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kittysmitty
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# Posted: 1 Apr 2018 10:17am
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Just wondering what has more strength for a carrier beam.
two 2x8's
three 2x6's
or a 6x6
Planning a small building and wondering what is stronger.
Thanks
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socceronly
Member
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# Posted: 1 Apr 2018 10:39am - Edited by: socceronly
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There are definitely people on here who can give you some fantastic answers, but I am sure they will need more information than this because depending on what you are doing the answer could easily be, and likely be, none of those.
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kittysmitty
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# Posted: 1 Apr 2018 12:29pm
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Ok, I don't think it makes a difference, but, let's say it's a 8 foot overhead beam in a workshop. Supported on both ends and will be used to hoist, with chain blocks, various items.
so which of the above will carry more weight? Thanks
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snobdds
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# Posted: 1 Apr 2018 12:40pm
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Two 2x8. The wider the board the better when standing on edge. Only if they are properly tied together and the grain of the wood is opposing each other.
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ICC
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# Posted: 1 Apr 2018 02:27pm - Edited by: ICC
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Assuming the species, grade, etc. are the same, the beam calculator states --- two 2x8 will beat out three 2x6 by maybe 8 to 10% with the single 6x6 about in the middle. For that application, I would rather use the triple layer built up beam and the single 6x6 would be my last choice. The 6x6 could actually be considerably less strong than either the twinned or tripled beam as timbers can hide defects we can't see from the outside. A layered beam will spread the defects (knots, etc) around and gets an increase in rating over a solid timber because of that. The numbers were run using actual dimensions; 5.5x5.5 for the 6x6 for example with the load concentrated at a single mid span point.
The weak point may be where the hoist is hung from the beam center. You would want something that shares the load between all layers, not a bolt through one 2x. A welded metal bracket that wraps the built up beam would be good. If a vertically drilled hole was to be used with a single (forged?) eyebolt then I might prefer a 6x6 with no knots in the central span area. How the hoist is connected to the beam is a potential point of weakness. Even a heavy chain wrapped around the beam may be better than a poorly bolted bracket or eyebolt.
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socceronly
Member
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# Posted: 26 Jul 2018 08:52am
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Quoting: kittysmitty Ok, I don't think it makes a difference,
Sorry I totally misread what you were doing.
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Nate R
Member
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# Posted: 26 Jul 2018 09:42am
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Or, use LVL and get even more strength. 
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Eddy G
Member
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# Posted: 26 Jul 2018 10:37am - Edited by: Eddy G
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Quoting: Nate R Or, use LVL and get even more strength.
I second that 
Not just for strength but its easier to work with and in some cases gives you more head room.
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Borrego
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# Posted: 26 Jul 2018 10:37am
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What about a 6x8? Might as well go for it if you're going to use it as a hoist.....
Don't forget to block it at the ends so it doesn't want to roll.....
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Fanman
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# Posted: 30 Jul 2018 05:49pm
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Two 2x8's will be nearly 70% stronger than three 2x6's. One 6x6 will be 22% stronger than three 2x6's. This assumes, of course, that the long side of the thinner boards is vertical, and that we're talking standard dimensional lumber where a 2x8 is really 1.5x7.5, etc.
Beam bending strength is proportional to the width of the beam, but to the cube of the height.
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ICC
Member
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# Posted: 30 Jul 2018 08:01pm - Edited by: ICC
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Fanman, it may be counter-intuitive, but engineering tables indicate that a three-layer 2x6 will bend less (have greater strength) than a one piece 6x6 timber, even with the lesser actual width of the surfaced 2x material. . The tables and design information published by the Southern Pine Inspection Bureau and the Western Wood Products Association bears this out.
Using the tables from here, (table 1 and table 3) we can compare Fb (fiber stress in bending) for southern pine. The higher the Fb value, the greater the strength, the greater the resistance to bending. A southern pine, #2 dense grade 2x6 has an Fb of 1200 psi while the same grade sized 5x5 or larger, is assigned an Fb of 975 psi. There are similar differences in other grades and other species of wood.
In general, once the wood is cut thicker than 4 inches there are different rules applied and different (reduced) values used. These differences include a reduction in the Fb value for timbers 5x5" and larger. The E and Emin values (Modulus of Elasticity) are also reduced in the same larger timber sizes.
It is interesting to note that the less than 4" thick lumber sizes also gain in strength values when they are used spaced less than 24" apart. These increased values have been used when we use the rafter and joists sizing tables where spacing is 12, 16 19.2 or 24" OC. There are tables that simply provide the Fb and E values for a single stick of lumber. The AWC joist and rafter calculator also is designed to accommodate the greater Fb and E values when joists and rafters are repetitive.
That repetitive strength increase also applies to built up beams that use three or more layers, as long as all three layers are fully supported at the support points. Incidentally, these built up beams do NOT need to be fastened to each other with nails, bolts, screws, or glues; they simply need to be fully supported equally. We usually do secure them with a few nails to help keep them aligned, but that does not lead to an increase in strength values.
In a similar vein of thought, we should be careful when we rip one wide piece of lumber into two or more narrower pieces. It is easy to reduce the lumber grade at least one full level; like turning a #1 grade 2x10 into two #2 grade, or worse, 2x6’s. How close a knot is to the edge has great influence of the grade. Grading lumber is complicated, that’s why the grader is the highest paid mill worker.
It may also be of interest to note that back in 2013 the values for southern pine were devalued. This was to more accurately reflect the reduced strength of our plantation grown southern pine.
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Larryhere40
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# Posted: 17 Dec 2022 09:39pm
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Any idea on how many 2x8’s would be used to support a 16’ span, have enough strength to support 1/2 ton in the center? Approximately! Thanks, can’t seem to find an answer anywhere! Thanks Larry
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ICC
Member
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# Posted: 18 Dec 2022 02:10pm
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Depends on species, grade and how much deflection (sag) you can have in that beam.
4 or 5 hem-fir #2, 2x8's work. That will be much the same for any commonly available construction lumber. Whatever it is being used for be sure the beam can't roll over under load.
Hasta luego
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