O.k. maybe this is a dumb question to some but I’ll ask any way. :/
Do sistered 2×12’s have the same span strength as a 4×12? In terms of using one or the other as a structural beam, does it matter?
thanks in advance to all who reply.
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The sistered 2Xs aren't the same dimension as one 4X so that changes it. I'm guessing but if they were both the same dimension and the 2Xs were glued I'd say they were as strong or stronger than one 4X.
If I were sistering 2Xs I'd sandwich in a piece of plywood, with glue. That's a big improvement in strength for a small cost.
Someone with an engineering table will probably be along to put some numbers on the question.
I'll let my curiosity get the better of me here...What does the plywood do structurally in this case?'Man who say it cannot be done should not interrupt man doing it' ~ Chinese proverb
Something I've always wondered about-The old time carps around here swore by putting valley tin between sistered members.Any validity to doing that?
Interesting thoughts....yes, valley tin would make a stiffer beam. Just how much stiffer would depend on the gauge. Only the metal at the very outside edges in contributing to the bending strength, but the 2x members prevent any lateral buckling of the tin, thereby enabling it to do its job. This is just a really small flitch plate. I reality, a 4x12 (trimmed to 3") vs. a pair of 2x12's could be the same strength, but probablility enters here. There is a higher likelihood of a knot or other weakness in the larger member. The timber codes account for this in various ways, and we put a larger safety factor on the larger members. Glue would contribute nothing to the sistered 2x12s. If the beams are braced laterally, very few nails are needed from a structural stability standpoint...prescriptive methods provide standard nailing schedules.
Thanks-I always wondered about the tin thing even though I don't use it for anything.I'm suprised the glue and nails don't add much, but I guess that makes sense if you think about it. Does it help prevent twisting?Always thought if one nail was plenty, 25 with glue was better. Maybe I've wasted my time?
MAYBE 12 gauge steel, but there is no way that valley tin will do anything. By the time a deflecting load sufficient to require more than the lumber will provide puts stress on a center of valley metal, that metal will be tearing out around the fasteners.Put this one on the shelf with old drunk carpenters tales.
Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!
The bending strength of mild steel is 36,000 psi and the bending strength of most building lumber is 1000 psi give or take. Therefore, a 1/32" peice of metal is the bending equivalent of 1" of lumber at the same depth. However, the metal is useless by itself as it will quickly deform laterally. The wood keeps the metal in place in this condition. Since we are talking about constant material from top to bottom, any shear flow is within the individual members, and nail fastening is of little consequence. Flip the sandwich the other way and the tin is useless at the members center. Put the tin on the upper and lower surface and it can now do something....but only if the fastening schedule has adequate shear transfer. It is in this case that you would see tear out due to deflection. Put on 2x4 atop another between two sawhorse and draw a bunch of vertical lines. Note that the line at the center of the member would stay aligned while you deflect the setup, and the lines at the far ends would move significantly apart from one another. This creates a visual of the concept of shear flow. The further apart the lines, the more nails/glue is needed. Therefore, far more nailing is needed at the ends of said member than the center. Two 2x4s thus arrange without nailing are only as strong in bending as two 2x4s. Nail them properly together and the strength will increase significantly, being a function of the depth squared. Those drunk carpenters had no notion of any of this, but did in fact have a concept that worked.
glue in the sister joint helps the two members share the load better than just using fasteners. a sheet of tin would prevent that happening and force the shear on the nails to handle it all.I can imagine that they were thinking that the metal added some tensile strength, but it needs to be at least 1/8" to really do anything. Valley tin ain't it.
Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!
A couple of clarifications:I am an apprentice old fart but not old yet.I am sober as a judge. (Although some judges might tip one now and then):)
Comment on plywood/lumber box beams, practical aspects.
Designed a 36 ft span beam to support roof and 2nd floor for house brother was building about 15 years ago, 36 inch height, 3ea 2x6s on top and bottom, 3/4 ply webs, glue and screw.
Material cost was less than 20% of what a glulam would have cost him, my design time (free) was about 3 hours, did it during waiting time while on jury duty.
Brother's time was probably longer, his only comment was - 'for the time I spent building that thing, could have bought the glulam and rented a crane' -- ach well...
