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Author Topic: Using oak for construction beams  (Read 2204 times)

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Offline Dionvh

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Using oak for construction beams
« on: October 26, 2018, 09:31:00 AM »
Hi there,

I am about to build a treehouse for which I want to use two beams as foundation for a platform. In the forest on my land there are some felled oak trees for which I don't know how long they have been lying there. The oak trees are about 70cm diameter and I would like to use them as the two construction beams bridging a space of 4 meters. They hold up quite some weight 2-3000kg, so I want them to be strong, that's why I am thinking of oak. 
I would like to start constructing as soon as possible so I don't have time to let them dry for months or years. So my question is, how would I know if I could use these oaks that are in my forest for construction beams? How bad is it if the oaks are still quite humid from the inside while already using them as construction beams? I don't care for cracks, but I do care about strength.

Is there anyone who could advise me in this?

Thanks,

Dion

Offline Don P

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Re: Using oak for construction beams
« Reply #1 on: October 26, 2018, 05:42:48 PM »
That sounds like better wood for siding or some other non structural use. Wood can actually lose a large percentage of its strength before we can visually detect it.
You could proof load the timbers, that is load them to double the design load.

A centered point load produces double the bending moment of a uniformly distributed load.
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Offline Brad_bb

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Re: Using oak for construction beams
« Reply #2 on: October 27, 2018, 12:54:42 AM »
Building with "green"  or "wet"  timber is not a problem so long as they are not sealed such that moisture cannot escape over time.  You don't want them in a sealed area where mold or rot can form.  

Whether they are still good or not will take some investigation.  Are they in the White oak family or red oak"?  White Oak is far more rot resistant due it's closed cell structure.  It will let moisture out in the right conditions, but will not let it back in.  Red oak will rot far more easily.  You'll have to cut some off the end and inspect for rot.  Cut back until you have clean wood.  Keep in mind that while the sapwood (newer wood on the outside) may be rotten, if it's white oak, the heartwood (mature darker wood) will likely not be rotted.  Checking or shrinkage cracks must be evaluated for use of the timber.  Are the checks continuous or discontinuous?  Is there spiral grain or straight grain?  Do you intend to use them as vertical posts or horizontal beams?Posts can have more defects and be a lesser grade wood than a horizontal beam.  Will you be sawing these logs?  If so, you can remove the rotten sapwood and use only the heartwood.
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Offline Dionvh

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Re: Using oak for construction beams
« Reply #3 on: October 27, 2018, 03:17:00 AM »
Building with "green"  or "wet"  timber is not a problem so long as they are not sealed such that moisture cannot escape over time.  You don't want them in a sealed area where mold or rot can form.  

Whether they are still good or not will take some investigation.  Are they in the White oak family or red oak"?  White Oak is far more rot resistant due it's closed cell structure.  It will let moisture out in the right conditions, but will not let it back in.  Red oak will rot far more easily.  You'll have to cut some off the end and inspect for rot.  Cut back until you have clean wood.  Keep in mind that while the sapwood (newer wood on the outside) may be rotten, if it's white oak, the heartwood (mature darker wood) will likely not be rotted.  Checking or shrinkage cracks must be evaluated for use of the timber.  Are the checks continuous or discontinuous?  Is there spiral grain or straight grain?  Do you intend to use them as vertical posts or horizontal beams?Posts can have more defects and be a lesser grade wood than a horizontal beam.  Will you be sawing these logs?  If so, you can remove the rotten sapwood and use only the heartwood.
Thanks for the info. I was not sure but I just looked up what family oak they are, it's called the English oak (quercus robur), as I am located on the spanish-french border. Do you have any info on this species regarding construction wood? I plan to use them as horizontal beams, spanning 4 meters. I will be bringing the logs away to be sawn into large beams. 
Thanks,

Dion

Offline Dionvh

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Re: Using oak for construction beams
« Reply #4 on: October 27, 2018, 03:18:59 AM »
A centered point load produces double the bending moment of a uniformly distributed load.


Thanks for your answer, I am not quite sure what you mean by this? 
I was wondering about the bending of the oak beam when it's still "green" and spanning a gap of 4 meters.

Thanks,

Dion

Offline LeeB

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Re: Using oak for construction beams
« Reply #5 on: October 27, 2018, 08:27:42 AM »
4 meters is not that large of a span. A 70cm diameter log is a pretty good sized log. Do you want beams for the look or do you feel you need them for strength? You would get all the strength you need from 30 x 15 beams if they are uniformly loaded. How you fix them to the trees would be a big issue. By milling them this size you could also get some flooring planks out of the deal. 
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Offline Brad_bb

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Re: Using oak for construction beams
« Reply #6 on: October 27, 2018, 08:55:57 AM »
We have no idea of your design-length and width or shape, or your planned method of construction.  We can't give any opinion on sizing etc without more info.  A sketch with some dimensions would be good.
Anything someone can design, I can sure figure out how to fix!
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Offline maple flats

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Re: Using oak for construction beams
« Reply #7 on: October 27, 2018, 09:19:35 AM »
Whatever you use for the main beams, be sure to allow movement where they anchor to the tree(s). One attachment should be fixed and the others dynamic (able to allow the tree or limbs or trees to sway in the wind.
As for oak, I can't help on the strength, but I can tell you that white oak that has been laying on the ground in an area where it is in standing water 3-4 months of the year and rots in about 2" is still hard and solid inside (good firewood. I have no idea what the strength might still be.) One thing that may help, is to make a 3 or more layer laminated beam, that will generally result in a stronger beam.
logging small time for years but just learning how,  2012 36 HP Mahindra tractor, 3point log arch, 8000# class excavator, lifts 2500# and sets logs on mill precisely where needed,  Peterson ATS upgraded to WPF mill, maple syrup a hobby that consumes my time. looking to learn blacksmithing.

