Presentation on quantified risk assessments and management recommendations and when not to prune (most of the time)

[Island Lescure]

I found it an interesting read.

 

From the Treeworks website:

Crown Reductions: An Inevitable Response to Uncertainty

WWW.TREEWORKS.CO.UK

---

Scottish Arboricultural Association Winter Seminar and AGM - 25th January 2020 Paul Muir, Principal Arboricultural...

 

Pdf:

 

Share large files. Get comments. Make it great. | Hightail Spaces

SPACES.HIGHTAIL.COM

---

Hightail Spaces lets you send unlimited files, of any size, to anyone. Collaborate with your team and evolve your work.

 

[daltontrees]

I was at the presentation. And yes, static load testing (or more importantly the mathematical modelling that is used on the results) indicates that reductions are rarely the appropriate risk reduction response. There is a very narrow window between trees having enough extra strength that reduction is not necessary, and being so bad that removal is the appropriate action.

 

The presentation supported the issues relating to t/R ratios discussed on Arbtalk recently in response to an article by Frank Rinn. Essentially t/R as a predictor of failure is almost meaningless after a tree reaches maturity.IU asked Paul Muir afterwards if it was possible to correlate static load testing safety factors and QTRA probabilities of failure. He didn't really answer the question, but did suggest that if the SF is over 1.5 there is no risk of failure (P = 0) and if it is 1 or less then failure is inevitable (P = 1).

 

I thought about this before, during and after the presentation and I believe it is far from being that simple. However, to try and define the correlation would take a whole paper in itself and access to the mathematical model. I personally don't believe the whole range of probability of failures from 1/1,000,000 to 1/1 can simply be accommodated between SFs of 1 and 1.5.

 

But even if generally that simple, does this mean that there is no role for estimating probability of failure for QTRA assessments? Well, no. I expect a full static load test would cost several £00 a tree, which is rarely justified. And the modelling is based on some assumptions and inputs that aren't always appropriate. And in many situations it would not be physically possible to do the tests. The tests are also applicable only to rootplates and stems. And I don't think there is any means of taking account of prognosis.

 

It all comes down (for me anyway) to one issue which has not to my knowledge been fully resolved by UK case law and that is, at what point in extreme weather does the  duty holder (occupiers liability etc.) not have to be responsible for foreseeable harm or damage? Some would say (and Paul Muir alluded to this too) it is around mid Beaufort Scale 9.

 

And there is still a right time for reductions as a risk management response, but mainly when it is acknowledged that it will tip the tree into decline and when there is little alternative means to preserve some of the benefits the tree provides.

[daltontrees]

I just checked this, and the wind loading to be assumed for buildings (which is what Paul Muir was referring to) varies across the UK but for central Scotland it's about 54mph which is top of Beaufort Scale 9 (severe gale). The lowest UK speed to be assumed is around Oxfordshire 45mph, top of Beaufort 8.

26 minutes ago, daltontrees said:

It all comes down (for me anyway) to one issue which has not to my knowledge been fully resolved by UK case law and that is, at what point in extreme weather does the  duty holder (occupiers liability etc.) not have to be responsible for foreseeable harm or damage? Some would say (and Paul Muir alluded to this too) it is around mid Beaufort Scale 9.

 

[Island Lescure]

On 05/02/2020 at 15:38, daltontrees said:

I was at the presentation. And yes, static load testing (or more importantly the mathematical modelling that is used on the results) indicates that reductions are rarely the appropriate risk reduction response. There is a very narrow window between trees having enough extra strength that reduction is not necessary, and being so bad that removal is the appropriate action.

 

The presentation supported the issues relating to t/R ratios discussed on Arbtalk recently in response to an article by Frank Rinn. Essentially t/R as a predictor of failure is almost meaningless after a tree reaches maturity.IU asked Paul Muir afterwards if it was possible to correlate static load testing safety factors and QTRA probabilities of failure. He didn't really answer the question, but did suggest that if the SF is over 1.5 there is no risk of failure (P = 0) and if it is 1 or less then failure is inevitable (P = 1).

 

I thought about this before, during and after the presentation and I believe it is far from being that simple. However, to try and define the correlation would take a whole paper in itself and access to the mathematical model. I personally don't believe the whole range of probability of failures from 1/1,000,000 to 1/1 can simply be accommodated between SFs of 1 and 1.5.

 

But even if generally that simple, does this mean that there is no role for estimating probability of failure for QTRA assessments? Well, no. I expect a full static load test would cost several £00 a tree, which is rarely justified. And the modelling is based on some assumptions and inputs that aren't always appropriate. And in many situations it would not be physically possible to do the tests. The tests are also applicable only to rootplates and stems. And I don't think there is any means of taking account of prognosis.

 

It all comes down (for me anyway) to one issue which has not to my knowledge been fully resolved by UK case law and that is, at what point in extreme weather does the  duty holder (occupiers liability etc.) not have to be responsible for foreseeable harm or damage? Some would say (and Paul Muir alluded to this too) it is around mid Beaufort Scale 9.

