HOW DOES A SHALLOW FACE INCREASE BARBER CHAIR POTENTIAL?

[murphy4trees]

Someone on Treebuzz started a thread called "Why 1/3 notch depth?" ... Why 1/3 notch depth? - https://www.treebuzz.com/forum/threads/why-1-3-notch-depth.46614/


I have said for years that is one of the rules that has largely gone unquestioned and unexamined in the industry (along with the 10% hinge width).
Another example of mass formation psychosis, with the overwhelming majority of the poppulation (in this case arborists) believeing and practicing according to what they have been taught, without understanding the "why". They just do as they are told and pass that on to the next generation, without any idea of the many factors involved in chosing an optimal face cut depth, and many not even having the faintest grasp of the broader principles involved. So almost no one has a complete understanding of the "why" of it. That includes the "instructors".

I've never seen anything in detail written about it, nor do I recall seeing any videos about the subject. One thing that I have heard and would consider common knowledge in the industry, certainly among experienced fallers (which there aren't may of at TB) is that a shallow face cut will increase the trendency for a barber chair. I've seen it myself and that's the kind of thing you only have to see once to realize the truth of and the increased hazards involved. But that still doesn't answer the "why" of it. And that of course is only part of the answer of why 1/3 notch depth.

When someone mentioned the increased tendency for barber chair at TB, there was some that discounted the idea and when Sean (southsound) asked HOW DOES A SHALLOW FACE INCREASE BARBER CHAIR POTENTIAL? no one had much of an answer. The two that tried were about incomprehensible. I was actually surprised that Sean had to ask. I thought there was more "common knowledge" on the subject on the west coast. Apparently not.

I published 3 videos on barber chair and as part of those productions I tried to purposely barber chair a number of trees, many of which never made it to publication. I actually learned the most from those trees that I tried to BC, but couldn't. IMO I undersatand the many complex factors that go into chosing a notch depth as well as anyone, and I can definitely answer the above question about shallow notches causing barberchair with a complete uderstading. I told the guys at TB that they don't have a clue, and they are just spouting nonsense, which didn't win me any points in popularity, but I'm just telling the truth!

Here at the house there are many with a much higher level of skill and experience in tree falling, than most of those guys at TB.
SO I would expect that you all can answer that question a lot better than they did. Does anybone want to take a crack at it?

 

[Bart]

General answer -not expert-

Balancing a few factors

placement of hinge pivot - gives 2/3 of trunk width as wedges lever arm - to not tensile pop the hinge fibres when wedging
- not a Reg over center magic cut that tips on it's own, presumed not back leaning, for vertical spar
- place hinge to balance location vs amount of hinge width you get to have i.e. kind of near trunk center is widest hinge

talking about front leaners knocks out the wedging rationale

small face on front leaner maybe lowers the shear forces by spreading the hinge and the release strap apart more

not comprehensive but perhaps a starting point.

 

[TINYHULK]

Couple other factors would be if the stem is being pulled on heavily by a tag line or has a heavy lean. By making a shallow notch you increase the potential for the wood to sheer and barber chair due to not having a lot of hinge wood and the hinge being so far forward. Barber chairs happen where the compression and tension fibers meet. if your having to cut through all of the tension fibers and then in to the compression fibers you leave to chance that there is a great enough force for the stem to split. Can always do some deep side cuts to stop the issue. Been doing some shallow notches with deep side cuts lately on this big oak tree I’m having to rig every piece down. Been successful so far

 

[Jomo]

Seems to me a deep face cut initiates a bend in the hinge quite a bit earlier than a shallow face cut.

That vital bend is totally absent in a barberchair.......

Jomo

 

[Bart]

I've never considered before that deep side cuts would focus the forces into the hinge perhaps to induce hinge fiber failure first, whereas a stronger wider hinge can transmit more (enough) force up the tree column to induce shear failure (barber chair split). If I understood it correctly.

You learn something new every day. Kudos.

But on the location of where tension fibers meet compression fibers, isn't that a moving target that shifts location as you cut? Ie if you formed a hinge in the fore half, the meet point would be somewhere within that wood and if you formed a hinge in the aft half a meet point would be somewhere within that aft half? Doesn't compression and tension have to exist all the time until hinge failure?

Or is the meet point of interest the original location before you start cutting?

