WOOD PLYWOOD SANDWICH FIELD BUILT BEAMS
user-442272
| Posted in Construction Techniques on
Building beams from dimension stock is a common practice as is making up a beam by making up a sandwich of plywood and glue between layers of dimension stock. I can find enough information about the various species of wood to calculate the load carrying capability of a beam made up of dimension stock but can find no info on the strength added by making a plywood sandwich. Does anyone have a source?
Replies
According to my lumberyard guy it doesn't make any difference at all. Gluing dimensional lumber together has an impact on load. (I'm not sure if I agree with it but I'm confidant this will get the ball rolling.)
Edited 6/29/2005 12:18 pm ET by TMO
The reason you can't find a lot of info on these is that no one wants the liability for designing them.
And why should they? If there's no income from the designing of the beam, there's no incentive to provide engineering.
You could always hire an engineer to design one for ya. But by the time you hired an engineer and spent all that time and money, it would be cheaper and faster to just buy an LVL or glulam.
Boss,
I posed a question similar to this a few weeks ago and got few responses. I was hoping to get one from you so I'll ask here.
I'm doing a front porch shed roof that will have a beam on the posts w/ about a 20' span. I've come to the conclusion that engineered lumber is the way to go.
The main question I had was, are LVLs or glulams ok in such an application? The roof of the porch will certainly protect the beam from direct water but since this isn't conditioned space are there caveats to the use of such a beam?
I remember that thread. I thought your question was answered well enough that I didn't respond. Basically, the engineered beams have to be protected from the weather. As long as they're wrapped in weatherproof material, I don't see a problem with it.
Babies don't need a vacation, but I still see them at the beach... it pisses me off! I'll go over to a little baby and say "What are you doing here? You haven't worked a day in your life!"
Thanks for the response. I just wanted to make sure as I value your opinion.
If you are talking about a 20' clear span, you are gonna need some HUGE engineered lumber, or you need to use steel. There is no intermediate support?
Re exposure, there are exterior LVLs, etc, but a more standard strategy would be to just to protect the bean from weather exposure by using standard boxing type methods.
Matt
Thanks for your response, however, I asked for a source of information. If there is no one that knows where to send me that's ok as well. I knew I could contact a design engineer and pay for his services. Liabilities aside, Where does he get his information.
Fishing is fishing, you can look like an idiot with an alder pole, string, bobber, hook and worm or with all the latest gear. Its the catching that isolates the idiots.
"I asked for a source of information....Liabilities aside, Where does he get his information."
I tell you this at the risk of sounding smart-a$$ed.
An engineer would refer to the latest version of the NDS (National Design Specification for wood construction). It gives specific guidelines for how to calculate the dowel bearing strength of nailed and/or bolted connections, etc. Last I knew a copy of the NDS cost around $300.
Then you'd have to study it for a while, because it's incredibly tedious and hard to follow. You'd have to be really good at math to understand the formulas they use.
After that, you'd have to get the design values of the lumber and plywood you wanted to use.
Then hopefully you'd come up with a solution that was correct, assuming that you understood everything and didn't make any mistakes or overlook anything.
So that's why I said you might as well just buy an engineered beam. Trying to learn to calc one isn't really practical.
A fine is a tax for doing something wrong. A tax is a fine for doing something right.
It looks like it is $50.http://www.forestprod.org/awc/index.html#t01-05%2Bt02-05And it looks like you can download the previous edition for free.http://www.awc.org/Standards/ndscommentary.html
I have a book called "Wood Technology in the Design of Structures" by Robert J. Hoyle, Jr. which has a chapter on the design of lumber-plywood box beams. I can remember puzzling over this twenty five years ago and gained enough confidence to overbuild by a factor of two a couple of these beams. I know that both beams have held up well but I can't imagine doing this again. It was not easy or cheap and was a huge liability. But of course I was young and foolish.
I don't know if you can still get this book but it is filled with a lot of fascinating information if you have the background or the time to try to understand it.
I could be wrong, but everything I've ever heard indicates that the plywood is a non-factor in determining beam loading. Its sole purpose it to pack out 2X lumber to convenient dimensions.
That's the way I understand it anyway. I'd bet that the ply adds a wee bit of strength to the beam, but I wouldn't even bother factoring it in.
