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socceronly
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# Posted: 25 Jan 2019 00:07
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I noticed a few posts ago someone mentioned keyed beams. Didn't find too much info.
I am curious, as a hypothetical question... what do you get if you stack a 6x6 on another 6x6 as a beam? Say you connected them with a bunch of timberlok screws or heavy bolts.
Is it just a mess or is it a stronger beam?
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socceronly
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# Posted: 25 Jan 2019 00:14
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Ha, I actually see Don P is in this thread 
http://forestryforum.com/board/index.php?topic=24971.0
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KinAlberta
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# Posted: 25 Jan 2019 22:53 - Edited by: KinAlberta
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Fascinating discussion!
In addition to bolts would modern glue (PL?) and/or double sided tooth timber connector plates (some bolt through) add much resistance to slippage?
For a quick to build 60-70’ atv bridge over some soft sometimes flooded ground I’ve wondered about stacking say 2 - 2x12s (for 22 1/2†depth) and then putting say 2x12s down either side to straddle the stacking seam to prevent slippage. (Yes, the diagonal boards would make far more sensebut that’s a lot of work!)
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ICC
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# Posted: 26 Jan 2019 00:42
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Construction adhesives have their usefulness. However, laminating lumber or timer as in making a stacked beam is not one of the best uses. Most retain some pliability, a little flex, which makes them great for things that might change with moisture level, humidity, and so on. I believe that is used to glue a stacked beamed there will be slippage longitudinally. As Don_P said in that link the two (or more) stacked timers/lumber will want to slip. Somewhat the way pages in a book slip when the book is bent.
A commercially produced glulam built up beam is made with special resins cured high pressure and high temperatures. They use selected machine stress graded lumbers. There are manufacturers of glulams that will make them with an arch design special for bridges.
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Fanman
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# Posted: 26 Jan 2019 09:26
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In theory, if you stack two beams that are somehow fastened together so they work as one solid beam, you get eight times the bending strength. Two beams not fastened together, you get two times the strength (just like two beams side by side, fastened or not).
If you're bolting them together, they act as one, but you have to deduct some strength for the holes, so more than 2X (probably) but less than 8X.
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socceronly
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# Posted: 27 Jan 2019 16:27 - Edited by: socceronly
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Quoting: Fanman In theory, if you stack two beams that are somehow fastened together so they work as one solid beam, you get eight times the bending strength. Two beams not fastened together, you get two times the strength (just like two beams side by side, fastened or not). If you're bolting them together, they act as one, but you have to deduct some strength for the holes, so more than 2X (probably) but less than 8X.
A few things are not clear here, in re reading DonP's post from the manual.
A lot of numbers getting thrown around, I would like to clarify for myself and others reading this in the future as people might make some dangerous assumptions from these numbers.
When you say 8x theoretical which supposition from the manual is that coming from? In re reading it I am not sure, so bear with me.
In the manual posted it said that the two beams would act 4x stronger than the original beam. So are you deriving 8x based on the two beams? Or was the manual referring to 4x the strength of single beam as thick as the two together?
When it subsequently uses % like 65%, what is that a % of exactly?
Single beam is 1
Double beam fastened is 4 (square of the depth rule?)
Now is the effective % a multiplier of what?
4 or 8 times the original beam? or a % of a solid beam of the resulting depth?
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Fanman
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# Posted: 27 Jan 2019 19:58
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Oops, sorry, I misspoke.
A beam twice the depth will have eight times the stiffness (resistance to deflection, i.e. it will deflect 1/8 as much), but only four times the breaking strength. That's not from any manual, but from the basic beam bending equations taught in engineering school.
The 65% mentioned in the website linked above is compared to a solid beam the same size as the composite beam.
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socceronly
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# Posted: 27 Jan 2019 20:22
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Thanks.
Is this correct?
2 6x6 = 65% of a 6x12 (or 75% if keyed with bolts)
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Fanman
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# Posted: 28 Jan 2019 17:57
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Quoting: socceronly Is this correct? 2 6x6 = 65% of a 6x12 (or 75% if keyed with bolts)
I couldn't say. 65% or 75% is presumably based on testing; without the actual data it's hard to say whether it would be true for your real world situation.
Theory only says that two unjoined 6x6's will be 50% as strong as a solid 6x12, and two perfectly joined 6x6's (like an engineered laminated beam) will be the same strength as the 6x12. The real world is somewhere in between, hence the 65 or 75%.
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KinAlberta
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# Posted: 13 Mar 2020 22:11 - Edited by: KinAlberta
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A year later I still I think this is a fascinating discussion. I can see a number of uses for relatively low cost but deep beams. Like my ATV bridge mentioned above. Or a cabin built on permafrost or otherwise poor ground.
A stacked beam of two or three 2x12s sufficiently locked together, thus making the beam 24†or 36†deep, should be able to span a good distance and hold a fair bit of weight.
I’d considered a DIY wood trust system but unless sheathed and sealed from the elements, rot would eventually destroy it. PT stacked beams would Seem to be so much simpler and safer.
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ICC
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# Posted: 13 Mar 2020 22:19 - Edited by: ICC
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One issue with beams that are very tall but narrow (stacked 2x12 as above) is they need to be held in place in a way to prevent bowing in the lateral direction as load is applied in a downward vertical direction.
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Brettny
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# Posted: 14 Mar 2020 07:04
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I'm in the same boat as you ICC. A 24in tall beam that s long enough to actually need to be 24in tall and only 4-6in wide is going to act as a wet noodle. Even a 16ft 2x6 twists a lot.
Once you start stacking lumber past 12in you should be looking at engineered lumber or a steel beam. Personally I find steel a lot easier to work with than wood. For instance you can weld two pieces back together at almost the same strength as it was originaly.
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SpyingOnMyKeystrokes
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# Posted: 15 Mar 2020 22:49 - Edited by: SpyingOnMyKeystrokes
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Quoting: socceronly I am curious, as a hypothetical question... what do you get if you stack a 6x6 on another 6x6 as a beam? Say you connected them with a bunch of timberlok screws or heavy bolts.
Wouldn't a double or triple 2x12 beam glued and screwed together work better and provide better load strength to fit within a 12" beam depth?
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SpyingOnMyKeystrokes
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# Posted: 15 Mar 2020 23:02 - Edited by: SpyingOnMyKeystrokes
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Quoting: KinAlberta A stacked beam of two or three 2x12s sufficiently locked together, thus making the beam 24†or 36†deep, should be able to span a good distance and hold a fair bit of weight.
With that beam depth, an LVL might be a good solution, and although this should be an engineered beam, I wonder if some glue and screw or thru-bolting pattern could be calculated to suffice?
I saw a video with a farmer making a fairly deep Glu-Lam roof-ridge beam out of stacked 2x4's for his barn. He put a layer of glue between each 2x and screwed each layer to the previous one (If I'm remembering correctly) and then used long all-threads on both sides torqued down at some spacing over the entire length of the beam to clamp until dry.
Here's a guy making one with Plywood layers:
https://www.youtube.com/watch?v=1J5ZZ1ujU7o
Massive Glu-Lams are beautiful in finished product, as both beam and column. The different grains in the layers look great.
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