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I'm currently building a swing with 4x4 posts based on a basic design I saw online. For the beam, my area does not carry 4x6 in any big box stores or lumber yards and no one knows why and cannot order them either. I really tried.

So my question is:

Should/can I replace the 4x6 with either a 4x4 with a 2x4 sandwiched together or should I use a 6x6 instead? The sandwiched one would be easier for the swing hardware I have as the bolts are only so long.

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    some detail on the length of the beam would be useful. It might be a 4x4 is good enough for a shorter span, but isherwood's idea of 2 or 3 6x2 set vertically sounds good
    – Mr. Boy
    Sep 2, 2022 at 10:01
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    Welcome. Please take the tour so you know how to best use this site.
    – isherwood
    Sep 2, 2022 at 15:07
  • @TigerGuy, answers go down there. Also, sure you can. It's not difficult or dangerous in this case. It's just more hassle.
    – isherwood
    Sep 2, 2022 at 15:39
  • The direction of the split definitely matters, but even when these are not stacked, it really depends on the connection if they have the same strength and stiffness as a monolithic beam. Only strong glues can achieve this, hence laminating beams. The difference is that bolts can only connect points, whereas lamination connects entire surfaces.
    – MiG
    Sep 2, 2022 at 15:47
  • @MiG: If one were to construct a beam by fastening an upright 2x4 to a pair of 2x4's above and below it, stresses on the inner 2x4 would be concentrated at the connection points, but a beam with a 2x4 as a "web" could be pretty with a moderate number of point connections. Using a continuous lamination would distribute force more evenly along the web, thus allowing it to be made of thinner material, but using thicker material will reduce the number of connections required.
    – supercat
    Sep 2, 2022 at 20:58

1 Answer 1

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You could, but a pair (or triplet) of 2x6s would be more appropriate (and probably cheaper).

The strength and stiffness of a horizontal structural member are far more affected by its height than by its width. Therefore, for your plan to work, you'd have to effectively and durably laminate the two together so they can't shift with respect to one another. This would take many robust fasteners. The shear force on a bouncing beam is significant.

It would be simpler to sandwich two or three full-height 2x6 boards. You'd still need to screw or bolt them together well, but a little flex between them is less of a concern.

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    Some Titebond II or liquid nails would go a long way to make the 2 2x6's act as a single beam. A few screws for back up and clamps while the glue dries. It'll be fine. Sep 1, 2022 at 21:49
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    Adhesive applied in the field should never be considered structural, in my opinion. It's rarely approved by code, and it tends to let go after a few seasons outdoors. I like both of those products, but not for this.
    – isherwood
    Sep 1, 2022 at 22:55
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    To me it's a belt and suspenders approach, Maybe I understated the number of screws needed, but I still maintain that an adhesive is not a bad idea. Titebond III might have been a better recommendation since it's pretty resistant to water, better then Titebond II. But let's face it, this is a SWING SET, not a major structural part of a house that's exposed to the weather. If it started failing, the owner would almost certainly notice and address it. But I respect your comment and will heed it in the future. Sep 2, 2022 at 0:50
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    far more affected by its height than by its width. I do not remember the formula for load capability, but it contains h^3, e.g. an upright 2x4 can take 8 times the load of an 2x2.
    – hlovdal
    Sep 2, 2022 at 9:01
  • Never depend on adhesive alone. Adhesive makes nails better, but it doesn't make up for less nails. Use it with nails.
    – user19565
    Sep 2, 2022 at 15:30

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