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Author Topic: Tree strength  (Read 626 times)

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Offline TroyC

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Tree strength
« on: October 24, 2021, 09:21:11 PM »
Let's say I have a 32' YP log with a 12" top and 24" bottom ( hypothetically). Then I buck log into 2 16' logs that will fit my mill. If I cut 8"x8" beams from the center of each log and center the pith on each, would there be any difference in strength between the trunk section vs the top part of tree? Just curious because the top 16' would weigh less getting it up on the mill.

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Re: Tree strength
« Reply #1 on: October 24, 2021, 09:52:07 PM »
@Don P can probably give you a very detailed answer but I don't see there being a difference given you are mostly working around juvenille wood.  
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Offline Runningalucas

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Re: Tree strength
« Reply #2 on: October 24, 2021, 09:56:23 PM »
Great question, never thought about it.  What I've seen, and considered when sawing larger cants, is whether the center of the log is rotten or not.  I've sawn some Western Hemlock that looked great, but in the butt log all the way up to 20' can have large portions of the pith rotted out.... Hence, why when building where larger beams are necessary, I usually build laminated beams.
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Offline beenthere

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Re: Tree strength
« Reply #3 on: October 24, 2021, 11:01:23 PM »
Sans any decay/rot in the butt log, would expect an 8x8 in the first 16' to be quite similar to the 8x8 in the second log.
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Offline Don P

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Re: Tree strength
« Reply #4 on: October 25, 2021, 09:28:53 PM »
I agree with the others for the first couple of logs. With boxed heart you are pretty much dealing with the same core wood in either log. As the tree gets both older and taller it puts on different density wood.

From Haygreen and Bowyer
Quote
It is common for density to vary significantly within the tree.

In many species butt logs tend to have higher density than logs cut from higher in the main stem.. However in some woods such as tupelo gum and yellow cypress the wood near the base of the tree may be lighter than normal wood. Generally, in softwoods the density decreases with height and increases with distance from the pith. In large softwood logs the density often increases outward from the pith and then reaches a fairly constant level.
At the end of the day as long as the density falls within "normal" and a stick by stick grading I'd bet would show more variability, random distribution of defects, than whether it was an upper or butt log.

Something interesting to watch on the next whack or two.
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Re: Tree strength
« Reply #5 on: October 25, 2021, 09:53:59 PM »
Would the juvenile wood be any bigger diameter in the lower log than the upper log? I have noticed that some pines have much more juvenile wood than others.
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Offline Don P

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Re: Tree strength
« Reply #6 on: October 25, 2021, 10:13:46 PM »
I was wondering the same thing, I need to study some ends closer. Most beam bending stress is carried on the outside. It would certainly be better if those faces were more than say 20 or so rings out from the pith so there is a strap of mature wood. That is not addressed in any grading that I'm aware of but juvenile wood does break lower.

In a graph in the book showing yellow poplar density by #of growth rings and height, by the time the tree was about 35' tall it was putting on higher density wood at a much lower ring count, which says to me that juvenile core is getting smaller. The widest area of lower density so I believe juvenile wood was around 16' up. So by that graph YP was getting the largest juvenile core in both of the first 2 logs. It would be interesting to see that graph for other species... but trees have an annoying habit of not studying, every one does its own thing  :D.
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Offline Ianab

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Re: Tree strength
« Reply #7 on: October 25, 2021, 11:50:27 PM »
Would the juvenile wood be any bigger diameter in the lower log than the upper log? I have noticed that some pines have much more juvenile wood than others.
The juvenile wood is mostly based on years, so the amount would depend on how fast the tree was growing. A young tree might be growing faster and putting on more diameter in the first years, when the core of the butt log was first being formed. Later it gets more competition and the top logs might grow slower?
No research to confirm this, but I don't think it really matters. The design tables for any materials always have a good safety factor built in. If you are allowing a 2X safety factor, and the beams are +/- 20% in strength, well that's one reason why the safety factor is built in to the calculations. 
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Offline Don P

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Re: Tree strength
« Reply #8 on: October 26, 2021, 07:51:00 PM »
I believe I've read somewhere that various forms of reaction wood can be off by 40% so it does bear paying attention to.

I'm curious and haven't geeked out today. First lets see how much load a 16' #1 YP 8x8 can safely support. Looks like max allowable load in bending is 3100 lbs

Then I'm going to imagine drilling a hole down the center of the 8x8 all the way through its length and see how much calling that core nothing reduces the capacity.

Hmm, this is kinda cool. Drilling out a 4" hole down the center of the length of an 8x8 reduces the section modulus about the same as if it were a 7-3/4" x 7-3/4" beam... like I said, the core isn't really doing the major work, think about the shape of an I beam and what that says about where the stress is in a beam. Allowable bending load 2825 lbs. By removing a hole 4" in diameter through the beam it loses 175 lbs capacity. But also in there, if you undersize a beam just 1/4" scant it can drop its capacity a couple of hundred pounds.
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Offline jpassardi

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Re: Tree strength
« Reply #9 on: October 27, 2021, 07:29:25 AM »
You're right on Don. The ability of a beam to carry a bending moment and deflection is affected most by the outer-most top and bottom fibers. Obviously material type also. And this is the reason I beams are made as you noted. The flanges do most of the work. They get you the most moment of inertia for the buck.
Without getting too Engineer geeky:  :-\ An I section will carry significantly more moment per weight (cost) compared to a rectangle but an I shape will reduce the shear capacity. Shear capacity seldom dictates in most beams though. Bending moment and deflection typically govern especially as beams get longer.
The longer and more slender the beam the more susceptible to lateral buckling along it's length and this is why you see lateral bracing along their span.
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