Brother's time was probably longer, his only comment was - 'for the time I spent building that thing, could have bought the glulam and rented a crane' -- ach well...
It's not your fault that he's slow. I'd have been most grateful for the engineering help. It would be up to me to decide how to build it and set it efficiently.
Okay I've got the same question but this time with headers over a window.
We have some windows in the bottom story with just the doubled 2by6 top plate (the 2by dim vert for 4" of wood) acting as a header. With a story and a half above this and a 4' span this seems a little bit on the week side.
I've got space between the top plates and the window for maybe a piece of 3by material. Could I pull out the window pop in a 3by6 and have the same amount of strength as a 7" deep piece of wood?
I'd prefer not to have to sawzall out the existing top plate-header and install a deeper 'real' header if I can help it.
Thanks,
Daniel Neumansky
Restoring our second Victorian home this time in Alamdea CA. Check out the blog http://www.chezneumansky.blogspot.com/
Oakland CA
Crazy Homeowner-Victorian Restorer
madmadscientist- I didn't really follow what you were describing, but stacking members on top of each other (if that is what you were asking about- a 3" topped by a 2" and another 2") doesn't give anywhere near the strength of a similar depth solid beam, unless they're really really well bonded, like a gluelam.
Side by side (as the op is describing) is a different story, that can be about the same strength as a similar width solid beam.
Or were you asking something else?
k
madmadscientist- I didn't really follow what you were describing, but stacking members on top of each other (if that is what you were asking about- a 3" topped by a 2" and another 2") doesn't give anywhere near the strength of a similar depth solid beam, unless they're really really well bonded, like a gluelam.
Yes different than OP's question what you describe above is correct.
Shoot I didn't think it would be that simple...but
Daniel Neumansky
Restoring our second Victorian home this time in Alamdea CA. Check out the blog http://www.chezneumansky.blogspot.com/
Oakland CA
Crazy Homeowner-Victorian Restorer
I think you need an engeneer for this one. If I was pushed, I would say you need a reg. header, which means sawing out the top plates to make room for a sufficient header.
Your buiding inspector didn't catch this one?
Nope old house the BI won't make you change the original stuff unless its REALLY bad or if you are already doing something to the wall. Like if I was going to install new windows they would probably require a 'real' header.
Daniel Neumansky
Restoring our second Victorian home this time in Alamdea CA. Check out the blog http://www.chezneumansky.blogspot.com/
Oakland CA
Crazy Homeowner-Victorian Restorer
Daniel, IIRC isn't there a rim joist above your header in question?
The formula for strength in bending is bh^3. Multiply the width (b) of your beam by the cube of the height, and you have its strength in bending relative to other shapes of the same material.
3x6 on the flat plugged into the above formula gives you 148.5.
That's about the same as a 1/2" x 7" on edge, by the above formula.
The formula doesn't take lateral stability into account, which would make a difference, but either way a 3x6 on the flat isn't very strong.
Daniel, IIRC isn't there a rim joist above your header in question?
Nope no rim joist unfortunately...the floor joists bear fully on the top plates and there is blocking at the outside ends in every other joist bay.
Well #### the SE spec'd 6by headers for the door openings at 32" wide...maybe I could lower all the windows 3" and slip a 6by6 under the double top plates...I'm feeling lazy I guess been spending all my free time finishing these really nice stain grade doug fir 5 panel doors for the downstairs and I'm getting antsy for some significant progress...
Daniel Neumansky
Restoring our second Victorian home this time in Alamdea CA. Check out the blog http://www.chezneumansky.blogspot.com/
Oakland CA
Crazy Homeowner-Victorian Restorer
"What does the plywood do structurally in this case?"
Personally, I think it does little but help with shear.
Can you cry under water?
Boss, do you have any drawings or tables for box beams handy?
They used to make up headers for very wide openings, 40' and more, using plywood sheathed box beams. The framing was bridge trussed 2X4 style.
I've never seen any sort of box beam tables. It would be interesting to see what they looked like.
Those who live by the sword get shot by those who don't.
