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Old 12-12-2012, 07:39 PM   #16
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Stacking beams


The topic of stacking beams has been discussed several times on this forum over the past few years. I wrote a lengthy message discussing the concept of horizontal shear and how that relates to the strength of stacked beams, you may want to do a search if you are really interested in this topic.

The essential fact is that two beams stacked on top of each other WITH NO means of connection are stronger than either beam alone, but not by much, and the exact amount is difficult to calculate. If the beams are properly connected using fasteners, the composite beam acts exactly the same way that a single beam of the same dimensions would have, in other words two beams which are 3.5 inches square (4x4's) joined together vertically have the same strength as a beam 3.5 inches wide and 7 inches deep, provided the fasteners used to connect the beams can carry the horizontal shear developed due to beam loading. Determination of the required fastener schedule is tricky, and beyond a DIY discussion.

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Old 12-12-2012, 07:53 PM   #17
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Stacking beams


From a pure structural standpoint, it does not take much of a plywood layer on each side of separate beams/lintels that are stacked from a flexural and deflection standpoint if you have enough screws/nails to transfer the shear between the units and create a composite beam with as much as 4x the engineering properties of a single beam/lintel. It works well if sources of materials are limited and can be very effective in reinforcing existing structures if loads are relieved before the veneer attachment.

We did similar tricks when reinforcing existing rocket test stands (no time or room for major mods with new more powerful engines) where channels were commonly used as beams and we emptied the fuel tanks to lessen the gravity loads during installation of plates and create the strength that was needed for firing when the upward load were critical after the fuel tanks emptied. - Same principal, different materials, different methods (welding sheets instead or nailing wood) and opposite load directions.

Engineering principals work in almost every structural application.

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Old 12-12-2012, 08:01 PM   #18
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Stacking beams


If it's for a group of windows depending on the look your going for you can put 2x4's between the windows to help carry the load.

For large open spans replace the ply wood or OSB in the beam with a steel plate. It would be hard to hide the lag bolts for like a window but can be done with for thought.

But when it comes down to it I would talk to an engineer.
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Old 12-12-2012, 10:02 PM   #19
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Stacking beams


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The topic of stacking beams has been discussed several times on this forum over the past few years. I wrote a lengthy message discussing the concept of horizontal shear .....
Duh. I should have figured that. I will search, and I thank you.
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Old 12-12-2012, 10:06 PM   #20
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From a pure structural standpoint, it does not take much of a plywood layer on each side of separate beams/lintels that are stacked from a flexural and deflection standpoint if you have enough screws/nails ....
Like many things in life, that seems almost counter-intuitive. Aren't you still relying on the plywood to convey the loads, and isn't plywood pretty weak, relatively speaking, for this type of loading? Working on those rocket test stands must have been a blast!
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Old 12-12-2012, 10:26 PM   #21
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Stacking beams


^ the screw/ nail schedule is an important factor in regards to plywood being used as a connector along with thickness, 5/8" being the minimum and ideal size in some cases.
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Old 12-12-2012, 11:10 PM   #22
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Stacking beams


Dan H: I have searched at length, by your name and various permutations of "beam blah blah". I am old and running out of time........ any idea of the title or a key word in the post where you laid out horizontal shear? thanks. i'd love to read it.
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Old 12-15-2012, 01:21 AM   #23
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Stacking beams


Found Daniel's discussion of horizontal shear, finally! See strengthening floor joist

Now, Daniel, where do I go to learn the intimacies of ensuring that I have properly addresses horizontal shear when stacking two beams? Engineering school? Surely some wood organization lists general rules of thumb for nail size and pattern, plywood thickness, etc.... any ideas? thanks.
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Old 12-15-2012, 08:05 AM   #24
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Stacking beams


Stacking beams is relatively uncommon, and the horizontal shear computations are usually done individually for each installation. The reason they are usually done custom is because the horizontal shear that develops between two components depends on the load and the geometry of each piece. You can find the formula in any standard mechanics of material textbook, however actual application to a real problem is a bit harder than just plugging in numbers.

Once you know the horizontal shear, you select the fastener spacing to provide an adequate safety margin, usually 100 percent, since fasteners are cheap and failure is a bad idea. In other words, you compute the maximum allowable spacing between fasteners, then use half that, so if the allowable spacing were 12 inches OC, you would use fasteners 6 inches OC. The allowable shear per fastener is typically obtained from Wood Products Laboratory or similar white papers on the subject.

As I said, this method of strengthening beams is relatively uncommon, and so far as I know there are no "rules of thumb" similar to the carpenter rules that have been developed to size headers.
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Old 12-15-2012, 12:08 PM   #25
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Stacking beams


OK. Understood. I did some Googling last night and it lead me to believe that this was more engineer territory. So, I've learned what I needed to and will just buy bigger beams. Thanks for the info; it is always interesting reading. I did learn that it is permissible to rip PSL's down in width (to 1 3/4"), which is helpful linearly, as you mentioned earlier.
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Old 12-15-2012, 03:04 PM   #26
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Working on those rocket test stands must have been a blast!
I'll bet it was rocket science as well.
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Old 12-15-2012, 03:17 PM   #27
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Once you know the horizontal shear.
In simple terms, how would we calculate the horizontal shear between two 4x2s laid flat, one on top of the other, and uniformly loaded?
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Old 12-15-2012, 04:17 PM   #28
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Stacking beams


The formula is t = VQ/I where t is the shear flow at the neutral axis, V is the maximum vertical shear in lbs, Q is the statical moment of the beam (for rectangular beams Q = Ay' where A is the area of the beam above the neutral axis, and y' is the distance from the neutral axis to the centroid of the area above the neutral axis), and I is the moment of inertia of the section.

So let's take your example. A pair of 2x4's laid flat has dimensions of 3.5 inches wide x 3 inches deep. If the 2x4's are 10 feet long, and are loaded with 20 psf, the total load is 200 lbs, and the maximum vertical shear is 100 lbs, which occurs at either end. For such a beam, Q = 3.9375 in^3. Moment of inertia I = 7.875 in^4. Shear flow at the neutral axis at the ends = 50 lbs/in.

This means that at the ends, the boards need to be nailed together to develop 600 lbs/ft shear capacity (12 inches x 50 lbs/in). A 16d nail has an allowable shear of 96 lbs, so you would need 6.25 nails per foot near the ends. If you use two rows, you would need to space the nails 12/3.125 = 3.84 inches apart, in practice you would use 3.5 inch spacing, two rows of nails. Towards the midspan of the boards you need fewer nails since the vertical shear is lower towards the midpoint (it is zero at the midpoint of the span).

This example shows why it is rare to connect flat boards this way. Far simpler to nail the 2x4's together oriented vertically and let the wood handle the horizontal shear stress. For boards oriented vertically, horizontal shear is irrelevant, and you only need enough nails to hold the two pieces together sufficiently to carry the composite member around.
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Old 12-15-2012, 05:25 PM   #29
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Many thanks, DH. That filled in a little gap for me. (literally!).
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Old 12-15-2012, 10:41 PM   #30
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Stacking beams


good work, holzman. i just bid on that last job you we asked to, but i bid half of what you did. "have equations, will engineer."

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