Fibre buckling or constriction - Opinions Please

[daltontrees]

At first glance I wold have guessed fencing wire. The tensile strength of wood is so much greater than its compressive strength that buckling occurring almost 50% of teh way around the stem seems unthinkable without an associated vertical shear, rather like a hazard beam partial failure. But on closer look at the photos, the raised ridge of wood seems to be resolving itself downwards in a slow curve on the right hand side, commensurate with shear.

 

Birch have a life fast die young approach. I think if the tree had been hollow it would have snapped under such force. A bit of buckling is trivial, but the tree might always have this weakness.

[Tony Croft aka hamadryad]

At first glance I wold have guessed fencing wire. The tensile strength of wood is so much greater than its compressive strength that buckling occurring almost 50% of teh way around the stem seems unthinkable without an associated vertical shear, rather like a hazard beam partial failure. But on closer look at the photos, the raised ridge of wood seems to be resolving itself downwards in a slow curve on the right hand side, commensurate with shear.

 

Birch have a life fast die young approach. I think if the tree had been hollow it would have snapped under such force. A bit of buckling is trivial, but the tree might always have this weakness.

 

weakness is not a term to be used in this case, or most buckles that have been bulged. Particularly in birch this is a very normal late developmental adaption. We dont see birches into their grand old age which can be much much longer than your average joe expects. Exercise some caution when applying terminologies that may elevate risks beyond the reality, we do it too much as an industry.

 

If youve never took the time to study ancient birch you would never know, an ancient birch is an incredible site and its form beyond imagination:001_cool:

 

An ancient birch has many many such buckles and twists, till so many have formed a latticework of 45 degree bulges appear, and it has its advantages, as do all the odd things trees do over the course of a long long life:001_cool:

[daltontrees]

Sorry Tony, but I'm not sure I follow. I said and meant weakness in the purely biomecnhanical sense, and the Birch's ability to keep compensating for this by adaptive growth means that it is probably not a source of elevated risk.

[Tony Croft aka hamadryad]

Sorry Tony, but I'm not sure I follow. I said and meant weakness in the purely biomecnhanical sense, and the Birch's ability to keep compensating for this by adaptive growth means that it is probably not a source of elevated risk.

 

I get you...

 

I don't personally get the shear idea though, not from a near horizontal arrangement, pure buckle IMO.

 

As you know Shear strength is the weakest link in the biomechanical properties, then Compressive and the strongest being tension. So shear is the obvious call:001_smile:

 

but shear is 45 degree or vertical, not horizontal

[Gary Prentice]

Tony, the trees probably 30-35 yr old, definitely no older.

[daltontrees]

i SUSPECT WE ARE TALKING ABOUT SLIGHTLY DIFFERENT THINGS. sEE THE ATTACHED FOR WHAT i MEAN. Sorry about the low quality drawing, I sketched it on an envelope and then thought' that'll do'. Hopefully Adobe will let you rotate the view.

 

In the first picture the wind force F is causing compression and tension as expected and as normal. In addition the centre of the stem is in shear as the dead weight of the tree, displaced over the centre of gravity temporarily, bears down on the compression side and pulls up on the tension side. What I meant was that a diametral crack (see second picture) could be started by this shear force. But once it causes separation of fibres, radial tension would ebb released and the crack would open for as long as the Force F remains. It would of course exacerbate the fibre buckling. If you turn the diagram suideways, it is basically like a hazard beam.

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[Gary Prentice]

I can follow that. There's a lot less literature, that I've found so far, on solid rod failure compared to hollow tubes/stems.

[daltontrees]

I can follow that. There's a lot less literature, that I've found so far, on solid rod failure compared to hollow tubes/stems.

 

It does seem hard to get any decent stuff about solid stems. There's plenty to be had about homogeneous rods, but I can't apply it to trees because they are very very definitely not homogeneous or isotropic. And that's just when they are in good condition!

 

Have you come across the Wagener (1963) strength loss formula that is featured at the back of Matheny & Clark's 1994 book? When looked at alongside the usual tiresome t/R ratio debate it seems to make relatively good sense and is closer to the engineeering formula for tube structures than anything else.

[Gary Prentice]

It does seem hard to get any decent stuff about solid stems. There's plenty to be had about homogeneous rods, but I can't apply it to trees because they are very very definitely not homogeneous or isotropic. And that's just when they are in good condition!

 

Have you come across the Wagener (1963) strength loss formula that is featured at the back of Matheny & Clark's 1994 book? When looked at alongside the usual tiresome t/R ratio debate it seems to make relatively good sense and is closer to the engineeering formula for tube structures than anything else.

 

Yes. oh yes.

 

Writing an assignment currently on strength loss formulas, a critical analysis. I've looked at Wagener, Smiley and Friedrich, Bartrams revised and Wessolly's SIM/SIA.

 

Wagener and the others use bending theory compared to Matthecks shell/plate buckling/kinking and shearing hypothesis. I was reading a NASA article at the weekend about shell buckling - for some unremembered reason, which was quite interesting, about building the lunar module.

[10 Bears]

Writing this now, I'm wondering if the adjacent high canopies may be causing an increase in wind speed rather than providing shelter!

 

Interesting thread. Getting back to this point you raised with regards to wind loading, this could be the root cause for the force "F" as shown in Jules' envelope diagram.

 

The Venturi Effect, is a fluid dynamics concept where liquid/wind is forced through narrowed gaps, taking the overall volume from an area of low pressure eg wide bore firemans hose, to high pressure eg narrowed pointed nozzle. You also have to consider flow/constriction rate and back pressure to understand it fully - but that's a bit much for this basic explanation. The net result though, is the same volume, under higher pressure and increased speeds.

 

So, without seeing the location in full or the surround area, your idea that the other canopies may be causing an effect is actually plausible. It could of course be exacerbated by building position and winds being funnelled through 'canyons'.

 

Having seen the area in detail - do you think this is a possibility as the source of the force?