Actually, a piece of plywood is itself a beam. Rip a strip of, say, 3/4" ply to 9" and place it on edge between to points. You can stand on it. Add another and laminate them. You AND a fat buddy can stand on it. Et cetera.A built up beam can be equally strong, or stronger than a solid beam of lumber (of the same overall dimensions). Ergo, two pieces of dimensional lumber sandwiching a piece of ply is stronger than the two pieces of lumber alone. Of course. And possibly much stronger, depending on the materials.
Toad, not to be smart, but I'm aware of the fact that plywood has some strength and therefore adds some strength to a built up beam. Even without finding a fat buddy to stand on some scraps with me.
But there's a big difference between me and my fat buddy, and, for instance, a garage header carrying a floor system as well as a roof load somewhere in snow country.
I believe the original poster is trying to find what the actual design value is for the plywood in a beam and I don't think it's really a scientific thing. Too much inconsistency in the manufacturing of plywood, I'm guessing. 3 ply vs 4 ply vs voids etc. Like I said, I'm sure it adds some strength to a beam, but I'm not sure it's measureable.
Further, if he's trying to engineer something himself, don't you think he's better off using span charts without the plywood factored in and using the plywood anyway as a belt and suspenders approach? I'm guessing that there may be a good reason why he can't find any span charts that factor in the plywood. Not that they don't exist.
what the actual design value is for the plywood in a beam and I don't think it's really a scientific thing
Yep, that's my experience/understanding, too.
Even with PL between the parts, what is the "adhesion" of the parts; how uniform is the adhesive; how thoroughly did the adhesive "set," etc. is what has seemed to "kill" field-made beams.
The liability of it drives it, too--just what is "stitch nailing," how many "bent overs" are you allowed, etc. Much simpler to get the factory handbook or table for a gluelam or lvl.
Now, as to OP's original question--I don't know if there's a single (or even any) good source. As previously pointed out too, the application matters. A pair of 2x10 can be just fine for spanning 10' on a porch. Can be, that's dangerously undersized, too--the details matter.Occupational hazard of my occupation not being around (sorry Bubba)
>> Actually, a piece of plywood is itself a beam. Rip a strip of, say, 3/4" ply to 9" and place it on edge between to points. You can stand on it. Add another and laminate them. You AND a fat buddy can stand on it. Et cetera.. << Oh yea, that's right - that's already been invented - it's called an LVL. BTW - don't listen to desilpig - he is just a framer that builds beams and other stuff like houses for a living. You do make a good point though; can I hire you to convince a building inspector or 2 that making my own LVLs is acceptable and also I need some backing on some other equally good, (and valid) theories? Your services will be required right after the BI has written out the "paying paper". If you can't be immediately available I'll just stick to accepted methods and standard materials.
Or maybe this web site name should just be changed to "Fine Virtual Home Building". :-) Or, how about "Fine Theoretical Home Building".
Also, how about filling out your profile? - you have been around for a while and we'd like to have an idea of who you are. Matt
Hi, Dirish. Actually, I was going to respond to Diesel's "I'm aware of the fact that plywood has some strength and therefore adds some strength to a built up beam" with ..'That's AKA an LVL.' But you beat me to it.It wasn't clear to me that the original poster was doing this for his own edification or for a customer. But my advice to him for a customer would have been to go by the book. No need to have his arse on the line.Me, I've experimented w/ built-up beams, and plenty of other unconventional construction techniques. More than most schooled engineers, I've no doubt. And I've studyied plenty the technologies of construction. Not that what engineers believe and practice is wrong--just incomplete. Probably Deisel has more skill and insight in designing beams than the average papered engineer. But in answer to your question, no, you may not hire me to convince a guv'mint building inspector that yours/my LVLs are acceptable..for the same reason that you will get nowhere using my expertise/opinions to contradict the establishment on even more pressing issues..Like why the hell does anyone need them in the first place? Anyway, sorry about my profile. But I bet you already know at least as much about me as I do you from your "profile".. I.e., I'm a male. I'm in construction. And, oh yea, I'm in Florida, along with a million others here in construction. Does that help?
