200year old cedar in Dorset..

[Paul Melarange]

How so?

 

Hi Tony

 

Static load tests enable an objective assessment of tree stability to be made. The tests result in the calculation of a factor of safety. A tree with a factor of safety of 1.5 (one and a half times stronger than it needs to be) or more would be considered as safe enough to be retained without further work. A tree with a safety factor of less than 1.5 would require either crown reduction (to lessen the wind load) or felling depending on the circumstances.

 

A static load test consists of three stages: stage one involves pulling the tree with a winch. During this procedure sensors positioned on the stem measure changes in the length of the wood fibres both in tension and compression. ‘The degree of compression or extension in the marginal fibres by the applied load is used as an indicator of the resistance to fracture’ (Detter, 2012) Tilting of the root plate is also measured.

 

The second stage involves wind load analysis. Details of expected wind speed and wind characteristics, for the area in which the tree is located, are obtained using published weather data. The trees resistance to streaming i.e. it’s surface area, porosity etc (the crown acting as a sail) is calculated.

 

The third stage involves the evaluation of the previous two stages using specialist software to produce a factor of safety.

 

The use of decay detection equipment such as Picus and Resistograph, although potentially very accurate, only enable a subjective management decision to be made. This is because the main criterion on which assessment of tree stability is based, using this equipment, is hollowness and the application of Mattheck’s rule: t/R ratio <0.3, which, as we know applies (loosely) only to trees with a full crown. So what about trees that have been reduced or that are retrenching? Difficult to know isn’t it? Just because a tree is hollow doesn’t necessarily mean it will fail. Furthermore, recent research indicates trees with a large diameter may be able to get away with having a residual wall that is significantly less than t/R ratio 0.3.

 

I didn’t have the opportunity to inspect the cedar at Kingston Lacy. However, based on Jeremy’s video, which included extracts from Mick Boddy’s report, I think that it is quite likely that a static load test would have provided the evidence necessary to justify the retention of the tree.

[Timbur]

My dad works for the NT at KL and visited at the weekend. He bought me a small section of the tree as we've chatted about it before.

He researches for the estate, nothing to do with felling!

It's a shame it's gone as it was rather stunning.

Tim

[treeseer]

Having used tomography the past 4 winters I can agree how the images can be shocking and misleading due to operator error, software issues, and flashy colors. Does anyone remember the jiggle that changed the fate of the Anne Frank Tree?

Frank Rinn, who develops and uses these tools, calls over reliance on such results "voodoo"

The Sep 2013 pic shows out was in need of pruning, maybe 10% off the crowded and overextended tips. The sight of that log confirms that condemnation was a horrible error. Why did it happen?

 

1. Too many consultant reports with mushy language full of CYA can be interpreted several ways. Edgy administrators will focus on the worst case imaginable, disregarding the rest.

 

2. Too many of us working arborists are skilled at rigging but not at retrenchment pruning. Dismantling uses big machines and earns big money now. The appeal of retrenchment works is too subtle and drawn out, not as exciting.

 

3. "Defect"-based formulas are deeply flawed. Target rating is overhyped. How many visitors are milling about the garden in a severe windstorm?

 

etc. In the US, the Freedom of Information Act (FOIA) enables citizens to wrest such reports from agencies that want it all hush-hush. Don't Brits have a right to see reports on public trees? Or are you still treated like serfs by the privileged classes?

 

All respect to Mr. Barrell for standing up and telling the truth. Please "Like" his video, and press for honest answers. Without looking at where this went wrong, it is bound to repeat.

Edited January 15, 2014 by treeseer

[Amelanchier]

Thanks Paul - I think we might have a difference of opinion as to what constitutes ''objective' insofar as whether one empirical test can be considered to be more objective than another.

 

How was the 1.5 safety factor determined?

 

In any case, I'm not sure the issue here was the availability of mitigating evidence...

[Paul Melarange]

Thanks Paul - I think we might have a difference of opinion as to what constitutes ''objective' insofar as whether one empirical test can be considered to be more objective than another.

 

How was the 1.5 safety factor determined?

