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Old 12-11-2012, 10:40 PM   #1
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Stacking beams


In looking for window headers, it was suggested to me to stack two beams, one of which is not quite of sufficient stiffness to carry the load. I don't like the idea real well, and here is my thinking: If you stack two identical beams atop one another, does the lower one really add much? It seems to me that the second one adds very little. (Assume a uniform load on the beams.) If the upper beam, A, bends, it will bend more in the middle than anywhere else, which puts a point load on the lower beam, B. A beam w/ a point load in the middle carries about 1/8th as much as it would if loaded uniformly. (That is not a statement of fact, but rather my understanding only.) IF that is true, then beam B does not add much initially, but will add more and more as the beams bend, albeit marginally increasing. That said, if there is a window below these two beams the window would likely crack before the beams acted as one. Now, if you gusset the two beams with plywood on each side, you still have not added much, as plywood is not the greatest beam material because alternate lams go in the wrong direction. Thus, it seems almost pointless to stack two beams, and several magnitudes better to just buy a taller beam. Anyone know this stuff for sure? Got numbers? Thanks.

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Old 12-11-2012, 11:11 PM   #2
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Stacking beams


Not making much since to me.
Most often this is what's done.
http://www.bing.com/images/search?q=...1B03E8&first=1

Just how wide is this window?

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Old 12-12-2012, 12:33 AM   #3
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Stacking beams


joe: yes, that is a typical, single-beam header. my problem is that i can get 7" psl's real cheap, but a single one is not quite strong enough to carry the load i have. it was therefor suggested that i stack one beam on top of another, to (very approximately) simulate a taller, 9" or 11", single beam. i know a 9" tall beam will carry the load, and can just go buy one and be done w/ it. however, this has become an academic challenge to learn about stacking beams. i don't think two separate beams, stacked atop each other, will even approximate a taller manufactured beam because you likely can not really get the two to "become one" (borrowing from Grasshopper....). so, this is just an academic curiosity that i'd like to learn about. i have an engineering handbook that i am going to look at, but, not being an engineer, it is easy to get misguided by one of those handbooks. btw: the beam will need to be 6' 8" wide; nothing unusual.
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Old 12-12-2012, 12:43 AM   #4
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Stacking beams


You keep saying beam. I've never seen a single "beam" used in modern carpentry.
For a 2 X 4 wall it would be 2, 2 X 10's with 1/2 plywood or OSB sandwiched between them.
Whole lot less work then what your suggesting.
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Old 12-12-2012, 01:41 AM   #5
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Stacking beams


"beam" = psl, as i mentioned in the second post, or lvl, etc. engineered lumber is scores of gobs stronger than dimensional lumber, size for size. even double 2x12's would not carry my loads. in reality, dimensional lumber headers are fairly weak members, though often adequate, readily available, and relatively inexpensive.
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Old 12-12-2012, 10:25 AM   #6
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Stacking beams


really the best way to make the stacked header as one is to glue/screw/nail plywood to both sides of the stack and form a bonding structural connection between the two.if the two headers are 7" each then a 14" piece of plywood would be needed. that in turn leads to which type of plywood would be best?? even without plywood if the two headers sit tight to one another then the top header starting to deflect would have the resistance of the lower header pretty much immediately, the plywood helps to keep any outward kickout force from the separated beam from occuring. the downward force on the header can turn sideways(twist) if there is no resistance to keep it from doing so.

Last edited by hand drive; 12-12-2012 at 10:28 AM.
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Old 12-12-2012, 02:31 PM   #7
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Stacking beams


I have no input for you jklingel but just thought some clarification would get you more answers for your questions.

You have a 6'8" span and a beam comprised of 2 2X12's isn't strong enough therefore the idea for stacked beams, is that correct?
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Old 12-12-2012, 03:36 PM   #8
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Stacking beams


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Originally Posted by hand drive View Post
really the best way to make the stacked header as one is to glue/screw/nail plywood... Yes. Glue, screw, subdue and tatoo; do everything you can. I still wonder if the plywood does much, though, as it is not very wide (2 1/4" max, and then you may as well buy a bigger beam) and still alternate lams are in the wrong orientation.

.... if the two headers sit tight to one another then the top header starting to deflect would have the resistance of the lower header pretty much immediately... Correct, but I still think you are getting primarily a point load on the lower beam. That is what it FEELS like to me, anyway. Wish I knew exactly.
Thanks. See after bullets.
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Old 12-12-2012, 03:38 PM   #9
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Stacking beams


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Originally Posted by drtbk4ever View Post
I have no input for you jklingel but just thought some clarification would get you more answers for your questions. * OK. Send money anyway.

