Which is stronger?

pburchett · Sep 21, 2015

Which is stronger? Two 2x6's nailed together to make a 4x6 or stacked on top of each other to make a 2x12? The reason I ask is because I need to beef up the truss carriers in the pole barn. Thanks

Zachary Hoyt · Sep 21, 2015

An actual 2x12 will be more rigid than either, but if the two 2x6s are securely fastened I think they would be more rigid on top of each other than side by side.
Zach

Brendon-KS · Sep 21, 2015

When loaded purely in the vertical direction (load parallel to the longer side) the 2x12 will be
much stronger than a 4x6. A 4x6 will actually be no stronger in this directing than two individual
2x6's. The key to this, however, is fastening the 2x6's edge-to-edge securely enough that there
can be absolutely no yielding in the joint - probably will need to use some fishplates along the
sides to ensure this.

dahlin · Sep 21, 2015

when I needed to strengthen rafters in my garage We took 3 2x6's and sandwiched 2 pieces of OSB board between glued with Gorilla glue and 3/8th carriage bolts 2 feet apart its held so far. Randy

WESnIL · Sep 21, 2015

My memory is that the strength is proportional to the square of the vertical dimension. In this case your vertical dimension is 12 so 12 squared equals 144. Your other choice of material is 2 x 6 material so 6 squared equals 36. You are doubling the 2 x 6s so that yields 36 + 36 or 72. Based on this one 2 x 12 still is 2x as strong as two 2 x 6s nailed together. That same principle is important for those building rack wagons, etc.

dpendzic · Sep 21, 2015

the key to the strength of a beam is its section modulus--a 2x6 on edge has a section modulus 0f 7.563 so two of them side by side would be 15.126----a 2x12 on edge is 31.641, but to place a 2x6 on top of a 2x6 without an adequate shear connection to make them one is quite difficult

David G · Sep 21, 2015

Side by side does little, I also think 2 stacked without lashing together does little.

Ranchingelec · Sep 21, 2015

Sandwich a piece of tin between the boards
That will help keep the beam from sagging.
Ranch

evilboweivel · Sep 21, 2015

I would nail a piece of 1/2" plywood 12" high, 16" would be better on both sides of the truss using titebond glue and
plenty of nails fastening the plywood to the truss.

Ron Muhlenkamp · Sep 21, 2015

A vertical 2x12 is much stronger than a 4x6, but the key is that the 2x12 is one piece. To get equal strength from 2- 2x6's would require a glue
joint as strong as the solid wood.

ScoutB · Sep 21, 2015

(quoted from post at 19:49:16 09/21/15) My memory is that the strength is proportional to the square of the vertical dimension. In this case your vertical dimension is 12 so 12 squared equals 144. Your other choice of material is 2 x 6 material so 6 squared equals 36. You are doubling the 2 x 6s so that yields 36 + 36 or 72. Based on this one 2 x 12 still is 2x as strong as two 2 x 6s nailed together. That same principle is important for those building rack wagons, etc.

It's actually a cubed relationship. If stacked the top member is in compression and the bottom member is in tension and the joint is the neutral axis. The joint is important but not because of shear so much as keeping the compression member from buckling. A 2x12 would be a better choice.

dpendzic · Sep 21, 2015

its actually a squared relationship---the strength of a beam is determined by its section modulus and for a rectangular beam it is width X height squared divided by 6

Brendon-KS · Sep 21, 2015

The joint between the two 2x6's fastened vertically edge-to-edge is very much in shear because it
is on the neutral axis. "Neutral axis" only means no tension or compression, not the absence of
any stresses. The beam only acts like a solid 2x12 if the two individual 2x6's are prevented from
moving relative to each other and this depends completely on the shear strength of the joint.
Think of what happens to a soft-cover book when it is bent - the pages slip against each other and
become offset and the two-element beam will try to do the same thing.

ScoutB · Sep 22, 2015

(quoted from post at 23:45:15 09/21/15) its actually a squared relationship---the strength of a beam is determined by its section modulus and for a rectangular beam it is width X height squared divided by 6

I guess it comes down to how you define strength. In this case I'd call it resistance to deflection since he's trying to hold up a sagging structure. Deflection calculations use moment of inertia, width x height cubed divided by 12. I always use "I" when comparing section properties but "Z" works just as well. It's just "I" divided by the distance to the neutral axis, hence the cubed vs. squared relationship.

ScoutB · Sep 22, 2015

(quoted from post at 00:03:38 09/22/15) The joint between the two 2x6's fastened vertically edge-to-edge is very much in shear because it
is on the neutral axis. "Neutral axis" only means no tension or compression, not the absence of
any stresses. The beam only acts like a solid 2x12 if the two individual 2x6's are prevented from
moving relative to each other and this depends completely on the shear strength of the joint.
Think of what happens to a soft-cover book when it is bent - the pages slip against each other and
become offset and the two-element beam will try to do the same thing.

Agreed, if the ends aren't attached to each other. I just assumed the ends would be fastened to the walls, making your book analogy like a soft-cover with bindings on both sides. Not terribly useful as a book but the pages can't slip relative to each other. If the ends are loose the top beam is in tension at the joint and the bottom beam is in compression at the joint. There would be two neutral axes (one for each beam) but the joint would have stresses as you mentioned.

Owen Aaland · Sep 22, 2015

(quoted from post at 21:48:01 09/21/15) I would nail a piece of 1/2" plywood 12" high, 16" would be better on both sides of the truss using titebond glue and
plenty of nails fastening the plywood to the truss.

The beam over the 16' door in my shop is made up of 9 2x6s, three wide and three high with 1/2" plywood glued and nailed. It supports 48' trusses of 3' centers. No problems in the first 30 years.

caterpillar guy · Sep 24, 2015

Take a good white oak 2x12 and for get the rest of this nonsense. It will be there when you are gone. The 2x6's won't do the job. They will help to reinforce a weak location. We always use 2 2x12's one on each side of the pole then set the truss with a stub between the 2x12's to nail the truss to. We also put a 5/8 bolt through the pole with a short piece of plank on the inside and a steel plate on the outside with about 4 bolts through the 2x12's for extra strength. Have never had a building come down yet some are 50years old. All are over 25years old.
The 2x6's stacked on edge would be stronger than a 4x6 configuration of them.

RobCons · Sep 24, 2015

Pburchett how long are the carriers your talking about? Span tables are readily available on line for various species of wood.

Which is stronger?

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