Try "
Plywood Design Specification, Supplement #2
Design and Fabrication of Plywood-Lumber Beams (Form S812 - 24 pages)
located at the american plywood association: http://www.apawood.org/level_c.cfm?content=pub_searchresults&pK=box%20beam&pT=Yes&pD=Yes&pF=Yes&CFID=7712033&CFTOKEN=94423064
Thanks.I downloaded it, and will look it over when I have time.
Committee - a group that keeps minutes and wastes hours.
Using "plywood box beam" to search on Google, this is one of the better descriptions:
http://books.google.com/books?id=X3OPshEm8Z4C&pg=PA24&lpg=PA24&dq=Plywood+box+beam&source=web&ots=f9s-shpO7s&sig=z0I74hBNrEEwDgexOfwEVI-4Gks&hl=en
There are drawings and photos of them in older books on carpentry, as I recall. I'll see what I can find on the web.
Maybe one of the other BT carpenters, architects or engineers will see this and have a reference he/she can share.
Edited 5/27/2008 2:02 pm by Hudson Valley Carpenter
Plywood was always used to make up the 3 1/2 inches for flush nailing .
It does add more than a 1/2 inch which was proven one time altough Ive never seen it in a chart. Its not a lot .
What always got me is a gluelam is not as strong when they use 2x4s and turn them flat and stack them as made up beams from yp 2x12s considering the same dimension. They seem to sag more often. Ive noticed that in beams in the middle of basement garages time and time again. Id get there to rock them and they werent truely straight. If they had a 1/4 inch or more Id call about it . I called a lot .
The builder would always say that it was on the plans . We would always pick it up and fix it with either plywood and lumber or steel.
Tim
Edited 5/29/2008 3:13 pm by Mooney
The builder would always say that it was on the plans . We would always pick it up and fix it with either plywood and lumber or steel.
Tim, I don't know if I'm reading this right. The builder said the sagging beam was in the plans? And you fixed it?
Sorry for the inquisition but that just grabbed my attention, especially that you would go there to sheet rock and end up 'fixing' a sagging beam.
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Edited 5/29/2008 6:34 pm by Ted W.
The beam was on the plans and yes I would fix it but Im not your run of the mill rocker.
I could do the builders job but I made more money subbing . Thats the way its always been. Subs make the most money.
Tim
"Subs make the most money."Maybe in Arkansas.... Bob's next test date: 12/10/07
Mebbe you were in the wrong trade for it too. Mebbe its changed.
We talked about it here a lot . Unless a builder had a lot of jobs going on he didnt beat his subs . But that depends on the builder and the subs too. I had three crews subbing under me . Hangers , tapers and painters.
There was a plumber here that made the most money every year . He had 3 service trucks and 3 crews doing installation with trucks . He put in septic tanks and did rotor rooting . He had a ditch witch and put in services digging trenches and footings with a backhoe. He made three times what I did Im pretty sure and I beat builders that were here at the time .
We didnt have track builders per say like the 100 house additions going in all at once so I dont know anything about that . I knew what it cost to build and what they sold for so it wasnt hard to figgure what was left for the builder . The builder doesnt build anyway most of the time . He hires you . A few put in footings but hire them dug . Anyway you can figgure a complete houses subbed out which they did and see whats left. Figgure the houses per year they build and its not rocket science.
I know they were paying some really good money to frame the big houses when the steep roofs came to town. Lots more money than the 4/12 pitch goverment jobs.
I always thought the builders made their money on the cost plus jobs if they could get paid the plus . Ive never understood why anyone should pay cost plus but I like getting it .
Tim
Edited 5/29/2008 8:30 pm by Mooney
For a long time builders quit building goverment houses because there wasnt any profit in them. So carpenters built them per say that did most of the building .
My Brother was a super over housing tracks in NW Arkansas building those 100 house additons . He said they were making money building the small ones .
Of course Ive never seen the prices in this area Ive read about on this forum.