 

[TINYHULK]

You bring up a good point Bart. I’m still learning aswell. My understanding of compression and tension is that they are always present based on the forces around it. But if you get a barber chair on a heavy leaning tree with a 1-3 face cut. That barber chair happened where compression and tension fibers met so the compression and tension fiber location did not change with the notch depth. Maybe that’s where a deeper notch is beneficial. By going deep enough you pass the stems compression fibers and fall into the tension fibers making it safer. For me I bore cut when I drop a tree and side cut on a limb that is heavy enough to barber chair. If I have an 8in dia limb I’ll cut 1.5-2in on each side matching my undercuts location. Kind of a cut and sweep maneuver so all of them line up. I think some people have called it a post cut. Basically your leaving a square in the middle of the round stem. If you think about where a barber chair could/would happen. By doing some side cuts the limbs barber chair would have to split from side to side. It can’t do that because all of the wood that was cut with the side cuts would act as a holding wood so it can’t barber chair. Easier example would be to watch some of August hunicke’s most recent videos. He does it a lot

 

[murphy4trees]

The split that causes a barber chair ALWAYS starts at the end of the back cut... The shear forces between tension and compression wood can open up a split on a heavy horizontal limb, but have nothing to do with a barber chair.

I asked the same question on FB and some guys were saying the same thing bout the split starting in the plane between compression and tension due to shear forces. That's nonsense... the plane between compression and tension changes as you cut. You can actually see that it exists in the hinge itself. There are ripped fibers under tension in the hinge and there are ripped fibers under compression in the hinge. The former looks all spiky.. the latter looks flat.. The back cut creates the back of the cut, which will always be under tension... so there is another widespread misconception exposed.

 

[Marc-Antoine]

I don't think that the tension/ compression thing is relevant in the barberchair. We see that frequently in the explanaitions, but each time, it itches me.

First, a thing to consider is making a difference between the concept of fibers built under a compression or a tension load and the actual force exerted on the established fibers.

The fiber's morphology is different from the start because they are constructed to fight the constrains exerted on the tree ( gravity, wind, ...) and so their own properties differs. This leads to some serious internal forces in the wood and often to prexisting cracks which are a big weak point in the structural integrity of the trunk. Some split apart just by bucking fire wood logs out of them. No question they will barberchair at the first occasion.

Now the load applied to the wood. During the felling process, the surface between the compressed area and the tensioned area (regardless of the fiber's morphology) is a moving frontier which can be or can not be related to the cutting front. All cases are possible. You can even have only compressed wood (tree in equilibrium on the uncut wood or on the hinge and a wedge on the back) or only tensionned wood (front leaner with a deep dutchman in the notch). In the last, the conflict between tension and compression isn't even in the critical area at all (cutting front) but somewhere between the hinge's front and the dutchman's edge. The dutchman is still a major cause of barberchairing.
An other clue is that the transition between tension and compression isn't brutal, like a toggle switch, but rather very gradual. The load is at the max on one side, decreases progressively, reaches zero, inverts and then increases again toward the opposite side, The variation is a constant slope across the considered section.

How I see it:
We can consider this as a bending problem. Under a side load, every piece of wood (or whatever you want) will bend by a certain amount due to its flexibility. This amount varies with the load, the physical properties of the material and the dimensions. Length = lever arm, width (side to side) and thickness (back to belly when loaded). The width doesn't do much, it's a factor one: put two lumbers side by side and you have to push only two times as much to bend them together by the same amount. But the thickness is an other matter, it comes in the formula at the cube. So a lumber twice as thick needs a push 8 times bigger. That's why you can build a ceiling with just some boards as a support instead of beams, as long as you can maintain them strictly vertical. Given this, back to the tree.
The tree (say front leaner, strong wind or a pull by a machinery) has its trunk flexing under the load, not by much, the full diameter is somewhat hard to bend. Start cutting. A small notch weakens the trunk, the flexing increases at the base, but barely noticeable by eye, the trunk is still plenty strong. Backcut. The load doesn't have changed much but as the resisting diameter begins to be reduced significantly, the flexing increases drasticaly (recall the cube in the formula). The deformation is very local, just near the kerf and cushionned by the wood elasticity. The bend in the trunk above stays the same. The remaining wood want to flex more and more due to the loss of its fibers. On the other hand, the already cut wood have no reason to bend anymore, beside being glued with the uncut wood. Even more, its own thickness increases as the cut progresses and it becomes more and more difficult to bend and to follow the local curvature of the uncut wood. Actually, it just wants to relax as if it wasn't loaded at all. At one moment, the spring back affecting the cut wood and the flexing of the uncut wood overcomes the strength of the "gluing"and both separate. It takes a while to show because the deformation is only local and gives a very short lever arm, thus it needs the accumulation of a lot of stress.
The upper half springs back from curved to straight, while the lower half increases its curvature radius and pushes the trunk's butt uppward. Some giant forces are at play here.
The split is progressive though. The dislocation starts just at the base and once initiated, the transition point is pushed forward along the trunk at a fast rate. The heavily curved part under it, the normaly curved part above, and, under too but on the back, the not curved at all part.