Offline terrifictimbersllc

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Re: Using oak for construction beams
« Reply #8 on: October 27, 2018, 11:55:06 AM »
I built a treehouse which sat upon the frame shown in the picture below.  I would guess the house weighed about 4000 pounds.  This picture was made after the treehouse was taken down.  

The two horizontal cross-members in the picture are red oak, 15 feet long and 4" x 12" cross section.   The span is approximately the same as your planned 4 meters.    There was no issue of the strength of the timbers.  

The horizontal 15' 4x12 on the back side was held to the tree with a 3/4" stainless steel lag screw.   There was rot between this 4x12 and the tree.  There was little rot anywhere else, as this under-structure was shielded from rain and snow by the house.  

This house stood 17 years and endured several hurricanes.  The support frame was designed to move, which it did by an inch or two sometimes.  I took it down because we were not using it anymore.  But 17 years was good timing anyway, because of rot.  For a very long life the structure must be designed so that parts only get wet where they can also dry out quickly.  




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Offline Dionvh

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Re: Using oak for construction beams
« Reply #9 on: October 27, 2018, 04:37:32 PM »
I built a treehouse which sat upon the frame shown in the picture below.  I would guess the house weighed about 4000 pounds.  This picture was made after the treehouse was taken down.  

The two horizontal cross-members in the picture are red oak, 15 feet long and 4" x 12" cross section.   The span is approximately the same as your planned 4 meters.    There was no issue of the strength of the timbers.  

The horizontal 15' 4x12 on the back side was held to the tree with a 3/4" stainless steel lag screw.   There was rot between this 4x12 and the tree.  There was little rot anywhere else, as this under-structure was shielded from rain and snow by the house.  

This house stood 17 years and endured several hurricanes.  The support frame was designed to move, which it did by an inch or two sometimes.  I took it down because we were not using it anymore.  But 17 years was good timing anyway, because of rot.  For a very long life the structure must be designed so that parts only get wet where they can also dry out quickly.  



(Image hidden from quote, click to view.)

That's great info, thanks. Were the horizontal beams green oak? Am wondering if the green oak will bend in vertical directions.

Offline Dionvh

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Re: Using oak for construction beams
« Reply #10 on: October 27, 2018, 04:38:48 PM »
Whatever you use for the main beams, be sure to allow movement where they anchor to the tree(s). One attachment should be fixed and the others dynamic (able to allow the tree or limbs or trees to sway in the wind.
As for oak, I can't help on the strength, but I can tell you that white oak that has been laying on the ground in an area where it is in standing water 3-4 months of the year and rots in about 2" is still hard and solid inside (good firewood. I have no idea what the strength might still be.) One thing that may help, is to make a 3 or more layer laminated beam, that will generally result in a stronger beam.
Thanks! Yes I will use a treehouse attachment bolt and there will be some space between the beam and the tree.

Offline Dionvh

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Re: Using oak for construction beams
« Reply #11 on: October 27, 2018, 04:41:17 PM »
4 meters is not that large of a span. A 70cm diameter log is a pretty good sized log. Do you want beams for the look or do you feel you need them for strength? You would get all the strength you need from 30 x 15 beams if they are uniformly loaded. How you fix them to the trees would be a big issue. By milling them this size you could also get some flooring planks out of the deal.
I'm guessing that's 30cm x 15cm?
Not for looks, just for strength as these two beams will hold most of the weight of the treehouse. How long do you think those flooring planks will need to dry? Or could I use it right away as green oak?

Offline Dionvh

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Re: Using oak for construction beams
« Reply #12 on: October 27, 2018, 04:42:17 PM »
We have no idea of your design-length and width or shape, or your planned method of construction.  We can't give any opinion on sizing etc without more info.  A sketch with some dimensions would be good.
Will make a sketch tomorrow and add a photo of it.

Offline terrifictimbersllc

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Re: Using oak for construction beams
« Reply #13 on: October 27, 2018, 05:08:52 PM »
That's great info, thanks. Were the horizontal beams green oak? Am wondering if the green oak will bend in vertical directions.
I sawed them myself out of fresh logs. I don't remember the exact timing but probably the house was sitting on them less than 2 years after they were sawn out of the log. So since they were 4" thick they would have not been completely air dried before the house was put on them.  

