DIY Laminated Plywood Beam

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I just know somebody here has experience . . .
A friend wants an arbor over her patio for vines to climb on. We envision an open structure with four posts supporting two main crossbeams. These crossbeams would support "rafters" on two foot centers. One of the rafters will carry a porch swing, but that rafter is near the posts, not near the center of the span. No other significant loads other than the weight of the vines that will eventually cover the rafters. Main crossbeams would be 20 feet long, with 18 feet of clear span between the posts. Rafters will be 12 feet long, with 8 feet of clear span and 2 feet of overhang on each end.
My plan is to buy 3/4 plywood intended for sub-floor and cut it into strips 7 7/8" wide. These strips will glue and screw together with Titebond II, and the screws will serve as clamps until the glue dries. I'll stagger the joints by using 2, 4, 6 and 8 foot strips.
I plan to protect the lamintate beam from the weather by enclosing it in a 3-sided box of pressure-treated lumber.
Here's the question. How thick does the laminated beam need to be?
DonkeyHody "Every man is my superior in that I can learn from him" - Thomas Carlyle
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Sun, Jul 1, 2007, 5:33pm (EDT-3) snipped-for-privacy@bellsouth.net (DonkeyHody) did query: I just know somebody here has experience . . . A friend wants an arbor over her patio <snip> One of the rafters will carry a porch swing, <snip> My plan is to buy 3/4 plywood intended for sub-floor and cut it into strips 7 7/8" wide. These strips will glue and screw together with Titebond II, and the screws will serve as clamps until the glue dries.<snip> I plan to protect the lamintate beam from the weather by enclosing it in a 3-sided box of pressure-treated lumber. Here's the question. How thick does the laminated beam need to be?
Yep, got plenty of experience. Whatcha want to know about?
But if you put the swing there, and later find out that's a bad place, it's gonna be a bitch moving it. I'd rather have a stand-alone swng, just in case.
Why screws? You figuring on taking it apart later? I'd nail it.
How you planning on lifting those beams in place?
JOAT If a man does his best, what else is there? - General George S. Patton
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The screws are to draw the layers of plywood together, each acting as a small clamp. To lift it, I thought I'd stand one of the rafter boards on end and screw it alongside the post. Put an eye-bolt at the top and use a block and tackle.
DonkeyHody "In theory, theory and practice are the same, but in practice, they are not."
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I'd be more inclined to use WEST System epoxy and clamps...forget about screws.
I'm sure Lew can add to my suggestion.
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Robatoy wrote:

I would not touch the above with a fork.
Even with epoxy, plywood is a lousy choice for this application.
With an 18 ft unsupported span, I'd sister a couple of 2x12 Doug Fir timbers together with epoxy and clamps.
Faster, cheaper, better.
3 for 3 isn't bad.
Lew
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Thanks to all for the advice. I'm re-thinking the plywood idea. The reason I considered laminates in the first place is that 20 foot lumber in any size bigger than 2X6 is pretty scarce around here. I'll price a factory glue-lam.
DonkeyHody "Every man is my superior in that I can learn from him." - Thomas Carlyle
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DonkeyHody wrote:
> Thanks to all for the advice. I'm re-thinking the plywood idea. The > reason I considered laminates in the first place is that 20 foot > lumber in any size bigger than 2X6 is pretty scarce around here. I'll > price a factory glue-lam.
SFWIW, right after WWII, materials were still difficult to get and expensive.
Some how, my dad managed to collect some kind of pallet that used 4x4 runners, about 12 ft long, that were probably "Ironwood" or something similar that required drilling pilot holes for every nail.
He used those runners for posts, then ran #9 galvanized baling wire overhead to provide a place for the vines.
That puppy lasted at least 20 years.
Might be an idea or two in there that you might find useful.
Lew
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"Don",
As an Architect, maybe I can add a little insight...
I would question the use of 'multiple' plywood laminations if for no other reason, the material strengths are indeterminate (the two main things needed for a rational structural design is 1) knowing accurately the loads one is to carry and 2) knowing accurately the ability of the material to carry those loads). While the wood generally used for plywood would have known properties, remember that plywood is generally used in an application (as in decking) that loads it in the plane of the laminations and in your case, you are loading it perpendicular to that. Couple that with the gaps that sometimes occurs in interior laminations (which is more critical in your application) and much uncertainty enters the equation - exactly what you don't want if a rational structural design is the goal.
