Building beams

[TisaWee Farm]

I understand that a built-up beam is layers of 2X with 1/2 plywood in the middle.   Can you use OSB instead?   Is the plywood structural, or just to make up the space?  Trying to figure out how to NOT have to use 6 sheets of expensive plywood!

CC

[MarkAndDebbie]

Don't know about the OSB. WHere I am I used PT plywood because of termites. OSB doesn't come PTed (that I know of). I have seen photos on this site that didn't run the plywood the whole way - just a spacer at the bracket. Youngins maybe?

[youngins]

I assume your talking about this:

[MountainDon]

I am curious about this myself.

I believe most of the strength in a built-up beam comes from the 2x material. Most, but not all. Looking at John's built-up beam primer   http://www.countryplans.com/builtupbeam.html    we see he uses plywood. In the example, a built-up 6x12 beam, it is stated "A three joist beam with 2x12's and two layers of 1/2" plywood as shown above will have roughly the same strength as a 6x12 beam of the same lumber and grade."  Deducing from this it would seem that the plywood must contribute something to the strength, unless the phrase "roughly" means about as strong as a tripled set of 2x's without plywood??  

Now, with all that said it still might be possible to build a built-up beam with OSB spacing as long as the 2x  dimensions were robust enough for the post/pier spacings.

Then again, what is the percentage saved over the entire project if OSB is subbed for the plywood in these beams?    0.0056% or what?


Here's another thought... Most of the strength in a beam or joist is derived from the upper and lower portions of the material. This is why steel beams are shaped like an " I " , there's more material there in the flanges. That's where the strength is. The web between keeps everything aligned.

Now back to the built-up wooden beam, the 6x12, in the example. Would using two 2 inch strips of plywood, one at the upper edge, one at the lower edge, in place of one 12 inch (11 1/4 actual) strip of plywood provide virtually the same strength? And would it be worth the extra effort for the dollars saved?


Waiting to hear from the experts.....

[John Raabe]

Mountain Don is right. The way to get the most strength with the least material is to have the plywood (or OSB) as a spacer at the top and bottom of the built up beam.

These are really home-made laminated beams and the plywood or OSB is more to flesh out the sizes to match standard hangers and brackets designed for solid timbers than they are to accumulate added strength. These layers also serve as continuity splices to weld together joints between the 2x material and allow site built beams of any desired length.

I would suggest that using plywood will be more stable if there will be weather exposure (and especially if you if want both materials to be PT). OSB would be fine, however in a dry environment such as a sealed crawlspace or interior beam.

In the 60's and 70's there was lots of information available on how to assemble built-up beams from 2x materials. Here's a rather beefy box beam that puts a lot of material at the top and bottom the same way an I-beam does.

[TisaWee Farm]

Still thinking outloud here.....

If a 2x4 is $2, and a 2X12 is $11,  could I do the following..... (you've got to picture this as a sandwich....)

2X12, 1/2 plywood (full length and width - for stability), 2X4's put at each edge with 4 inch gap in the middle (or whatever it works out to), 1/2 plywood again, and final 2X12.  

That'd save nearly a dollar per foot of beam.  In my case, $92.   I'm not sure it is worth that, though, if it compromises on the strength in any way.   Or is more time-consuming to create.

[John Raabe]

It's an interesting idea but I wouldn't do it for your 28' project. I would use either the 6x12 (or a glu-lam equivalent) as the best option, or, alternatively, the homebrew laminate beam shown in the Enchilada plans. Try to get any joints in the 2x stock to line up over the piers if you can and it will be plenty stiff enough for your cottage load.

[georgevacabin]

Hello All,

I'm at the "how do I make a built-up beam" stage.    I want to build the 14x24 so the beams will be 24ft.  Got that.  In pricing materials yesterday at Home Depot (I know, I know) I found that the best they could offer was a 4x4x12 vs. the 4x8x24 called for in the plans.  

Soooo, :

1. Do I sandwich the 4x4x12 together to create the beam?

2. How do I get the joints connecting the two 12 footers to line up over a pier?  In the plans it looks like the joint of two connected 12 footers would fall in between the 3rd and 4th pier.

I apologies if these are not the brightest of questions  :-/

[georgevacabin]

uhhh.  My apologies.