Tim
Its most certainly a regional thing. I've watched builders put 50 house a year up and net 20% on every one with one supertintendent and two laborers. At 60k per pop, that's a modest 3 million net and that didn't include development money from the subdivsions lots if he was the developer. He also made money in the title work and loan orgination end.When I was busting my butt trying to make money as a sub, I could have been buying ten acre plots and selling out every lot before they were finished. I just didn't know how to get the deals funded. Now I know...but the parties over lol! Bob's next test date: 12/10/07
"I've watched builders put 50 house a year up and net 20% on every one with one supertintendent and two laborers.Now I know...but the parties over lol!"What time period is the "party" you're describing taking place in?I would guess that the money the builder is making is not because of the service he offered (or the "product" he created), but it comes from his investments returning a great rate.If it takes 12 months to go from land purchase to closing, and the market appreciates 15% during that time, his profit is largely a function of the market. Maybe I'm wrong, but I would be surprised to see builders netting 20% in flat market conditions. A sub is not really "investing", he is just delivering a service/product so he's not able to make that kind of return.
Jon Blakemore RappahannockINC.com Fredericksburg, VA
These guys don't build fifty custom homes a year and not factor in their markups and forget about carrying costs and inflation. Bob's next test date: 12/10/07
In that case it would have to be regional.
I didnt know what I do now either.
Comes with age I think.
Tim
Glulams typically come built with a camber in them. Its not unthinkable that the frames installed them upside down.I've installed some fairly hefty glulams and never saw one sag a 1/16". Maybe they would with roof and drywall load... Bob's next test date: 12/10/07
I always eye balled them before I covered them.
I havent had any experience with them in several years . It might be better I dont know . None I ever saw had a camber up under load.
I put metal corners on them so they had to to pretty good . If they were sagging I backed off because they would move more . I couldnt have that and neither could my builder. I could have furred them straight but thats not the deal. If they are bowed down I never thought they were done .
My real point was it was 2x4s glued up laying flat making a beam. Ive seen 2x12s sandwiched with ply hold. [glued of course]
Ive fixed them glueing 2x12s to each side and nailing a good pattern. Let the jack down the next day and they dont move at all. I had a builder that came back in with steel beside them.
The ones I saw fail were always in garages holding the middle . So they were holding one end of the joices and a center wall in most cases. The extra weight would have been the ceiling above the center wall and of course the wall itself . They bowed before people and furniture got there.
Tim
What does the plywood do structurally in this case?
Goes back to why plywood makes such a great exterior material, particularly for box beams, when it's held against the framework by nails.
Each of it's laminations have shear strength. Putting them together with glue makes them super strong, when fastened so that twisting isn't possible.
Before laminated beams became available (a great example of plywood's shear strength) it was common practice to make up ridge beams for big cathedral ceilings using triple 2X12's with five ply 1/2" CDX laminated between them. That method offered the best means to join the 2X12s over long spans without losing a lot of strength. The beam was, in effect, an unbroken double 2X12 laminated sandwich.
Using pipe scaffold, we built them in place, piece by piece, as we assembled the roof.
I've also made up double 2X12X20' with a plywood sandwich to extend their span capability beyond the usual 16' limitation. Adds enough shear to make the beam quite rigid.
Edited 5/27/2008 2:24 pm by Hudson Valley Carpenter
Well, I thought I was following along until your answer to Mike below.Do you mean that the plywood keeps the 2x from twisting? Or the other way around?And assuming that I'm even on the right track, isn't this much like what a flitch plate would do? (Aside from being bolted as opposed to nailed like the plywood)If so, when would you choose one method over another? Materials on hand? Span of the beam?'Man who say it cannot be done should not interrupt man doing it' ~ Chinese proverb
Do you mean that the plywood keeps the 2x from twisting? Or the other way around?
Plywood doesn't have much shear strength unless it's kept from twisting or torquing, so it's mainly the 2Xs that keep the plywood from twisting...although using glue helps to make each member less likely to twist/torque.
I've never consulted an engineer on this question so I'm speaking only from experience and observation of results. My testing methods were always simple and practical. Jump on the center of the span with both feet and feel the deflection. A modified Mr. Whipple system. ;-)
I swear officer, I've never filtched anything.
Edited 5/27/2008 10:18 am by Hudson Valley Carpenter
Not enough to be worth considering, IMOThe strength needed in a header or beam like this is gained in the height far more than in the width.If the header/beam runs moire than 8', the joint in the plywood negates any gain by focusing to a stress point.
Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!
At the risk of starting a war here...
Plywood adds nothing to the strength of a beam. Half the fibers are in the wrong direction for resisting bending stresses.
Take a 1x2, and a 1x2 piece of plywood. See which one bends easier.
Okay, I'm in for an argument. :D
Stand a strip of plywood (9" wide-ish) on edge with the ends supported and press down on the center. Granted, it may not be as rigid as the same dimension of construction grade lumber, but it doesn't melt like butter in the sun either. Therefore, it does add strength. --------------------------------------------------------
Cheap Tools at MyToolbox.netSee some of my work at AWorkOfWood.com
9" wide you're not going to be able to bend any kind of wood. That's why I suggested a narrower strip. Same physical properties regardless of width.
Melt like butter in the sun? You mean solid wood cupping? I'll give you that plywood doesn't cup like solid wood. That doesn't seem like a major consideration in designing a beam.
Boss H is right, plywood does add some shear strength. It's a rare beam that's governed by shear though.
9" wide you're not going to be able to bend any kind of wood. That's why I suggested a narrower strip. Same physical properties regardless of width.
Melt like butter in the sun? You mean solid wood cupping? I'll give you that plywood doesn't cup like solid wood. That doesn't seem like a major consideration in designing a beam.
Boss H is right, plywood does add some shear strength. It's a rare beam that's governed by shear though.
I'm not refering to cupping or shear. What I'm saying is that plywood is stiff, albeit not as stiff as regular wood. You stated earlier that plywood by itself adds no strength, but that's far from true. It adds a lot of strength. And not just shear, but in flex as well. Sistered 2x10s with 1/2" plywood in the center is a lot stiffer, and will span much wider, than without the plywood.
Conversly, if the plywood was replaced with 1/2" lumber (say a 1x planed down to 1/2" thick), it would be stronger still.--------------------------------------------------------
Cheap Tools at MyToolbox.netSee some of my work at AWorkOfWood.com
You are correct, it is an exaggeration to say plywood has NO strength in bending.
The modulus of elasticity of framing lumber, which is the measure of stiffness in bending, is between 1,100,000 and 1,600,000. LVL's are 1,900,000 or 2,000,000. Steel is 36,000,000.
I don't know what plywood is but probably a bit more than half of solid wood, let's say 700,000. So plywood does add some strength, but it's generally not figured in when calculating beam strength.
Take a 1x2, and a 1x2 piece of plywood. See which one bends easier.
If the plywood is kept from twisting, it'll be much stronger. That's what the 2Xs do, hold the plywood so that it only acts to carry shear loads.
It's the same principle as a box beam, just reversed and enhanced by lumber which is also being used for shear.
Edited 5/27/2008 9:20 am by Hudson Valley Carpenter
Wow! Maybe not such a dumb question after all. Thanks for all the info and feedback. Perhaps more specifics are in order.I have a span of 16'5". I am planning on making this a structural ridge beam support similar to the fhb article in the Dec 07 issue. He uses sistered 2x12s on a span of what appears to be 18'. I have a rise of 2 1/4" for every 12" on a run of 6'. I was planning on finding a reclaimed 4x12 but am having trouble locating, so Instead was going to use the sistered technique with a a plywood sandwich, then laminate and stain (the beam will be exposed). Now after reading this post I'm wondering if I shouldn't just use a box beam. Any suggestions.thanks again,stizzo
Stizz, you're still here?
ROFL.. We can get to rambling sometimes and forget who even posted the original question. At least this time we managed to stay on topic. :D
Used as a ridge a box beam may crush in from the lateral pressure.
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Cheap Tools at MyToolbox.netSee some of my work at AWorkOfWood.com
Edited 5/27/2008 10:08 pm by Ted W.
Thanks, Ted W......but I forgot to add some info that might help elaborate; this beam is so I can remove the rafter ties and will sit directly under the 1x6 ridge board (old house), so i don't think there will be much lateral pressure, just vertical right?