Don't get me wrong, we're not talking some major critical issue here (it's not holding up a second floor of a house, etc) - but, it seems reasonable to not get into areas that a trained structural engineer would avoid, no? There are better ways, anyway.
One basic thing to remember in structural members is the *depth* of a member -- as this adds rigidity far more than width (i.e. a 2x12 is far better than a 3x8 or 4x6 (all with the same cross-sectional area)). It's a 'moment of inertia' (Ix) thing (you can google around -- or just remember that the 'Ix' goes up with the square of the DEPTH and you can see the effect). That is - provided the width doesn't get too slender such that the beam can flex in the horizontal axis (as it will) - but that is what the rafters above will provide -- 'lateral support'.
Remember that a beam under load has tension stress in the bottom and compressive stress at the top and you can see that the wood fibers need to be oriented horizontally (which every other lamination in plywood would defeat that purpose - since the fibers would be oriented incorrectly) and you can see how solid, sawn wood would serve your purposes better.
I believe Lew mentioned a few 2x12s and I would agree that something like this would be a smarter solution. Also, the American Plywood Association (www.apawood.org) has some pretty good tech reports on built up beams -- and don't get me (us) wrong - one *can* build up beams with plywood, but it is generally done with a plywood panel sandwiched between sawn lumber (or, two panels in layers 2 and 4 of a 5 member sandwich).
Then again, we also (now) have LVL (Laminated Veneer Lumber), which is just a big hunk of plywood (but with the laminations oriented correctly and with documented strength values) or LSL (Laminated Strand Lumber (similar). These are (generally) about twice as strong, size for size, as solid sawn lumber but are, of course, more expensive. Being an exterior application, you *should* use pressure- treated or naturally decay resistant wood. I would not use a LVL or LSL without capping the top surface with a conventional 1x pressure treated piece - just to keep water from working it's way between the laminations (or, cap it with some copper sheet - would make a nice design element -- just paint the covered surfaces with some oil-based paint and maybe use a few copper washers to space the sheet slightly away from the wood surface for ventilation (letting it dry between rains) -- but, then, this begins to complicate things more than it should be. Simpler is to simply use multiple 2x solid sawn members (and, maybe, that exterior plywood inner lamination) and cap with a 1x6 pressure-treated 'rain cap'.
Loading? There wouldn't be much - fundamentally the dead load of the materials and vines, etc. It is possible that wind-loading could be your greatest load - especially if covered completely with vines. I would double the rafters where it carries the swing and run your bolts from the top of the rafters (vs eye-bolts screwed into the bottom of the rafter) -- there are 'edge distance' issues with bolted connections and driving a lag-type eye-bolt into a 1.5" member (nominal 2x width) maybe cutting things a little close. Better to use a conventional threaded (i.e. machine threads) eye-bolt hanging from the top (between the two 2x rafters) - which would load the assembly in compression and not tension (i.e. pullout) - which the conventional lag-screw type eye-bolt will do. Since running a bolt right thru the interface between the 2 2x's might could cause delamination over time, smarter to run the bolt thru a smallish 'cap piece' on top of the 2-2x's (and, with a little overhang, this could be a decorative piece, as well) and this would not cause delamination.
That's probably enough to get some thinking going...
-- john.
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John, You've given me plenty to think about. While I can't argue with your structural analysis concerning the orientation of the wood fibers in plywood, my experience has been that plywood had less deflection when loaded "edgewise" than solid wood of equal dimensions. I deduced (perhaps incorrectly) that the advantages any grain defect being so thin and localized offset the obvious disadvantage of half the layers being oriented in the wrong direction. Or maybe I was simply wrong about it being stronger in the first place. Good food for thought in any case.
I failed to mention one of the reasons I was considering building my own beams. For esthetic reasons, we wanted the elevation of the rafters to equal the facia board on the house, even though they won't be attached to the facia. The house is slab-on-grade, with long eaves yeilding a fairly low roofline. While I recognize the strength advantage of a "tall" beam, headroom under it becomes an issue pretty fast. So . . . I was looking to substitute some width (got plenty of room for that) for some height. In other words, I wanted to make the beam wider instead of taller, while realizing the diminishing returns involved in doing so. I understand the tradeoffs, but I can't calculate them. All the tables I can find assume loads far higher than these beams will ever see. Maybe it's time to abandon wood and go to a steel beam. We could wrap it in pressure treated wood to hide it.
Thanks again for taking the time for such a detailed and thought- provoking answer.
