Summer limb/branch drop

[elicokiz]

Thought some of you may be intrestested in this article fromhttp://www.northcoasttreecare.com/H4-ProtectionWood.html

 

Sudden Branch Drop

 

Given enough time, most branches will break. What causes them to break before they are dead or obviously defective has been an ongoing question for many years. Branches have been known to “suddenly” break and drop (SBD). The current understanding of this phenomena is summarized by Costello( 8) and Harris, etal (9)

 

Shigo puts forth the hypothesis that SBD is the result of internal wood cracks failing due to dryness(10). As stated above, cracks are commonly inhabited by wetwood organisms. Shigo observed that trees with cracks and the associated wetwood seldom failed. It was only when the wetwood dried out that the cracks materialized into a stem or branch failure.

 

To test this hypothesis, he bent stems with artificial cuts to simulate cracks. It was only when heat was applied to the cuts (to force drying) that the stems broke. He admits this was a crude experiment, but that it would be refined and further tested by engineers at the Univ. of N.H. I have not been able to find out if this was ever done.

 

The other part of this discussion is that as cracks progress as a result of loading, the branch or trunk tends to split into two “beams”. One beam may slide or move over the other. The wetwood may act as a lubricant. If and as it dries out, the friction may be more than the two beams can handle and fracture occurs.

 

In a discussion of fracture mechanics, it is reported that plant tissues are more sensitive to water content and temperature (than metals). Also, fracture toughness across the grain can be 20 times greater than toughness parallel to the grain (11). These thoughts may add some background to Shigo’s work.

 

After cutting thousands of stem parts, it is obvious that internal wood cracks are abundant. What is not so obvious is why don’t more limbs break? This may be answered, at least in part, by the fact that wood typically fails gradually and retains overall integrity well into the realm of irreversible condition before it actually breaks apart. Partial failures may exist for a long time (12).

 

 

 

interestingly we had a Morus drop a lever branch mid afternoon on Friday, it failed at the branch union.

 

regards

Dave

[David Humphries]

Thought some of you may be intrestested in this article fromhttp://www.northcoasttreecare.com/H4-ProtectionWood.html

regards

Dave

 

Thanks Dave

 

interesting read & good to see cited material in the references

 

 

 

.

[Ty Korrigan]

I've just got back from a call out.

A client with HUGE pops covered in Gui (Mistletoe)

Several branches dropped within days of each other, no wind or rain pushing on them.

It's been 35c here too.

The limbs have twisted a little as they broke, all are heavy and long laden with dense clumps of Gui.

Strange thing is, half of each break is clean, almost like a silky cut clean.

Then the last 50% is a rip with long twisted fibres.

It's the second call out this summer for branches dropping.

The other was a very tall and beautifull macrocarpa such as I have never seen before.

It is in a semi-wooded area.

Single stem, total clean with no dead matter having been thinned by another arborist a while back.

All the weight at the end of the branches.

3 have cracked off falling across a right of way.

Regards

Ty

[Tony Croft aka hamadryad]

No one knows why it happens, yet.

There are some good theories on here.

 

My extensive studies have revealed that SLD hardly ever happens, and there's f-all we can do about it.

 

 

I was all serious about my reading of this till you made me laugh!

 

good thread

[treeseer]

Thanks Dave

 

interesting read & good to see cited material in the references

.

 

It would also be good if more of that material was more easily available!

 

I'd love to see the Costello piece; he's a fine analyst.

 

That was written over 5 years ago; Mr. Phillips might express less uncertainty on much of that today.

[Amelanchier]

But I understood SLD was associated with dry (but could however be high humidity) STILL hot days.

 

I would suspect humidity rather than temperature.

 

 

Sent from the Arbtalk app...

[openspaceman]

Strange thing is, half of each break is clean, almost like a silky cut clean.

Then the last 50% is a rip with long twisted fibres.

 

I'm no expert by any means but:

 

That's describing a typical greenstick fracture from over loading. The top part of the branch is in tension, the bottom in compression. Most of the strain is taken in tension but when it goes all the tension fibres separate at once hence the straight break, the bit that was resisting the compression now fails in bending mode, often splitting both ways along the centre line.

 

The pictures I've seen of summer branch drop don't exhibit this but lower branches hit by the falling upper branch often do.

 

What they do seem to show is a progressive delamination from a point, just like the glue between fibres has failed. I think lignin plasticises around 100 C (hence steaming to bend wood) so it won't be hot enough for that., some sort of differential drying would add local strain.

[Matthew Arnold]

This rain today is going to cause havoc on the hyperextended/sunseeking limbs. We've had 6 come down of vary sizes and a couple which look like then are gonna be on the way down overnight.

Edited July 27, 2013 by Matthew Arnold

[Gary Prentice]

I'd certainly be interested, but don't remember any specific SBD research from here,

there is the International Tree Failure Database(ITFD) from Berkley - CA, which is a data base for generic tree failures.

 

 

 

.

 

My bad. I found the article I was thinking about. Journal of Arboriculture 9(4): April 1983, Richard W Harris. In the states too.

 

Sorry, I'd forgotten how long ago that I'd read it.

[One Buck]

Not sure if this is the same thing but this happened in York yesterday.

 

 

 

Narrow escape for York shoppers as huge branch falls from tree (From York Press)