Triple the beam. Bob's next test date: 12/10/07
"triple the beam"? Could you elaborate for me? Do you mean 3- 2x12's, 3- 2x12's with sandwiched ply or do you mean 3- 4x12's...... or is this a box beam reference?
I think he means 3 - 2x12's--------------------------------------------------------
Cheap Tools at MyToolbox.netSee some of my work at AWorkOfWood.com
Yes, that's what I mean. I don't understand why people want to jamb plywood inbetween every pair of 2x's. I've seen this done hundreds of times in areas when a third member could be just as easily added. I understand the need for it when it it critical to make the surfaces flush with something...as in a 3.5" wall...but it makes no diifference if it's an attic beam or a dropped ridge beam or a ridge. Just nail 3 2x12's together and check the span tables to see what the maximum span is. Bob's next test date: 12/10/07
I don't understand why people want to jamb plywood inbetween every pair of 2x's.
Jamb THIS Jimbo!
Uhh... should I jamg THIS also?
I agree, no need in using plywood except for dimentionally critical applications, such as headers in a 2x4 stud wall. I don't think he meant anything personal.
--------------------------------------------------------
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Edited 5/29/2008 2:55 pm by Ted W.
I didn't mean anything personal to anyone. I've seen hundreds of small headers up in attics sandwhiched with plywood in situation that didn't need any extra strength. I've even had carpenters working for me that wanted to do that too. When I ask them "why are you putting plywood in between those members?" they reply; "because we always put plywood in headers!". They don't understand the reason for adding them in outside walls and just assume that they need to do it everywhere. Sometimes these are carpenters that have ten or more years experience! Like I said...I don't understand. Bob's next test date: 12/10/07
what you should do is have an engineer indicate the right ridge beam for your spans and required live load, neither of which you have stated here. A pair of LVLs 14" wrapped with pine for finish would be my guess as most efficient.
Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!
As HVC said, two 2x anythings will only be three inches wide instead of 3-1/2".
As the other posters have said, if they were the same dimension they would have the same strength--if they are adequately connected to each other. What "adequately connected" means is completely dependant on the situation.
If the load is uniform and from above, and the ends of the 2x's are supported well, the connection between the two can be a lot less than if the loading is off-center or if the ends of the beams aren't supported the same.
Actual dimension differences aside, if I recall my structural classes, yes there is a difference between two pieces of wood of same dimension as one ... structurally speaking. Maybe not a lot, but there is a difference and a structural engieer would account for it in a design. And plywood will add a lot to a pair of wood members in terms of strength. There is a fastening pattern/schedule that goes with doing this. A structural engineer would be able to give you more specifics about all of this, so if you are pushing some limits, I'd encourage you to talk to a structural engineer. This is wood structures basics and he'd be able to help you out.
Just a bit of data to cloud the picture. Looking at a very old table for floor beams of No. 2 SYP for 50 psf loading (40 live, 10 dead) a 4 x 12 beam is rated to span approximately 8 inches more than a built up beam of 2-2 x 12s. Not a very significant difference, IMO.
I'd say that is significant. Bob's next test date: 12/10/07
I'd suggest 2 2"x12" have a better load bearing capacity than a 4"x12" in most cases. Built up beams composed of dimensional lumber are less subject to internal stresses that occur when the wood dries. Look at post and beam construction . It is a rare beam that doesn't develop cracks over time - significantly reducing their load bearing ability.
Actually the checks that develop in large timbers tend to increase the moment of inertia and occur along the neutral axis, so they increase the strength of the beam.
"Actually the checks that develop in large timbers tend to increase the moment of inertia and occur along the neutral axis, so they increase the strength of the beam."
That's interesting. I went and looked at some post and beam yesterday and sure enough the checks were all along the neutral axis in the middle third of the beam. None near the top or bottom cords. I know with I beams or trusses they remove un- needed material from this area, leaving just enough to tie the cords together, but I can't see how checks, which separate the top and bottom of the beam, would make it stronger. Surely, it would at best be in a spot where the surrounding wood could compensate for it and transfer the forces between cords?
Most timber framing is done with boxed-heart lumber, so when the timber dries checks develop where there is least distance from the face to the heart. The heart itself doesn't check, so continuity between top and bottom surfaces is maintained.