UKTC Archive

Re: Non invasive cable brace

Subject: Re: Non invasive cable brace
From: Scott Cullen
Date: Dec 28 2006 10:40:38
SC insertions....

----- Original Message ----- 
From: Jonathan Mills 
To: UK Tree Care 
Sent: Wednesday, December 27, 2006 7:22 PM
Subject: Re: Non invasive cable brace

I think that predicting the dynamic movement in a living structure is
very difficult. 

SC:  There are a very few researchers working on this.  Ken James at U. 
Melbourne and Mark Rudnicki at UCONN come to mind.  It is very easy to 
overthink this.  We know that certain branching and fork patterns tend to 
fail and we know that PROPER cable bracing reliably reduces that risk of 
failure with relatively minimal corollary risks.  Experience shows this.

Plus the added factor of weather makes the whole thing
appear unscientific. Does any one know the exact force a tree takes
when wind is blowing with or with out leaves? 

SC:  There have been some instrumented tests.  Few with full size trees.  
Claus says vehemently that the wond force can never be reliably known, but 
well, er, except by using his method of estimating the failure stress of the 
main stem.  Divided by a safety factor.  The problem with this analysis it 
does not specifiy the wind speed at which a working load is assumed and 
safety factor is the ratio of working load and failure load.  Wessolly 
suggests the load is essential information and can only be known at a given 
wind speed.  This requires two other parameters: crown size or surface area 
and drag coefficient.  Claus suggests that crown size is not reliably kowable 
and that drag coeffcient is an unknown constant.  In fact drag ciefficent is 
not a constant and varies with crown characteristics.  And will vary widely 
in leaf and out.  That last variable is not really material since we would 
brace for the larger or in leaf load.

Those links will give you some reading material.  See the Wessolly paper in 
1989 Arboricultural Journal for the basic wind drag equation.  Wessolly says 
Claus is wrong and that wind load is quite estimable.  

Does this have to be
measured be the cable strength is decided? 

SC:  But wile both Claus and Lothar make noises about needing to know the 
force on a cable to size it, in practice you do not.  ANSI A300 Part 3 
specifeis cable size by stem size.  Ken James's research indicates most 
cables are way over-specced for actual loads

Considering it was only 10
years since Claus M demonstrated the living structure of trees
intervention at this level seems a bit crude. Not to say some lovely
trees have had branches secured for a very long time with bracing.

SC:  We, as an industry, have been doing this for 80-100 years and have 
learned by experience.  It works regardless of what Claus has recently taught 
us.  We may in fact improve what we do but rather than crude, I would say 
PROPERLY done cable bracing is an elegant solution.

Is bracing a bit of tree anthromorphism?  Is there a good book or referance?

SC:  Well the whole of arboriculture is anthropocentric. ISA publishes a Best 
Management Practices for Tree Support Systems as a suppliemnt to ANSI A300 
Part 3.  Both good references.  The new German ZTV gives standrads for 
e;astic systems.  And ISA is translating to English.

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