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Author Topic: Using oak for construction beams  (Read 2184 times)

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Offline Dionvh

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Re: Using oak for construction beams
« Reply #20 on: October 31, 2018, 04:44:14 AM »
That is sounding much better.
 The beams will not bend appreciably due to moisture content at that span if sized correctly, under full load it should still be less then 10mm deflection. As the moisture decreases the wood will shrink in width and thickness but not appreciably in length. If the grain is spiraled (look at the bark, you don't want to see a corkscrew) then the wood will want to twist as it dries. That is probably a greater concern than bending. Historically on longer spans with beams that are shallow in relation to length the carpenters would put temporary posts under the beams until the wood had dried. You really aren't in that situation. If concerned making the beam deeper (taller) decreases deflection rapidly.

Next, are the trunks from which you will make the beams relatively free of limbs or are these shorter large limbed trees?

What I'm doing here is the same thing I do when approaching a tree or timber for structural use. Step outside of your immediate need and really look over the candidate dispassionately. Be conservative and critical, always ready to walk away and find another. I've seen people adjust the grade and play all kinds of mental games to suit the immediate need. In the end the math equations are not what supports the load, the timber you choose is what really matters. From there we can size it using the math and you can make it larger than that for increased security if desired.

When you saw the beams the loads are carried on the top and bottom. The middle holds the top and bottom apart, think of how a steel I beam is constructed. The most critical section is the strap of wood along the bottom edge. It should be relatively clear grain, free of large knots, the grain running relatively straight along the line of the beam. What the bending equation is looking at is the tearing stress in the center of that bottom most strap of wood, what is called the extreme fiber, the fibers furthest from the neutral, center, axis of the beam. You'll hear the phrase "extreme fiber stress in bending", that is the strap of wood we are talking about. As the grain twists naturally or around a knot it is easier for that extreme fiber to tear as load bends the timber. As decay sets in those fibers break short and easily.

Enough rambling for this morning, my building inspector should be heading for the office by now, oh joy :D
The tree trunks are about 4-5 meters and free of limbs. 
So as the beam twists when it's drying, will it move so much that it would move the decking board?
Wondering if it will even dry that much that quick when it's outside just supporting the platform. It's not getting heat by inside warmth. The treehouse is placed in quite a humid forest so I'm guessing it will not dry quickly. 
Thanks

Offline Dionvh

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Re: Using oak for construction beams
« Reply #21 on: October 31, 2018, 04:46:38 AM »
Its a treehouse.  The prime question isnt "will it last 20 yrs?" ... Its "will i get it done before the kids are 20?"


My dad promised the perfect treehouse and never even started on it, thats the part i remember.  My kids treehouse is an eyesore but the fact that i built it is what theyll remember.  The memory of your relationship is what theyll be measuring.  Not if you got your spans and loads right.
The treehouse will actually be used for guests, so no kids involved. The goal is that it will be a quality cabin for people to sleep in.

Offline LeeB

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Re: Using oak for construction beams
« Reply #22 on: October 31, 2018, 05:41:47 AM »
Look along the length of the log. Does the bark spiral around the log or is it reasonably straight? If it spirals a lot, the beams are likely to twist. A picture of the logs would give us a lot of the answers to your questions. 
'98 LT40HDD/Lombardini, Case 580L, Cat D4C, JD 3032 tractor, JD 5410 tractor, Husky 346, 372 and 562XP's. Stihl MS180 and MS361, 1998 and 2006 3/4 Ton 5.9 Cummins 4x4's, 1989 Dodge D100 w/ 318, and a 1966 Chevy C60 w/ dump bed.

Offline mike_belben

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Re: Using oak for construction beams
« Reply #23 on: October 31, 2018, 09:46:08 AM »
Alright, that changes things.  Sorry i assumed. Carry on.
Revelation 3:20

Offline Dionvh

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Re: Using oak for construction beams
« Reply #24 on: November 03, 2018, 02:51:27 AM »
Look along the length of the log. Does the bark spiral around the log or is it reasonably straight? If it spirals a lot, the beams are likely to twist. A picture of the logs would give us a lot of the answers to your questions.
Okay thanks. Will try to make a photo asap.
Dion


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