 21, 2008, 1:23 am

> 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

Posted by Bobk207 on February 21, 2008, 11:48 am

> > 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-

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....

>>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. <<<

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.

>>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? <<

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

Posted by Wayne Whitney on February 21, 2008, 12:42 pm

> 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

Posted by Bobk207 on February 21, 2008, 3:59 pm

> > 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

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

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