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Author Topic: Which oak is the strongest spanned?  (Read 1242 times)

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Offline Kwill

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Which oak is the strongest spanned?
« on: January 27, 2019, 08:30:41 PM »
Im in thdd process of startung my sawmill shed build and i need to span 16 ft. The rest of the building is being built out of cedar. I was going to use cedar for this beam but im thinking i might go with oak. The beam is going to be a true 4x6x16. I have red oak white oak and post oak on the farm. Which would be the best? Or will the cedar be fine?. It will be setting on a 8x8.
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Offline Firewoodjoe

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Re: Which oak is the strongest spanned?
« Reply #1 on: January 27, 2019, 08:37:14 PM »
White oak. We sawed 22 foot 6x6 white oak for a hay barn. I think it will touch ends before it cracks.

Online Southside logger

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Re: Which oak is the strongest spanned?
« Reply #2 on: January 27, 2019, 08:59:43 PM »
Perhaps @Don P can chime in with some of his magic formula, secret sauce, stuff.   ;D
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Offline barbender

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Re: Which oak is the strongest spanned?
« Reply #3 on: January 27, 2019, 09:15:51 PM »
DonP will have the real numbers. Strength means different things, but in general red oak is much more brittle than white oak.
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Offline Dave Shepard

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Re: Which oak is the strongest spanned?
« Reply #4 on: January 27, 2019, 09:20:57 PM »
Strength green, and strength dry are two different things. My house is supported on one half by four 2x8 hemlock, the other by an oak timber I'd equal cross section. The hemlock is straight, the oak sagged between columns. Everything above the oak in the house takes the same shape as the sag.
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Offline Don P

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Re: Which oak is the strongest spanned?
« Reply #5 on: January 27, 2019, 10:38:20 PM »
Oh crap, I'm the professor on Gilligan's Island :D
A beam built up of three or four 2x's is going to come from the 2-4" thick strength table which has better strength and stiffness (sag) numbers than the heavy timber 5x5 and larger table. That is why there are 2 different drop down calcs in my simple beam calcs, different strength tables. A built up beam is then given an additional 15% strength jump because the defects are better distributed across 3 or 4 boards than across one single timber. It's easier to saw and move 2x's too.

I don't know what the beam is holding up Kwill, is it a rafter or is it a beam holding up the end of a bunch of rafters of some unknown length, you'd have to sketch it with dimensions and post a pic. Somebody brought up the casket thread the other day. Mule was a character. I got to work one day where we are working on an old farmhouse. Mule is down at their barn standing in the back of his truck skinning a bear that's hanging from the beam over their side shed door opening. You've got to quantify load and span.

But, let me show y'all something. Go to this 2x calc;
http://forestryforum.com/members/donp/ddsimplebeam.html
Click select species and scroll to the bottom, #2 white oak, click it and then click "show result", look at the bottom, "Design Values", base design bending value 850 psi, look at the right, adjusted bending 977. Look above, 4.5" thick, and "Yes, 3 or more members". That is the 15% increase. Change it to "no", click "show result" and it goes back down to base design value.

Play with size, drop it to a 2x4 and notice the strength jump. That's pretty impressive, that size factor recognizes  the unit bending strength is higher in a smaller 2x than a wider one. Knots are smaller, the grain is generally straighter across more width of the narrower stick than a wider board. The average per square inch strength is higher in the narrow stick. It's still a little stick, it isn't a 2x12 but it is stronger per square inch. There is no such size factor increase in the heavy timber adjustment table. Nor is there the multiple member adjustment, and the base design bending strength and stiffness is lower in a heavy timber, go to that calc and compare strength numbers for heavy timber:
http://forestryforum.com/members/donp/beamclc06b.htm
Click #2 white oak Beams and Stringers and "Show Result", Bending is 750 psi, stiffness (E) is .8 instead of .9 for 2x #2 WOak. No multiple member increase or size factor increases in heavy timber. So although heavy timber is cool, from a strictly performance view, built up dimensional is stronger.

