Joist selection

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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
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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...
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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
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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
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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
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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
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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
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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
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I'm still wondering about your sanity! ;) What sort of deflection are you getting with a 2x8 sistered on?

Right. And you're doubling the cross section.
R
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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
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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?
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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
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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
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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
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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.

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
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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
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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
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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
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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 .... :( qVQ/I
cheers Bob
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