Joist selection

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

Posted by Bobk207 on February 18, 2008, 9:00 pm

> > > 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.

> > 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, 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

Posted by Wayne Whitney on February 18, 2008, 10:37 pm

> 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

Posted by Wayne Whitney on February 19, 2008, 12:12 am

> 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

Posted by Bobk207 on February 19, 2008, 2:17 am

> > 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

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

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