 

In any case, I'm not sure the issue here was the availability of mitigating evidence...

 

Hi Tony

 

I think we may well have a difference of opinion as to what constitutes an empirical test let alone 'objective'! But I have no intention of getting into semantics.

 

You ask 'how was the 1.5 safety factor determined?'. I assume you mean why is it 1.5 rather than 1 or 2? Below is a quote from Andreas Detter's paper 'Static load tests in arboriculture'. He explains it perfectly:

 

'If the resistance against failure matches exactly the expected wind load, the factor of safety of the tree would be 1. But according to engineering standards any structure must have sufficient strength reserves beyond the expected loads. Due to the level of uncertainty involved in any numeric approach, a factor of safety of 1.5 is required in the static load test'.

 

I hope this answers your question?

[Pete Bannister]

Fascinating thread and some really good posts.

On the matter of the safety factors Paul M has quoted:

‘Due to the level of uncertainty involved in any numeric approach, a factor of safety of 1.5 is required in the static load test’

A 1.5 factor in structural engineering is tiny given the probable accuracy of data on the material properties. 1.5 is approaching aviation engineering standards where material properties are pretty rigorously measured. FOS of 4-5 would be typical of those used in static structures (e.g. buildings). It does depend on how you arrive at your FOS and there are alternative methods but Id like to know exactly how the 1.5 FOS is arrived at

Paul M’s recommendation for static load testing is very sensible and surely this was a case where empirical evidence should have been applied. But hindsight is a wonderful thing

That said, 'load testing' had just been undertaken by the recent storm. Maybe this had revealed evidence of progressive failure? I don’t know.

Edited January 15, 2014 by Pete Bannister
typo

[Amelanchier]

You ask 'how was the 1.5 safety factor determined?'. I assume you mean why is it 1.5 rather than 1 or 2? Below is a quote from Andreas Detter's paper 'Static load tests in arboriculture'. He explains it perfectly:

 

'If the resistance against failure matches exactly the expected wind load, the factor of safety of the tree would be 1. But according to engineering standards any structure must have sufficient strength reserves beyond the expected loads. Due to the level of uncertainty involved in any numeric approach, a factor of safety of 1.5 is required in the static load test'.

 

I hope this answers your question?

 

It does. Semantics notwithstanding, the threshold is subjective...

Edited January 15, 2014 by Amelanchier
politik

[treeseer]

It does. Semantics notwithstanding, the threshold is subjective...

 

And so sadly enough is the range of evidence to consider. Diablos ex Machina--use with caution, lest the mechanical tail wag the mental dog.

 

The study is BIOmechanics. Response growth needs to be considered, and anticipated The tree should have a say in the matter; its life is at stake after all.

 

Barrell's reduction work 30 years ago just needed to be repeated. Where did the idea that pruning would not be acceptable come from?

 

Edited January 16, 2014 by treeseer

[David Humphries]

Ther NT's statement on why they took the decision to remove the tree.......

 

National Trust's South West Blog – Duke of Wellington?s tree at Kingston Lacy

 

posted on uktc by David Evans and here by Tommy Hutchinson, but thought it should go in this thread also.

 

 

The most interesting part below, where it appears the 'aesthetics' were the overiding factor, rather than the 'decay'

 

"....we accept that we may not have explained fully that the felling was because of concerns of the tree’s appearance as part of the historic landscape garden at Kingston Lacy if it underwent extensive surgery.

 

It was a difficult decision which involved contributions from many experts both within the Trust and externally and different opinions regarding the long term potential of the tree were proposed. While not everybody was in agreement, it was felt that the major surgery needed would have spoilt the appearance of the tree and therefore the garden in which it was a feature."

 

 

 

.

[SamWhiting12]

Good find David.

 

What I don't understand is why they didn't want to prune because of the aesthetics!! Its a historical tree, the attraction to the tree is its history not what it looks like. One of their information boards by the tree explaining why it looked the way it did, would once again, provide the tree with even more history making the tree more interesting.

 

Its like saying a piece of war time machinery is too rust to show any more, lets just scrap it so its not our problem anymore.

 

Poor decision IMO