DonkeyHody "In theory, theory and practice are the same, but in practice, they are not."
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DonkeyHody wrote:
> advantage of a "tall" beam, headroom under it becomes an issue pretty > fast. So . . . I was looking to substitute some width (got plenty of > room for that) for some height. In other words, I wanted to make the > beam wider instead of taller, while realizing the diminishing returns > involved in doing so.
Pretty straight forward, you can't get there from here.
The key issue is "Moment of Inertia".
The formula to compute "Moment of Inertia" for a rectangle:
I=(b)*(h^3)/12 where b = base dimension & h = height dimension.
As you can see, "Moment of Inertia" is only changed linearly with a change in width while the "Moment of Inertia" is changed by the cube of the height dimension.
"Moment of Inertia" explains why wide flange and I-beams have the shapes they do.
Greatest strength for the least amount of material.
Perhaps an example:
Assume: b=1, h=1 I=(1)*(1^3)/12=1/12
Next assume: b=2, h=1 I=(2)*(1^3)/12=2/12
Finally assume: b=1, h=2 I=(1)*(2^3)/12=8/12
Tall skinny beams win every time.
Lew
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I guess it's a little late in the conversation to admit that I have an engineering degree. I understand the concepts well enough, but I don't have the resources to calculate the amount of deflection I can expect. Note that my load is lighter than usual and my amount of allowable deflection is greater than usual. In this case, if the deflection isn't noticable to a casual observer, it's acceptable, since there's no drywall to crack or doors to bind etc. etc. I was hoping somebody would chime in and say, "Oh, I've done that lots of times. Make your beam 6 inches wide and you'll be fine."
I've about come to the conclusion that, while I CAN get there from here, the young couple's budget won't support it. So we'll probably just add a center post, which will simplify life considerably, even though it's not an elegant solution.
DonkeyHody "In theory, theory and practice are the same, but in practice, they are not."
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DonkeyHody wrote:
> I guess it's a little late in the conversation to admit that I have an > engineering degree. I understand the concepts well enough, but I > don't have the resources to calculate the amount of deflection I can > expect.
Beam deflection calculations are a bitch.
Never bothered making them.
Just de-rated the max allowable stress by a factor of 4 and got on with life when doing bending stress calculations.
Lew
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Lew Hodgett wrote:
| I=(b)*(h^3)/12 where b = base dimension & h = height dimension. | | As you can see, "Moment of Inertia" is only changed linearly with a | change in width while the "Moment of Inertia" is changed by the cube | of the height dimension. | | "Moment of Inertia" explains why wide flange and I-beams have the | shapes they do. | | Greatest strength for the least amount of material.
<examples snipped>
Interesting!
A bit of playing with my calculator has me wondering why not use 1x stock for rafters and joists, or at least _some_ rafters and joists? It would appear that if I resawed a 2x rafter and edge glued the two halves, then the resulting rafter could carry a greater load.
What am I missing?
-- Morris Dovey DeSoto Solar DeSoto, Iowa USA http://www.iedu.com/DeSoto /
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Not much....
You would need aditional blocking to account for the loss in lateral stability. You would also have a tough accurately screwing two edges of a subfloor (plywood) onto a 1-by.
-Steve
--
Posted via a free Usenet account from http://www.teranews.com


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Stephen M wrote: || Interesting! || || A bit of playing with my calculator has me wondering why not use 1x || stock for rafters and joists, or at least _some_ rafters and || joists? It would appear that if I resawed a 2x rafter and edge || glued the two halves, then the resulting rafter could carry a || greater load. || || What am I missing? | | | Not much.... | | You would need aditional blocking to account for the loss in lateral | stability. You would also have a tough accurately screwing two | edges of a subfloor (plywood) onto a 1-by.
Thanks, Steve. I'm obviously not an architect/builder guy :-)
Last week I worked out a gambrel roof design for a friend's garden shed (which started me thinking about offering these things in kit form). The 2x4 rafter segments looked like overkill for the 8' span, and now I'm thinking that 2x4 is only needed where sheets of the ply meet, and that the 'between' rafter segments probably could be 1x4.
CNC drilling pilot holes in sheathing and rafters wouldn't be a difficult - which should make it fairly easy to accurately screw plywood to rafters.
I'd not try using 1x in a larger structure because I suspect structural integrity would vanish fairly rapidly if there were a fire...