You have a 6'8" span and a beam comprised of 2 2X12's isn't strong enough therefore the idea for stacked beams, is that correct? Correct. Neither the the double 2x12's nor a single PSL is quite enough.
Thanks for the reply. See after bullets.
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Old 12-12-2012, 04:35 PM   #10
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Stacking beams


My understanding is if the two stacked beams are not fastened/glued or otherwise "stuck" together so that they are working as one, then it is not as simple as 1 + 1 = 2. If you place these two beams side by side, then yes it would be 1 + 1 = 2.

Regarding stacking: You would need to calucate the shear forces inbetween the two beams when placed under load, and compare that to the actual shear resistance of the method you use to fasten the two beams together.

If you do not fasten the beams together, the only shear resistance you are getting is from the friction between the two beams... which will ultimately give you a higher strength beam, but not one that is double in strength.


*Disclaimer: Have an engineering degree but do not practice in structural design. Hopefully someone who does can give you a 100% certain answer.
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Old 12-12-2012, 04:56 PM   #11
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Stacking beams


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Originally Posted by jmd87 View Post
My understanding is if the two stacked beams are not fastened/glued or otherwise "stuck" together so that they are working as one, then it is not as simple as 1 + 1 = 2. Agreed. Two stacked are not even close to one manufactured of that height, and my feeling is that the second one is adding about 12% (1/8), at least initially. If 1+1=2, then they'd only manufacture one thickness and tell you to stack what you need. Also, the Karate Kid would never cement all his blocks together and THEN break them; never happen.

Regarding stacking: You would need to calucate the shear forces inbetween the two beams when placed under load, and compare that to the actual shear resistance of the method you use to fasten the two beams together. Exactly, which is why I question just nailing and/or screwing. Gluing two stacked, if you could do as good as the manufactured stuff, would likely be pretty good.

If you do not fasten the beams together, the only shear resistance you are getting is from the friction between the two beams... which will ultimately give you a higher strength beam, but not one that is double in strength. Again, just a gut feeling, but I'd sure not want to rely on that friction adding much.


** My disclaimer is that I am NOT an engineer.
Roger that. See after bullets. Thanks for the reply. I am going to try to post on an engineering forum; just busy as h3ll building right now..... and pushing snow.
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Old 12-12-2012, 05:03 PM   #12
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Stacking beams


That reminds of me of a house I sold years ago. Original load bearing kitchen wall was cut out (balloon frame wall) with second floor joists suspended by 1x6 siding, to put a bathroom addition.

Stacked header, I am guessing, can work, but you are guessing on the load capacity of such a header. Top header will start to bend, taking the bottom header with it, until some balance is reached, or both fail.

Splicing two pieces with plywood will not work. Plywood is weaker than header materials, unless the whole thing is engineered.
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Old 12-12-2012, 05:27 PM   #13
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Stacking beams


Keep in mind that the existing "header/lintel" is already loaded and deflected due to the existing dead loads. Unless the loads are temporarily releived, the straight member under will just pick up the new load and will deflect. The question is whether you are concerned with strength or deflection mainly?

That is why many structures a jacked slightly before a reinforcement is added since the load is eliminated and the existing deflection minimized.

Dick
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Old 12-12-2012, 07:02 PM   #14
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Stacking beams


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Originally Posted by carpdad View Post
That reminds of me of a house I sold years ago. Original load bearing kitchen wall was cut out (balloon frame wall) with second floor joists suspended by 1x6 siding, to put a bathroom addition. Wonderful engineering on someone's part!

Stacked header, I am guessing, can work, but you are guessing on the load capacity of such a header. Top header will start to bend, taking the bottom header with it, until some balance is reached, or both fail. That is what I am feeling, too.

Splicing two pieces with plywood will not work. Plywood is weaker than header materials... Typical plywood, yes.
See after bullets. Thanks.
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Old 12-12-2012, 07:05 PM   #15
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Stacking beams


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Originally Posted by concretemasonry View Post
Keep in mind that the existing "header/lintel" is already loaded and deflected due to the existing dead loads. ... New construction.

The question is whether you are concerned with strength or deflection mainly? Have to consider both; window under header, so deflection will kill it.

Dick
See after bullets. Thanks.

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