[MountainDon]

No need to apologize. Agreed, 24 foot long 4x8's may be difficult to find. For certain the big box stores won't have them. They could order you a pricey LVL beam I'm sure. You might be able to get/order a 4x8 x 24 ft long solid wood beam at a full service "real"lumber dealer. But then you have to deal with the long and somewhat heavy beam.

There's another approach.

The best built-up beams are made by laminating planks, side by side, wide dimension in the vertical plane,  rather than on top of one another. More on this below. In the case of a beam for the 14x24 Little House, using a 24 foot length,  here's a better solution. You should be able to get 2x8 material, 10 footers and 14 footers even at a big box. The 14 foot length might be difficult in some locales; then choose a 16 foot and cut off a piece. Sometimes you can't help but have some waste material.



This is not the only way to build a beam, just a suggestion that will work. You can play with different lengths. For example, If you need to cut a 6 foot length, try cutting a 12 footer... the other section might be usable in aother layer or the second beam.

In between the 2x8 sandwich you would use 1/2 inch plywood, or similar, primarily to space the load bearing 2x's to fit standard 4x construction brackets, post bases, caps, etc. Everything gets nailed together with the nails being placed near the edges of the planks. No need to nail in the center really.

Also see John's illustration of a built-up beam

http://www.countryplans.com/builtupbeam.html

As John states the joints should either fall directly over a support beam, or be no closer together than 4 feet in adjacent layers of the sandwich.

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If you, or anyone else, wants see why a side by side lamination has more strength, more stiffness than a vertically stacked beam try this. Get some balsa wood from a hobby store. Model aircraft builders use it. A 1/8" X 1/2" would be scale to a 2x8 (close enough for this purpose) A 1/4" X 1/4" balsa stick would be approx scale to a 4x4. For illustration purposes place two supports 10 - 12 inches apart. Place one 1/4 x 1/4 stick on top of another and press down with a finger. You'll see the upper and lower pieces slide a little against each other.

Now take two 1/8 x 1/2 planks and place them on edge in a sandwich. Pin or tape them together. Press down or place the same weight on them. They will be much stiffer, as long as the wood is similar.

If you pinned the two 1/4 sticks together that will stiffen up the balsa beam. However then you are relying upon the mechanical fastener shear strength rather than the long wood fibers. With good sized lumber and ample mechanical connectors couple with sufficient support posts such a beam would be usable. But IMO, the built-up beam using 2x materials is better.

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BTW, balsa wood has a very high strength to weight ratio. Pound for pound it is stronger in some ways than pine or even oak. The more dense (heavier) the balsa the stronger it is. Scale aircraft builders use different densities of balsa in different applications. Sheeting wing is done with the lightest balsa while the wing ribs or fuselage members would be more dense material. Model aircraft builders may also use clear spruce sticks for some applications. Aircraft quality plywood is like nothing you will ever see in a lumber yard, like 1/8" thick with 5 plies; 1/32" with 3 plies.

Balsa info for the curious    http://www.skyhooks.ca/balsa.htm
http://www.sigmfg.com/cgi-bin/dpsmart.exe/MainMenuFV4.html?E+Sig
https://www.balsastore.com/store/categories.php?cat=27

Yet another hobby, though one that I haven't actively participated in a number of years.

Sorry, another lengthy post with more info than asked for / needed.  :-?

[georgevacabin]

And the fog is lifting    Thanks MD!  

Now I get it (sometimes it takes a little longer).  I could not figure it out mainly because I was thinking you would use materials of the same length (2-12 footers for example).  Thank you for the side view in your drawing.  It really helped.

As I keep looking at the plans and trying to visualize this in my head I'm sure I'll have more questions.

You all are great and I'm really excited!  

[georgevacabin]

I found a local lumber yard that has 2x8x24 pt lumber!  I will use these with plywood spacers.  

[MountainDon]

Just be certain to use correct type of fasteners for the PT. Sounds great

[georgevacabin]

Thanks MD.  I think I'm going to go with plywood the length of the beam vs. spacers.  When you say fasteners are you referring to nails?

[MountainDon]

[highlight]Hot dipped galvanized nails or stainless steel nails[/highlight]. Spiral or ring shank if you can get them. Also with ACQ PT wood any metal brackets should used should be approved for contact with ACQ PT wood... a heavier zinc coating. Or use a layer of #30 felt between.

[georgevacabin]

Thanks MD!  I'm reading your post everywhere on the forum.  Thanks for being such a help and putting up with the newbee questions!