Then to Dave's point, a green timber is about half as strong and much less stiff than a dry one. Propping a green timber at midspan as much as possible helps while it is drying, help it "set" straight.

That is how to compare the basic wood strength numbers. You have to input the load, span, and beam sizes to actually check the beam.
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Offline Kwill

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Re: Which oak is the strongest spanned?
« Reply #6 on: January 27, 2019, 10:42:18 PM »
It will be holding up 12ft 2x6
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Offline Don P

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Re: Which oak is the strongest spanned?
« Reply #7 on: January 27, 2019, 11:03:58 PM »
Restating what I think, you've got a 16' oak 4x6 holding up the opening on the face of a shed roof made of 12' 2x6's?
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Offline Kwill

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Re: Which oak is the strongest spanned?
« Reply #8 on: January 27, 2019, 11:09:03 PM »
Yes.
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Offline Kwill

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Re: Which oak is the strongest spanned?
« Reply #9 on: January 27, 2019, 11:15:15 PM »
Shed is going to be 12 wide by 24 or 28 long. With a 16 ft opening. 2x6 rafters 5x5 poles and 8x8 for the opening.
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Offline Don P

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Re: Which oak is the strongest spanned?
« Reply #10 on: January 27, 2019, 11:33:46 PM »
6', half the rafter, is bearing on the oak beam, half the rafter is bearing on the back wall.
6x16', the opening span = 96 square feet
I think I remember you have a 20 lbs wind/snow load + 10psf dead load= 30 lbs per square foot
96x 30= call it 3000 lbs bearing uniformly along the beam
http://forestryforum.com/members/donp/ddsimplebeam.html
back out and Enter;
load 3000
span 192
width 4
depth 6
#2 white oak
2 month duration
No
Size 2x6
No
No
"Show Result"
Nope

Make it a 4 ply 8x8, nail together 4 2x8's.
Click 8, 8, yes, 3 members, Size 2x8, show result
Bending is more than strong enough, sag can go to 1" under full load, prop it if you get a big snow but it isn't going to break. It'll sag about 3/8" under its self weight.


If you go solid timber go to the heavy timber calc here;
http://forestryforum.com/members/donp/beamclc06b.htm
It starts to work there as a solid #2 WO timber at 8x9.

I helped a friend saw up some poplar beams for his mill shed a little while back, roughly the same size, we have about 5 pounds/square foot more snow load, opening was about 17' I think, solid sawn poplar 6x12's going on top of treated 6x6 posts.

To think about your proposed 4x6 another way, it is the same as two 2x6's, spanning 16', and trying to support 8 2x6 rafters.
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Offline Kwill

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Re: Which oak is the strongest spanned?
« Reply #11 on: January 28, 2019, 11:28:27 AM »
I think i will take and mill  4 2x6 and put osb between them and screw and glue and come up with a 8x8 to set on top of the 8x8.
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Offline Don P

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Re: Which oak is the strongest spanned?
« Reply #12 on: January 28, 2019, 05:59:04 PM »
It's up to you but that won't get you where you should be. A 2x laid flatways in the buildup really doesn't contribute anything, the osb or plywood that people stick into beams doesn't contribute enough to matter. Glue makes it stiffer but not stronger and marginally. Screws are usually a bad idea, unless you use structural screws. Most deck and drywall screws are brittle where nails are ductile, the screw snaps unpredictably, the nail will bend all the way around itself without snapping. Drive a screw halfway in a board and count how many swats back and forth to break it, do the same for a nail.

The old saying among carpenters is "Deeper is cheaper". For bending strength the first thing you look at is the "section modulus" of the beam, the equation is breadth x depth squared/ 6. The squaring of the depth is the key there, depth builds strength fast. A 4x6 and a 2x12 have the same area, the section modulus of a 4x6 is 24"3, the Sm of a 2x12 is 48"3. For the same board footage, just because of the depth, the 2x12 has double the bending strength.