I think I'm having an off day... I guess the point I was trying to make is that we gotta stick with stupid stuff like codes. I guess we agree on that. Theories are nice, but not necessarily applicable where "the rubber meets the road". Seriously though, I do have some more respect for you now that you have stated that you have actul professional expierence. There are some admitidly very smart people here who havn't had the actual expierence to build much more than a shed or a deck - from what I gather... Also, I know building codes (and the weather) in FLA require you all to build some seriously sturdy stuff. The area I build in is in a 100 MPH wind zone - I bet you are way over that. Matt
Well, alright Matt..you may just be having an off day. But I'm a perpetual smartazz. 'Kay?Anywho, you certainly don't "know" I have professional experience. I've only said I have. I could be making it up. All you know is I seem to talk the talk. That's what earned your respect. That was enough.As it should be.My point: WHat's written in span tables and official guv'mint codes and ordinances (and engineering textbooks, for that matter) is only worth the paper it's written on. Unless it can be backed up w/ real world experience. And unless it's practical. Most ordinances are neither. So a practical person can safely ignore them. But for the sake of appearances, yes, we in the trades must pretend that the rulers who make the codes know best. Just like we, grown men, must ask for permission just to make a decent living, and pay them a FAT tribute on our earnings, thereafter. We do it for appearances, not because we believe it is 'right' or moral. In fact, we do it all the while knowing it is absurd.But I openly object to their absurdities. That's how I manage to live with myself afterall.And, Matt, I also live in a 100mph wind zone. But the minimum guv'mint requirements for construction here are actually LESS than what I would normally do in any wind zone. But then I'm just practical like that.
Ive got a customer that just built three curved laminated beams. He built a form on the floor and then glued and screwed bc plywood to fit. The ispector wont let the entire floor bear on it. But he will let whatever portion isnt ale to be supported by an lvl bem right behind it be carried. Its really cool looking. Ill try to get some pictures.
I'd like to see a pic....
Matt
Humm... I don't know... really, I'm OK with about 90% of what is in our code book - a lot of it is a bit overdone, but that's OK. Some is under done. I fill in those gaps where practicle. And I like to pass all my inspections - so it's a bit more than just for appearence.
>> Anywho, you certainly don't "know" I have professional experience. I've only said I have. I could be making it up. All you know is I seem to talk the talk. That's what earned your respect. That was enough. << I pretty much try to give people the benefit of a doubt until they give me a good reason to think otherwise. For now I'm gonna take you at face value. On the other hand, some of the crackpot stuff I've read around here (other threads) caused me to remember some guys names well...
Matt
I remember something in FHB many years ago. Guy built a header out of 2x3's w/ 1/2" plywood glued and screwed to both sides. Like a little wall 10" high.Have you seen the Black Dog's eyes
Staring in the fire?
Mike, think that's abox beam. They are covered in the IRC, couple of pages & I think the max opening is 6' or maybe 8'.
Joe H
A number of years ago we had quite a few buildings come down under the snow load- Not the old ones, mind you, that were built 50 or 60 years before the imposition of building codes- These were, among others, two fairly new schools and a bunch of metal pole buildings (engineered and stamped)- I imagine Matt would respect the professionals behind these failures- After all, they have experience in the real world-
Dave, I earned near an engineering degree in my college days--I know what the professionals are taught by the establishment. I've read the literature. I also study and observe in the "real world" just like the papered professionals. I'd wager I'm equally observant or more so than the average code inspector or private engineer. I can tell you, a good majority of these guys are, in fact, idiots.Professional, allegedly educated, guv'mint licensed, well-paid hacks. And I've seen hack jobs done by Harry Homeowners (like roof-carrying headers built like a 2x4 knee wall w/ the top plates split directly under a truss)..They sag, droop, crush windows and doors below them, but still stand. That's what I don't get. The ignorantly framed tree house lasts a hundred years but the fantastically engineered tower free-falls after a smallish fire smolders at the top floors. Know what I mean, Vern?
Now you have got me wondering... exactly what do you do for a living?
Matt
You mean, what haven't I done for a living..?
As my grandad used to say, "Thank god I'M pure"-
The engineers working on big projects typically have a strict budget that they have to stay within and they have to find every way they can to stay within the budget and still meet code.Many home owner amateurs just stick a bigger piece of wood in - just in case it needs it. It doesn't always meet code, it often isn't pretty, but it usually lasts.My ex-girlfriend often said that I coudn't build anything straight or level if my life depended on it, but that what I built was the best place to be in a storm.