Regardless, if I understand your concern, the question you are asking cannot be answered unless you give details of the weight and distribution of weight that will be placed on a 4 meter span.  Given that information, I am pretty sure another member here would be able to tell you what is the minimum cross-sectional area of a green oak beam which would support that weight without any concerns.

In my estimation,  each of the 4x12 beams I used supported about 150-175 pounds per foot along its length (the house was sitting on a floor made of 2x8 rafters which were evenly spaced on 16" centers across the 4x12 beams).  I believe, though I could be wrong, that these horizontal 4x12s could have supported many times this weight without sagging even if completely green.

p.s. now I remember you want to support 2000-3000 kg.  Is that on each beam or divided between the two beams?   Wondering if someone else knows how to calculate what size beam over a 4 meter span would have no trouble supporting such weight even if it were all placed in the center of the span??  Would a 6x12 (12 vertical) be plenty?
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Offline LeeB

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Re: Using oak for construction beams
« Reply #14 on: October 28, 2018, 09:12:58 AM »
Actually, somebody else does and he made it easy for the rest of us. DonP put a beam calculator in the toolbox. The 30cm x 15cm (6" x 12")beams I mentioned were actually figured for the full 3000kg load mentioned evenly spaced on a single beam so would be overkill. A 3x12 would would work. Somebody should probably check my figures though.

Ok,
Rechecked it myself and had input the numbers for a higher grade beam. Using numbers for mixed oak #2 timbers, a pair of 4" x 12" works. 

Design for Bending
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Offline LeeB

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Re: Using oak for construction beams
« Reply #15 on: October 28, 2018, 09:29:57 AM »
That sounds like better wood for siding or some other non structural use. Wood can actually lose a large percentage of its strength before we can visually detect it.
You could proof load the timbers, that is load them to double the design load.

A centered point load produces double the bending moment of a uniformly distributed load.
@Don P, let me see if I understand you on this. He wants to support 3000kg, assumed on even spacing. That would be 1500kg per beam on two beams. So if I read you right you are saying if you center load the one beam with 1500kg that would be the same as proof loading the whole beam to 3000kg evenly spaced?
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Offline Don P

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Re: Using oak for construction beams
« Reply #16 on: October 28, 2018, 10:37:28 AM »
I believe that would be a fair test, look at the link below. Study the moment equations in figs 1&7. A point load at midspan produces the same bending moment as double that load uniformly distributed.

https://awc.org/pdf/codes-standards/publications/design-aids/AWC-DA6-BeamFormulas-0710.pdf

This should all fall within the elastic range of the beam. When you are in the elastic range when the load is removed the beam returns to its original shape. Snap a line down the length of the beam, slowly load and then unload, repull the line, it should be the same.

I'm not very enthusiastic about this. This wood has been on the ground an unknown time. I can only assume years. Why was it left in the woods? This is a new sawyer, what is the grade going to be? This is a species we do not have design values for here, how are we coming up with allowable design values for a fresh timber much less a decayed one.

White oak is wood, wood will reach equilibrium with its environment. The notion that it is somehow sealed from the environment is incorrect. This wood is decay resistant it is not decay proof. Trees do not read books and books speak in generalizations and averages not individuals, the deposition of extractives is highly variable in individual trees.

If there is food there is a mouth that feeds on it. I don't know if you all have looked at micrographs of hyphae punching through cell pits and walls. Long before my eyes can see anything they have done a pretty good job of making swiss cheese of the fibers. A pick test is one method I use to roughly check in place decayed timbers, it is crude. I see no reason to use it on the supply side of things. Jeff put up a good pic the other day of a cant with both brown cubic and white rot very visible, how many people picked up on it? It was very far gone, if it made it into a beam the strength was a small fraction of published numbers.

When it comes to structure there is no good reason to use questionable materials, the cost of failure is high in human terms. I've had 2 floors collapse as I was working on old buildings. One broke on one end and dumped, I was sliding down the floor, oriented myself to hit the wall feet first and was feeling pretty good about things in the moment, then I heard cabinets, tables, chairs, china sliding behind me.

That doesn't mean the wood is firewood, use it in non structural places. Don't put yourself in a situation where you are trapped into using a questionable structural member. Look around, we are growing replacements every day, go get a fresh one :).

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Offline Dionvh

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Re: Using oak for construction beams
« Reply #17 on: October 30, 2018, 03:57:20 AM »
The felled oaks I want to use have not been lying there for long, they still have some old leaves on them. I'm guessing less than 1-1.5 years. They look like they're still in perfect shape, as they're also partly off the ground hanging in other trees and bushes. 

As for the bending, I'm not necessarily asking about the bending because of the load on it. I am wondering if it will bend on its self in a vertical direction, due to the wood humidity decreasing. On top of these 2 oak beams I am using other smaller beams and building a platform on top of that. I don't want the oaks to bend in a way that it will move my decking boards.

Thanks,

Dion

Offline Don P

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Re: Using oak for construction beams
« Reply #18 on: October 30, 2018, 07:46:12 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

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Offline mike_belben

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Re: Using oak for construction beams
« Reply #19 on: October 30, 2018, 01:10:47 PM »
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. 

Revelation 3:20


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