For the header, in #2 white oak, a built up 8x8 works, or a 4x10, or a 3-1/4 x 12 which has the least deflection. The 8x8 has 5.33 bf per foot, the 4x10 is 3.33 bf/', the 3.25x12 is 3.25 bf/'... whenever possible a deeper beam builds strength fastest.
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Offline Kwill

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Re: Which oak is the strongest spanned?
« Reply #13 on: January 28, 2019, 07:52:14 PM »
I wasnt going to lay it flatways
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Offline Don P

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Re: Which oak is the strongest spanned?
« Reply #14 on: January 28, 2019, 08:28:26 PM »
I'm rereading your last post, gotcha, there is an 8x8 header, what are the 4 2x6's for?
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Offline Kwill

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Re: Which oak is the strongest spanned?
« Reply #15 on: January 28, 2019, 09:02:40 PM »
The 8x8 are the poles the header is going to be setting on. The 4 2x6 was going to be the header. Screwed and glued together with osb between each board. 
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Offline Don P

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Re: Which oak is the strongest spanned?
« Reply #16 on: January 28, 2019, 09:39:22 PM »
Barely fails in bending, ~2" of sag at full load, ~3/4" sag under self weight.
Idea, what they tell us is that stacking say 2 of the 2x6's above the other 2 2x6's is no better than nailing them all side by side. However, if there is 12" wide osb between them and if it is well enough attached along the top and bottom edges to take care of the shear, then it would begin to approach the performance of a 4x12. More plies of osb or ply with seams offset would improve on it. The box beams designs I've seen that is sort of along those lines call for nailing around the perimeter on about 4" centers.
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Offline sealark37

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Re: Which oak is the strongest spanned?
« Reply #17 on: January 29, 2019, 03:03:44 PM »
I would cut the white oak into 2x4 stock and build up a truss to measure 4'x16".  Just me Regards, Clark

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Re: Which oak is the strongest spanned?
« Reply #18 on: January 29, 2019, 05:31:35 PM »
That definitely doesn't work. Take a phone book in both hands, bend it, watch the pages slide past one another, that is the shear problem I was describing above in stacking timbers. The problem is very much worse if trying to restrain the shear in 2" layers. A glulam beam can do that with good machining, waterproof structural glue that doesn't creep, and hydraulic clamping pressure, factory conditions.

If it were me I'd forget the hassle and just go to the woods and find a tree capable of making the beam. That's the beauty of having a sawmill.
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Offline nativewolf

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Re: Which oak is the strongest spanned?
« Reply #19 on: January 29, 2019, 07:36:45 PM »
@Don P Always an education.  After about the hundredth time of you explaining I am beginning to follow.  Depth is the key on these span questions it seems.  Really does seem like if you have a sawmill you just mill up the 12" deep beams from the right logs.  Heck just buy the right logs if you don't have a few hundred acres of woods, some great beam wood is pretty cheap.  I'd sell the WO today and buy 4 nice poplar or hickory.  
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Offline Don P

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Re: Which oak is the strongest spanned?
« Reply #20 on: January 30, 2019, 07:37:43 AM »
Thanks, but, umm, I read sealark's post too fast. What he proposed is actually a homemade truss rather than a homemade glulam beam. That is a viable option. On the apawood.org website there is a paper on box beams made of dimensional lumber and plywood skins on both faces with serious nailing around the perimeter. There is an old thread here, radar66 built one about 10 years ago. I think he came to the same conclusion in the end, it was a lot of work.
Off to put out fires, and light one to work by, both my pickups layed down yesterday, bummer!

Edit;
on the apawood.org website search for z416. You'll need to register but they don't spam
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Offline nativewolf

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Re: Which oak is the strongest spanned?
« Reply #21 on: February 01, 2019, 07:20:17 AM »
@Don P 

Good luck with the trucks.  Southern trucks don't like the cold either I guess....but chewing metal is a bit beyond decency. 
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