That's got to be the biggest crock of BS that's ever been posted here. Engineers use tested and widely approved standards to design buildings. They don't just throw a piece of wood up and say "looks good to me". I've seen many, many pathetically designed and built buildings done by DIYs. I've yet to see a poorly designed building done by an engineer.
Ninety percent of politics is deciding who to blame. [Meg Greenfield]
I didn't say that they were poorly designed, I said they were designed to a strict budget - there is a big difference.I am an engineer and I know what budget constraints mean to a project. Budget often forces me to leave out things that I consider important, but they aren't necessary to meet the requirements, so they are thrown out.There are structures that were built long before building codes came along that are still standing. There are also some well engineered buildings that haven't survived.A well engineered building is likely to survive. The ones put up by the DIY are an unknown. Some will fail with the first storm to come along, some will withstand more than the well engineered building.No crock or BS involved.
Your previous post didn't say a DIY was an unknown. It just slammed engineers and made it sound like DIYs weere the ones who were doing things right.You also implied that engineers cut corners in order to meet budget. I find the opposite to be true - That they tend to OVER-design due to liability laws. You also implied that if you "just stick a bigger piece of wood in" that means things are over-designed. If you really are an engineer, you know that isn't true.Your name wouldn't be George, would it?
If you want an army to fight and risk death, you've got to get up there and lead it. An Army is like spaghetti. You can't push a piece of spaghetti; you've got to pull it. [Gen. George Patton, Jr.]
No - my name isn't George - grin!I'm sure that there are engineers that are concerned about liability. In my experience, engineers - including myself - tend to over design because that is the just the way it should be done.Just sticking a bigger piece of wood in doesn't mean that it is overdesigned - if the starting point was good building practice then it was more overbuilt than overdesigned. I built my own woodshop - no code requirements in this part of the county. I put hurrican clips in because they should be there - even if code had applied they wouldn't be required. I thought that 3/8" plywood was too flimsy for the roof and 5/8" was heavier than I wanted to lift by myself. So, the roof decking is two layers of 3/8" plywood at right angles. I built the shop for me and liability had nothing to do with it - grin!The only drawings for the shop consisted of some pencil drawings that I used as a starting point and modified as I went along.
Why bother trying to talk to "Boss Hog"? It's like trying to have a conversation with a fog horn-
Get up on the wrong side of bed today? Or is there some legitimate reason you have your panties in a bunch?
Are you always an idiot, or just when I'm around?
Like I said-
So far you haven't said much of anything. If you don't like my posts, put me on your ignore list. But spare me the childish comments.Or at least tell me what the problem is.
The only way to win a war is to attack and keep on attacking and after you have done that keep attacking some more. [Gen. George S. Patton, Jr.]
Now boys.
You know what they say about wrestling with hogs...
pathetically designed and built buildings done by DIYs
Ron, there you go, talking about me DIY projects again, and here me be an deagead injenear two!
You gotta take the "designed' out of the quote though, there werent no design to it, jes' slapped up.
View Image
I think you should make up a calendar for sale with pics of your sheds etc. There is a certain primal beauty to them, especially with the moss and large scale logs etc.
Thanks, best pat on the back all day today!
Read your post 59971.36- If you can't see the reason for my post, I won't be able to explain it to you-
So you're calling me a foghorn just because you don't like one post?Like I said, put me on your ignore list. But get off my a$$.
I'd kick your a$$ but this is my best dress.
<<< "...me on your ignore list. But get off my a$$...">>>
I posted to someone else when you took issue with me- Take your own advice if you don't like my posts and ignore them-
<<< "...I'd kick your a$$ but this is my best dress..." >>>
Very adult, very grown up indeed- Blow it out your horn-
Ok, now just like my dad used to say...If I have to pull this thread over, I'm going to slap you both silly.Of course, right now that wouldn't take much slapping !LOLDave, Boss' tagline changes with every post he makes. Besides, I would have taken it as a defferential funny. A sort of extended hand, not an insult.I'm warning youse guys... Don't MAKE me come over there...=0)
Are we there yet ?
Have you updated your forum profile lately? Please Do!
Neither of you guys want Jeff to come over there, I think the COD airfare would be a killer.
