SC: insertions follow
----- Original Message -----
From: Jerry Ross
To: UK Tree Care
Sent: Thursday, December 21, 2006 3:03 AM
Subject: Re: Non invasive cable brace
I'd tend to agree Scott, if only arborists (and tree-owners) wouldn't
specify cables where they're not needed.
The problem is that in perhaps 60% of cases (should that be 80%?)
they're fitted across forks that would be perfectly capable of
supporting themselves without the cable.
SC: Yes, I'd agree that some %age are really unnecessary. The smallest
%age may be the forks that are in imminent risk of failuer. Stated another
way, those that are likely top fail at fairly low loading, say winds with a
short recurrence period. Those are also the ones that a whole group of arbs
say should not be mitigated at all but that the tree should be removed. I do
not agree that that group should be 100% of the cable braces installed. I'd
say the largest %age group are "insurance." Just which ones that are not
"imminent" are or are not "perfectly capable of
supporting themselves" and under what loading. Take any storm. You'll see
failed forks that were not "imminent" that were worth saving.
Un-braced, the tree will happily accommodate increasing stresses
resulting from ongoing tip-growth.
Add a brace, no reaction growth and the tree becomes addicted to the cable.
SC: Well that's the "elastic" hype. If you saw deGouret Litchfiled's
presentation in York on Oct at AA, that includes a number of manufacturers
with a fairly consistent set of sensibilities and marketing pitches. This
business of putting in an elastic cable and the tree will fix itself by
laying dowmn reaction wood and so some day will mot be "addicted" or
dependent on the brace. Well isn't that cute? The problem with the logic is
that we developed cable bracing solutions in the firts place because we
observe that forks fail. They fail after having been able to move and sway
freely and lay down all that reaction wood. Huh? Well they still fail. So
in the most basic analysis we have a decsion: 1) remove the tree, 2)
tolerate the risk, or 3) brace it permananetly, i'e. "addict it." Well, is
the tree too valuable to remove? IOW do we want to keep it? What is the risk
of failuer and what are the targets? If those questions point to bracing who
cares if it is permanent?
SC: In may experience having installed hundreds, maybe thousands of steel
cables and having seen many more, properly (see following) installed steel
cables have a very low failure rate over quite long lives. Inspection
intervals should be a function of observed failure intervals not some
arbitrary CYA nonsense. Inspection inrevals should also be a function of
"risk of harm" as Mike would say. High value sites, hogh traffic sites are
the ones that get tree care anyway. Somebody is on that site or in that tree
with some frequency anyway. Just have a look at the cable and the
attachments. A knowledgable inspector can see most conditionds from the
ground.
Which may or may not have been fitted properly
SC: That is a problem. But do we as profession say "don't install any,
just remove the tres or tolerate the risks becuase some are not properly
installed?" So we say "don't manage trees becuase there are incompent
providers out there?" In the US we have ANSI A300-Part 3 which provides the
standard for proper installation and materials. In Germany there is the ZTV
Guidance [though it has been noted that a principal author may happen to be
an elastic cable manufacturer and golly it only allows elastic cables]. One
can argue the important inspection is the installation inspection to see in
it was properly to standard,
and may or may not be
maintained adequately.
SC: See paragraphs above.
But if a brace is justified, other than in special circumstances (e.g.
where a valuable decay-prone species really needs extra support), I'd go
for steel every time.
SC: In my experience cables are most typically specified to add
supplemental support to weak forks and have little to do with decay. O would
not spec a cable just becuase a species is "decay prone." I could see an
argument to brace where there already is decay, it's not just a specification
I'm familiar with.
SC
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