-- Morris Dovey DeSoto Solar DeSoto, Iowa USA http://www.iedu.com/DeSoto /
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"Morris Dovey" wrote

That is an interesting idea, to predrill the holes in both the sheathing and rafters. I wonder how well that would work out in the real world. I suspect that there may be problems making things fit.
What a friend of mine did when he built things like this was to predrill the (countersunk) holes in the sheathing. He would put the sheathing in place and drill the pilot hole with a yankee drill driver. Ya know, a drill that works like a yankee screwdriver. It drills holes instead. This worked well for him.
He would then drive the screws with his cordless screw driver.
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Lee Michaels wrote: | "Morris Dovey" wrote
|| CNC drilling pilot holes in sheathing and rafters wouldn't be a || difficult - which should make it fairly easy to accurately screw || plywood to rafters. | | That is an interesting idea, to predrill the holes in both the | sheathing and rafters. I wonder how well that would work out in | the real world. I suspect that there may be problems making things | fit.
I do this type of thing fairly routinely with solar panel production and it works well for constructs up to 12' (I haven't used the CNC for anything larger) so long as moisture contents don't change much - with "much" being dependent on both materials and required precision of fit. I suspect that as long as the holes line up with about 1/32", there shouldn't be a problem in a framing/sheathing/subflooring context.
| What a friend of mine did when he built things like this was to | predrill the (countersunk) holes in the sheathing. He would put the | sheathing in place and drill the pilot hole with a yankee drill | driver. Ya know, a drill that works like a yankee screwdriver. It | drills holes instead. This worked well for him. | | He would then drive the screws with his cordless screw driver.
I have the large Yankee driver, which came with both drill and screw bits - and I've done what your friend did. It works, but these days I'd be inclined to do the drilling with a tailed drill and use a cordless drill (because of the clutch) to do the driving.
I keep the Yankee and a brace (currently fitted out with a #2 square bit) as backup - been meaning to build an "In case of emergency break glass" case for 'em. :-)
-- Morris Dovey DeSoto Solar DeSoto, Iowa USA http://www.iedu.com/DeSoto /
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I have three cordless screwdrivers. One is a Klein, another is a Stanley, and the third is a Dewalt 14.4v drill. I have a phillips bit (from Lee Valley) in the Stanley. :-) I also have chrome Yankee drill (like the telephone repairmen used). I was able to buy new old stock bits for it. I do use them all.
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I hope the original poster sees his original idea is a bit nutty. I don't think OSB interior flooring would hold up outside, and the orientation of the core does not appear add desired strength.
My buddy and I (who are not engineers) were re-doing his one story home and we got into taking out a supporting wall and remaking it. We made a support beam out of 3/4" plywood sides with 2x4's in the middle, and we used construction adhesive and framing nails from a gun to build it. It was about 18" tall and the length was only about 11' long at the most. The SOB was total overkill just to hold up the roof, it was very very strong with the plywood glued in like that - vertically. I told him if there is every a chance of a tornado or his large trees in his yard falling on the house, go stand under that SOB and you'll be fine.
Outdoors? I think the suggested dimensional lumber make a lot of sense.
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But it just so happens that I have a 24 foot beam laminated from 3/4" plywood holding up the porch of my shop right now. It has 16 feet of clear span with 4 feet of cantilever on each end. The beam itself is only 6 inches tall by 5 inches thick. That headroom thing again. It's painted with waterbase house paint and protected by the roof, but it's only 6 inches from the edge. I built it before I had access to the internet and all the sage advice I can get here. I remember I had read about a new glue called Titebond II that was supposed to be waterproof, but it wasn't available locally, so I used regular Titebond. I didn't even have access to load tables, but it was only supporting one side of a 6 foot roof section, so I did some seat-of- the-pants engineering. It's "almost" strong enough. There's just a bit of sag in the middle, but you have to sight down the beam to see it. It's been there for 13 years so far. From time to time I consider adding a center post to take out the sag, but it's not noticable, and I really like having the clear span, so I haven't done it. I'm pretty sure the beam needed to be taller. But I've seen no sign of deterioration, and it hasn't sagged any more after the first year. There may be better approaches to the problem at hand, but I'm not quite ready to dismiss my original idea as "nutty".
DonkeyHody " We should be careful to get out of an experience only the wisdom that is in it - and stop there; lest we be like the cat that sits down on a hot stove-lid. She will never sit down on a hot stove-lid again---and that is well; but also she will never sit down on a cold one anymore." - Mark Twain
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