O/T Lumber strength question

Starting to build a good size pole barn / shop. Lucky for me, my Father-in-law has a small sawmill. We were wondering: Is a 3x8 as strong as 2 2x8s nailed together? This is good heart pine with a minimum of knots. It would sure save us some cutting on the mill. These will be used as headers for the trusses to sit on. Any ideas?

I like laminated any more, the wood today is poor. Any knot will make a real weak spot in your wood. Nailing 2 together will span the bad spots.

Perhaps you have old growth wood & it will be fine....

--->Paul

NO lumber is stronger when it is laminated together in order to get one piece s strong as the two 2x8 you would have to be alot thicker than3"

I should have clarified, I mean a 2x8 in commercial lumber sizes (1.5 inches thick)

Two laminated together will be much stronger than a solid plank,like the above post stated if ther are knots the other board will help carry the weak spot.And also laminated will not warp,twist and check near as much as a solid plank .

Dustin IL

The two 2x8 will be stronger. The strength of the rectangular cross section beam is directly proportional to the moment of inertia, and the formula for that is I=bh^3/12. So for your 3x8 it's I=2.5*7.5^3= 87.9 in^4, while for the 2x8 it's I=1.5*7.5^3= 52.7 in^4 for each one, or 105.4 in^4 for the two beams together. So your doubled 2x8 will be (nominally) 20% stronger.

Well said Fred !! got that info at work but I sure don't know it by heart been selling lumber and a manager of a lumberyard for 16 years of my life and might I say it is slowed down now!!

Dustin IL

Since my engineering job doesn't relate much to my engineering degree, that's the one little bit of info I've retained. Hope business picks up for you. I'm doing my part around here. We're adding about 1100 sf to our house, finding that I can pretty much have my pick of contractors, and my contractor is hiring some very good subs without having to pay premium prices, and the subs are almost always there when they say they will be. So the construction slowdown is working in my favor.

JBMac: Big Fred beat me typing with the formula, so I'll note one other point--Height (depth) is a cube function, so it's MUCH more important than width in figuring strength. If you've got any leeway in how you saw your lumber or design your building, it's much more beneficial to use a deeper beam than a thicker one.

Fred I wish I knew of one addition going on around my store!! I also bet you are buying lumber cheaper than you have in years too !

Dustin IL

While I prefer the 2 2x8s, he may be comparing a single 3x8 actual to a pair of 1.5 by 7.25/7.50 inch beams???? I don't know how that would change your formula comparison.

--->Paul

It is indeed much stronger.

The arguments the Guys place seems to be of the assumption that opposite wood grains will lend more strength , and for that reason I could Agree ,, However If You set your timbers in a jacknotch on your post and donot rely entirely on barn nails to hold your timber to the post , I feel you would have a strong barn ,and You will have less Sawywr Time too ...

Pine is not the best kind of wood to use for framing, building codes here won't allow it. Hemlock, fir , spruce or hardwoods are much better.

Built many a house in my youth working for my Uncle with Ponderosa Pine. Never had a problem and all those houses are still standing up just like when we made them. Up here in Oregon we use Douglas fir because it the most plentiful.
Where I live in Calif. as a kid Ponderosa Pine was the best wood. And Real Red Wood for decks not that painted crap.
Walt

Saw a guy build a roof over a trailer house with pole barn design. The lumber was too small and too long of spans between polls. Looked good when he was done but a few years later it was swayed down between poles.
In my house I'm replacing beams that were poorly designed or constructed. Common construction process is to put 1/2 plywood spacers in window headers, they become structural if they're one pieces the same size as the header, but if you're spanning 15 feet and don't interlock the ends it's just a waste of wood and more weight.
What I replaced: 2@ 2x10x13 w/ 1/2 plywood not interlocked or glued.
What I used: 2@ 2x12x15 w/ 3/4 plywood interlocked and glued, screwed and bolted.
What I replaced: 3@ 2x8x12 w/ 2@ 1/2 plywood not interlocked or glued.
What I used: 3@ 2x10x14 glued and screwed.
What I replaced: Load bearing door way. 2@ 2x10x6. While the short span held up with the 2x10s the support boards were bowing under the weight.
What I used: 4@ 2x12x6 glued, supports were glued and pulled together, screwed and another board added under each side.
All had some structural problem, bowing, bending, crushing.
While this calculator doesn't address headers you can see what different dimensional woods and species will span.
You didn't say how wide or how many trusses will set on each span, but it's obvious that five 40 foot truses setting on a span are going to weight more that three 12 foot trusses.
calculator

If you're milling your own lumber, why the choice between 2X8's nailed together vs. 3X8? A scientific type below said the 2- 2X8's nailed together would be 20% stronger than 3X8- so instead of making 2X8s and nailing them together, why not just make 4X8's and be done with it? Saves even more milling costs.

If you are building a polebarn then the trusses would be setting on the posts? If you are using these for headers as you stated, depending on your truss span your header span and your live and dead loads 2- 2x8 may not be enough.

Steven

Did just what your looking at this past summer but used 3 - 2x10s. I understand that you have sawn lumber available but I found that treated dimensional lumber had become so cheap it just wasnt worth using green lumber. I did buy local sawn lumber for the siding. Nailing is important - use plenty.

Thanks for the input guys. It is southern yellow pine, what we have is old growth heart pine. 100% of all wood framed buldings in the south are made from it. By 3x8, I meant that it would be the exact demensions as 2 laminated commercial 2x8's (1.5" x 7.5" or whatever they are)

Thanks Again!

I think I used the wrong term, trusses will sit on these attached to the poles at the top as joists?