Got to thinking of this after I saw what someone did in my garage. I know that its common to sister larger joists onto undersized lumber when finishing an attic space or addidng up, but what about addidng joists on top of old ones. If you had 2×6 attic joists, could you rip off the roof and lay 2 x 4’s on top of them, maybe glued together and get the same performance as a 2 x 10?
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I'm not an engineer but I think this would work. When force is applied to the top of a joist, the joist wants to bend or deflect such that the bottom of the joist is in tension and the top is in compression. The middle section is under no strain (the reason why one is allowed to drill holes in the centre of a joist for pipes).
My logic is that the joint between your stacked joists would be under no strain and so it would work. ALthough for this to work I think the bearing ends of the stacked joists would have to be well connected together so that they act as one.
What do others think? Does what I say make sense?
E2Canoe,
You are correct saying that the top of the beam is in compression and the bottom ofthe beam is in tension; however, there are shear stresses in the middle portion of the beam. At midspan on a uniformly loaded beam the shear is 0. Shear is at its maximum at the ends. In order for the beam to act as one, you have to transfer the shear from the top member to the lower member, for the tension and compression to flow as you have described.
xxxxxx-
a rather unique handle there. Welcome to Breaktime.
Or seeing this is your first post is this another Breaktime regular masquerading under a different guise?
"sobriety is the root cause of dementia.", rez,2004
"Geodesics have an infinite proliferation of possible branches, at the whim of subatomic indeterminism.",Jack Williamson, The Legion of Time
I hang out in knots, just to learn my hobby of woodworkin. I am in breaktime because I enjoy learning about residential construction issues.
Just popping in with an engineers perspective. The strength of a member is proportional to its moment of inertia. For a rectangular section the moment of inertia is 1/12*b*h^3.
max bending stress in a beam occurs at its top and bottom of the section.
stress = M*c/I
c = distance from section centroid to outside edge of beam
M = moment in beam.
Bottom line, unless you can transfer all of the shear across the plane where the two boards come in contact in the stacked condition you will have the sum of the moment of inertia, not the full section. It would act as if they were side by side nailed together without this shear transfer.
For a rectangular section the moment of inertia is 1/12*b*h^3.
max bending stress in a beam occurs at its top and bottom of the section.
stress = M*c/I
c = distance from section centroid to outside edge of beam
M = moment in beam.
I got to tell you I didn't know that. :o)
Cheers"sobriety is the root cause of dementia.", rez,2004
"Geodesics have an infinite proliferation of possible branches, at the whim of subatomic indeterminism.",Jack Williamson, The Legion of Time
Hey ....I'm no engineer, either....BUT I think that you still would have each member act as an individual not as being equal to a 2x10.
To approximate the 2x10 unit the 2x4 would have to be epoxied to the full length of the 2x6. By yht time you do that you could have sistered in the 2x10's.
The sum of the parts does not necessarily equal the whole. Each peice will still act indepedentlly and will probably assist each other in some way.
Major considerations not given are the "span" and "load".
.............Iron Helix
Helix has it right. The pieces would have to be glued or otherwise fastened together to eliminate the shear plane. Take a 4x8 on edge and a stack of five 2x4's on the flat, all say 16' to 20' long. Set them up side by side on sawhorses at the ends. The cross sections would be identical, but just pushing down in the middle with your hands you'd be able to see immediately that the solid piece is a lot stiffer and stronger.
-- J.S.
By the way, I think I read somewhere that the resistance to deflection varies with the cube of the depth of the joist. However, it varies only with the first power of the width of the joist.
So that means using a 2x8 instead of a 2x6 is two and a half times stronger. And it also means that a single 2x8 is still stronger than sistered 2x6's.
We did a simiar thing in our hunting cabin--the rafters were just 2x4's @ 24" o.c.. We put a 2x6 under each one, crown up, and nailed little blocks towards the center bottom of the old rafters. Then we levered the news ones in place and screwed plywood gussets to the sides. I figured with the blocks they would act sort of like lenticular trusses. Not an engineer either, but the roof no longer sags ('specially where the big tree limb landed on it).
I have seen several cases where people renovating older homes were not satisfied by the bounce in the floors on the second story. Investigation showed that the joists were in good condition if a bit small by todays standard but not so small as to be unsafe.
Increasing the depth of the joists or increasing the number of joists was suggested. The fist choice was not seen as a good option because both the room above and below had relatively low ceilings. The second option wasn't seen as good because many of the bays between joists were occupied with ducts and a great number of pipes and, my specialty, wiring. Not that these utilities couldn't be moved, especially the wiring, but none seem willing to do it for free.
In both cases the lead carpenter suggested installing 3/4" plywood above and below the existing joists making them both an inch-and-a-half deeper and effectively into I-beams. This plan was run past the engineer who approved.
Once installed, they glued and screwed it in place with blocking at the joints even though they used T&G ply, the joists were much stiffer and the customer was satisfied. The first time I saw this the engineer came out to look at the situation before and after. He was quite surprised that the plywood stiffened the joists so much. Last I saw him he had a big SEG and was jumping up and down slightly giggling to himself. Of course he didn't mention that it was the senior carpenter who came up with the plan. Seems to be the way it goes.
I'm just an electrician so I would take this as just an option to further investigate. Be sure to consult with a reputable builder and/or engineer before settling on a method.
If you had 2x6 attic joists, could you rip off the roof and lay 2 x 4's on top of them
The only time I have seen this done, the engineer came up with it. But, in that case, the new joists were run 90° to the old ones. Both sets of joists were sheathed over in plywood. Being only a hammer carrier at the time, I did not get to interact with the engineer on the "whys" of the solution. I do suspect that the fact that the exisiting plywood sheathing was glued & nailed in (all the way out to the plates, notched around the rafters, no less) had more than a bit to do with the decision.
Now, back in structures class, oh so many moons ago, this topic was discussed. If you could fasten a 2x4 on the flat to the top of a 2x6 joist, its strength & stiffness change in the miraculous way that T shaped beams do (that would presume a method of fastening that was 100% continuous and uniform).
The simple answer to your question is NO.
no , but rip off that roof and just lay in what ever size you need next to the old and take your nail gun,start shootin...when you get to the out side edges thats where you may have to lay another piece "on the flat" to make your roof edge the same hgt. this is ok over the walls ,Be sure not to do some "funky shimming job" any thing you place here needs to be full length and "through and through"
HAPPY HUNTIN!
It's likely that you'll get something in between a 2x6 and a 2x10. More like a 2x7.5.
Either connect them entirely, or don't connect them at all. One connection in the middle could make things worse.
For $100 bucks an hour, spend an hour with a structural engineer (in the yellow pages). They're worth the money.