Well, I put way too much time to write this and I did nothing of what I wanted iinitialy. As often.
I hope it's understandable enougth, despite the faults too, I have yet to put the english dictionnary on my new pc.
Bed time.

Oh! and by the way, the horizontal split when cutting straight down the limbs is exactly the same phenomenon.
For the natural split of the limbs under load (no cutting), I have to think about it.

 

[Bart]

Marc, you are an edumacated dude. You should try to find the engineer video with the stress distribution diagram in the stem, just like a school assignment. He portrayed it as linear variation across the trunk width; probably correct if you reason it out. Real nerd stuff, but that's how it goes. There's Mohr to learn too (inside nerd joke)

 

[TINYHULK]

I would love to watch that video if y’all have any idea where it’s at or what it’s called

 

[Bart]

Bit of spare time. Found it.


edit - oops, that was the wrong one, though worth a watch for the basics. This was the video:


Nope. Third time's the charm:

 

[murphy4trees]

did you watch the third one???

He suggests cutting the notch on front leaners only 1/6th diameter, the using a plunge cut... that's only necessary when there is so much front lean that the bar might get pinched just making the notch, which is the exception rather than the rule.. The plunge cut avoids the BC, but we are still nowhere nearer to answering the question as Terry doesn't address it....


SO....
HOW DOES A SHALLOW FACE INCREASE BARBER CHAIR POTENTIAL?

any other input????

 

[murphy4trees]

I've never considered before that deep side cuts would focus the forces into the hinge perhaps to induce hinge fiber failure first, whereas a stronger wider hinge can transmit more (enough) force up the tree column to induce shear failure (barber chair split). If I understood it correctly.

I tried to point that out years ago to Burnham and others.. their belief was that the coos bay reduced barber chair only becasue it allowed the sawer to race through to holding strap between the two side cuts and that racing, meaning the speed of the cut, was the only factor that helped reduce barber chair. I rightly pointed out that by severing the sides of the log, there weren't enough fibers attached between the top and the stump to split the trunk... Just common sense when you think about it, but that assertion was met with incredulity. another example of something that's just seemed like common sense to me, that I had to explain to all the so called experts here. ONE OF MANY

 

[Burnham]

So...you HAD to explain it, eh? Well, let me relieve you of that responsibility, Murph...from now on, you should feel free to NOT explain anything ever again, hmm?

 

[murphy4trees]

So...you HAD to explain it, eh? Well, let me relieve you of that responsibility, Murph...from now on, you should feel free to NOT explain anything ever again, hmm?

you denied it was true at the time.. probably still in the archives.. one of the coos bay threads... should I look for it?

 

[theTreeSpyder]

i think can dial to conjure this beast from several factors to the same sum failure of BC, several ways.
>>Where massive force squares off against own mirror twin and neither gives way.
>>in high force ranges overcomes the wood container of the force overflowing the seams..
>>in lower ranges like topping small vertical or snapcut horizontal with same ratio-ed geometry w/o problem, in fact to great wins.
But not so much as raise the stakes to massive powerband beyond the wood limits as a 'container' of forces.
>>my whole view of a tree here is as a rigid weightless geometry, with the CoG as cannon ball size expression of all weight etc. placed at certain point.
The rest of tree just connects other forces to the CoG for forces to play out. CoG moves, then rest of passive tree gets carried around CoG type model.
.
For me the 1/3 rule always illustrated,
that then the hinge must be farther back than 1/3, and thus the rear tensioned fibers reaching more towards fatter center of stump area.
The wider the hinge the more side to side control, and thinner across for less 'buckling' in hinge.
A wide hinge starting at same pivot position vs. narrowed hinge at same pivot of compressed hinge fibers position, have both the same leveraged load so fold at the same strength in response. The side to side control can be accentuated by Tapered Hinge to the off-side(Dent term) of a sideLean.
Hinge goes from tensioned to compressed fibers gradually, so must be a switzerland of not stretched nor compressed fibers(just moved) fibers i think in the gradual/gradient slide across from compressed to tensioned; that is in range of leveraged distance multiplier between tension & compression torque on tree. The 'gradient' slide from compressed, thru switzerland to tension would be to a cosine scale of change, with outside extremes, center Natural; like a pendulum swing.
.
In hard forward lean bluff hinge forward of this thumbrule benchmark, in soft or back lean may bluff hinge back, each time chasing the CoG with the pivot of the compressed fibers towards CoG. But, really looking at an indvidual tree, stump might be more elliptical and if along the long axis to fall, then forward/backward making less difference to 'steering' in the 'straighter' side range of the non-pure round . Then too, any weakness, void etc. must be considered , per it's position too.
.
Extreme borders of high cos vs. high sine.
In any early close, have not built very much speed factor, that is the squared factor in E=MCsquared that helps enable throwing more positively forward thru obstruction.
In a shallow lean closing early can invoke this wall of massive downward force i think from very high cosine of linear gravity force downward
>>before sine cross-axis force can build very high to throw thru that wall, then X slower speed that also inhibits clean throw forward thru 'wall' of force.
While, at the other extreme of the paradigm; a hard lean with so much sine/leverage of so much force have to relieve it faster(i did by side trims or triangle left for backcut etc. anything to allow faster backcut as relief). And again, an earlier close before speed gained, to throw more positively thru, make things worser as a multiplier.
.
Between the 2 outer extremes/antagonists of too shallow and too deep of leveraged lean are all other types with hybrid mix of properties, depending on where in the 'gradient' between the 2 extremes/parents inheriting from. Then those range of effects adjusted each left or right on the scale of outcomes per wood species/condition i think in terms of.
.