Gee wiz, Art. You could at least have told them what a mean sumbeech I am when they get me angry.; )
Are we there yet ?
Have you updated your forum profile lately? Please Do!
You posted to someone else ABOUT me. That's why I responded. Anyone is free to respond to any post they want.My tag line changes with every post because that's my trademark. If you don't like them, don't read them.Do you plan to take the time to tell me why you're on my case, or just continue with the childish BS?
We have enough youth, how about a fountain of smart?
Let it go Boss... you're better than that. In fact, the way you carry yourself around the forum serves as a positive example for the rest of us. I re-read the whole thread again and really can't understand why he's got a beef with you. Just a bug up the azz I guess.
There's a heck of a lot of difference between the average house and a tall building. There aren't many things that can fail catastrophically in a standard sized house, but many, many in a tall building (or any large structure such as an indoor sports arena).In a house, if one rafter fails the roof will sag a little. Let a rafter fail in a sports arena and the entire roof can come in.Why can't they build large buildings the same way they build houses? Well, scale up a rafter that spans maybe 15 feet to span, instead, 150 feet. First off, you've got the Sierra Club on your tail for chopping down California redwoods to get logs that big, then you've got to figure out how to get them in place, since they weight about ten times what the equivalent steel would weigh.It's the nature of the beast that as you scale up in size the "robustness" of the structure diminishes. It's kind of an entropy problem, and engineers must fight hard to counter it in large structures.
>> common practice as is making up a beam by making up a sandwich of plywood and glue between layers of dimension stock. << Oh it is? I didn't know that, but I'm just a builder... You talkin super glue or what? BTW - We call those "headers" and the plywood is just a spacer to make the member the same thichness as the wall - No glue though. >> Does anyone have a source? << Yea, go to college and get an engineering degree and then pass the PE examination. Until you have done that either use standard methods that are supported by manafacturer info, span tables, etc, or hire an engineer to do the calcs for you and assume the responsibility.
Doug,
Here is the answer to your question taken from a Q&A in JLC.The Journal of Light Construction is in my opinion in the top three resources for residential building. As a framer, sider, foundation guy, generally ridiculously cool guy (:-) ) it has been the biggest help to me. I'm almost a professional now :-) If you don't subscribe, consider it. Visit the forum, just about everything I know I learned there, the rest was from making mistakes. The magazine is much better (from a technial standpoint) than FHB. The website is http://www.jlconline.com The Q&A is free, just go to the research tab.
I hope this helps.
Plywood in Built-Up Headers
Q. Do the layers of plywood in typical built-up headers add significant strength to the header?
A. The most important thing the plywood adds is thickness. Of course, the plywood does add some strength, but for several reasons engineers almost never count on this strength in their designs.
Only the layers of plywood with the grain oriented horizontally (parallel with the direction of the header) are really adding any strength. A quick look at the thicknesses involved shows that the additional strength is small. If half the layers in 1/2-inch plywood are horizontal, that’s 1/4 inch of extra material. Compared with 3 inches of 2x10, that’s an increase of only 8%. What’s more, you only get the full effect of this extra thickness if there are no splices in the plywood near the middle of the span, or better yet, no splices at all. For headers at openings wider than 8 feet, that’s not often the case. But it’s these longer headers that will most likely need some extra strength.
Combine these drawbacks with size limitations and the plywood almost never makes a critical difference in safety. What I mean by size limitations is that when I design a header, the numbers may tell me I need two 2x9s. Since two 2x9s are about 30% stronger than two 2x8s, the 2x8s plus 8% from 1/2 inch of plywood wouldn’t be strong enough. And I wouldn’t ask the framer to rip some 2x9s, I’d simply call for 2x10s. What’s more, he’ll probably use double 2x10s for all his headers, big and small. Because headers only come in certain depths, there’s usually extra strength in the 2x10s to begin with. And that extra strength in the 2x10s means that the small extra strength from the plywood is rarely important. But the thickness is helpful.
—C.D.
Tim,
Thanks for clearing that up.
But what the fugg happened to your font back there? It looks like the script written on the back of Frodo's ring! Only smaller.
;)
You see that thread about my latest catastrophe? The thread is titled "Uh-oh" and I think I put it in General Discussion. Poor concrete guy.... at least us framers have a sawzall in the trailer somewhere!