Early close is a Dutchman of sorts/towards that range, with or without side relief. Kerf Dutchman being earliest close close extreme.
Full face Dutch, like accidental kerf HORIZONTAL of face within a face; leaves the close down strength of fiber columns gives BC most strongly w/o any forward relief just as aloft in smaller stuff this is how you invoke the greatest response; same stars you don't let align in falling type logic of different ends of the powerband. We are cutting across inviting split up the layers more than ever before, and with high impact of change. Dent's Step Dutchman close on CoG side and push across to relief of still open side, add Tapered hinge to accentuate both compression close as pivot and other extreme of tension pull in Tapered Hinge, both torquing in unison against the sideLean for Swing, distance between again is leveraged distance. But full face across close, no relief, just freight train of impatient force, most likely in felling as accidental face within a face of horizontal kerf bypass slanted cut, so close is along the most stacked resistance of fibers, not at cross angle of flex(like if slanted bypass horiz in standard hinge closes across fibers at more flex angle instead).
.
These are all own observations built on what hauntingly seemed to match of what little was handed down (1man chainsaw sold in 1950+ so not too much info)as also fit all other mechanical models walked with, and could not prove wrong.





Nice films, ty Bart.
i think the hinge power is in the competing extremes, and only compressed part of hinge is the pivot, tension is the hold, distance between the 2 as the leverage given to the competing extremes for greater torque against tree movement in life, then whittled away to hinge as same mechanics.
The pivot of hinge takes the downward force (cos) and the lean force across(sine) and the tension fibers ballast of that (sine again).
This makes the pivot definitively the most loaded point. This pivot will be to the CoG side (front to back) and end(left to right) as the most efficient place for Nature to put the CoG force in the hinge as a pivot of the mechanix.
The lean kickback of split trunk is as like to breach side of rocket launcher etc. as on the power axis of the fall.
The sapwood cuts to side/winged takes away the leathery strip than can squiggle around the side and throw off fall towards that side.
Deeper into center of tree is generally the greatest width extremes to control side to side/steering.
Plunge cut to me is rear trigger to then faster fall w/o having to cut thru the last bar width or more, so not too slow a cut cause BC.
i think of our local Live Oaks as growing slower then forms like the 'late wood' of normal trees, then has it's own even tougher version of this late wood.
Miracle steel was found to have equal tension = compression strength; while wood thumbrule is tension strength = 70% compression strength.
We came up with stone arch, because stone is only tension = 10%compression strength. Also thus need binder fiber or rebar for cement etc.
i always thought BC was named for the bend over and slide back like an old time brown leather barber chair at a barber's?

 

[Burnham]

Oh, Daniel...you really are dense at times .

 

[Bart]

Thanks Kenny. The guy's no guru but I give him credit for applying the engineering to the videos. Unfortunately, he didn't know about the quest for the sword in the stone and only made his videos for basic information. He's clearly new to arborist work (don't watch him climb). Best quantitative analysis I've found so far, though.

Did you get the Mohr joke? It would transform some stress if you did, in principal (axis!).