 

And there is still a right time for reductions as a risk management response, but mainly when it is acknowledged that it will tip the tree into decline and when there is little alternative means to preserve some of the benefits the tree provides.

Thanks for your inputs! Looks like I had the wrong following preferences so only saw this today.

Did you mean "There is a very narrow window between trees having enough extra strength that reduction is not necessary, and being so bad that removal is the appropriate action."? So a tiny window where reductions are necessary/useful.

I am not doing many risk assessments these days so I am not keeping up to date with the changes in the methods but it was impressive to see the certainty of the decisions taken and the confirmation, for me, that reductions are generally not appropriate.

One thing I liked about the presentation was the consideration of factors other than just the tree condition such as if it is sheltered or part of group.

[daltontrees]

No I said " There is a very narrow window between trees having enough extra strength that reduction is not necessary, and being so bad that removal is the appropriate action." But that means what you thought it might mean i.e. it's either got a bit to spare and doesn't need reduction or has nothing to spare and reducing it might belp but will pushit into rapid decline and removal would be a better plan. 

 

That rather overlooks the retrenchment or last-gasp options that involve pruning, but I think we can all see that reduction as often specified by tree surgeons and consultants is either a work-generating option or a scaredy-cat reaction to defects which haven't been properly assessed. Many of these fancy-sounding VTAs that I see have quite simply not bothered to assess the strength of the remaining parts, which is after all in the core description of VTA.

 

Paul Muir admitted to there being imprecisions in the system, so it's not that decisive. Think in terms of a 50% margin of error.

[scottythepinetree]

Could someone explain how a tree safety factor is calculated? How do they get 1.5 or 15?

[jfc]

On 05/02/2020 at 10:40, daltontrees said:

I just checked this, and the wind loading to be assumed for buildings (which is what Paul Muir was referring to) varies across the UK but for central Scotland it's about 54mph which is top of Beaufort Scale 9 (severe gale). The lowest UK speed to be assumed is around Oxfordshire 45mph, top of Beaufort 8.

 

Hi Jules, I was at camperdown too, good day I thought.

 

Yes the wind speed he was quoting raised some issues for me. I think he was using 22m/s which is about 55mph. However that is a common wind (as the last week has seen). If i remember correctly he was talking about a 1 in ?50yr storm event as the cut off for the safety factor. So I don't understand how 22m/s relates to a 1 in 50yr storm, which would surely be nearer 40m/s. Have i missed something or got the wrong end of the stick?

 

I think this relates to your question about at what wind speed the owners liability is irrelevant.

 

jan.

 

[daltontrees]

On 13/02/2020 at 07:37, jfc said:

Hi Jules, I was at camperdown too, good day I thought.

 

Yes the wind speed he was quoting raised some issues for me. I think he was using 22m/s which is about 55mph. However that is a common wind (as the last week has seen). If i remember correctly he was talking about a 1 in ?50yr storm event as the cut off for the safety factor. So I don't understand how 22m/s relates to a 1 in 50yr storm, which would surely be nearer 40m/s. Have i missed something or got the wrong end of the stick?

 

I think this relates to your question about at what wind speed the owners liability is irrelevant.

 

jan.

 

Well, separate to what was said in the presentation, it is entirely possible that climate change might justify recalibration of what would be  considered a 1 in 50 year wind event. And I think it is only a matter of time before a court case challenges what is an acceptable threshold for duty of care and freseeability of failures. By rights there should be a review of wind loading for building design (BS6339-2:1997). If there was, the static loading assumptions would have to be adjusted.

 

Or to put it another way, trees that SLT results say are sound should be failing these days.

 

Attached meantime is my version of the wind speed map from BS 6339, I have converted the speeds from m/s to m.p.h. Paul Muir is based in Bristol (45mph) but Dundee is 52mph which is 15% higher. And speds need to be adjusted (for building design) for terrain, orientation, season and altitude, so there's nowhere near a one-size-fits-all speed.

mph.pdf

[daltontrees]

On 12/02/2020 at 15:29, scottythepinetree said:

Could someone explain how a tree safety factor is calculated? How do they get 1.5 or 15?

No, I can't. It is probably an immensely complicated calculation. But basically the tree is pulled a bit, and the lifting of rootplate and the bending of the stem are extrapolated by calculation to th amounts that would occur if the applied load was the same as the loads ina  1 in 50 wind event. These must be compared with critical values for tree failure. If the strenght critical values are 1.5 times the calculated deflections, there's a 1.5 safety factor.

[scottythepinetree]

On 17/02/2020 at 10:39, daltontrees said:

No, I can't. It is probably an immensely complicated calculation. But basically the tree is pulled a bit, and the lifting of rootplate and the bending of the stem are extrapolated by calculation to th amounts that would occur if the applied load was the same as the loads ina  1 in 50 wind event. These must be compared with critical values for tree failure. If the strenght critical values are 1.5 times the calculated deflections, there's a 1.5 safety factor.

Thanks for the reply. That would make sense (I think). It would be difficult to pull anything useful out of the presentation without knowing exactly how it is calculated beyond vague assumptions. Most studies would generally describe in detail the process so that results can be verified by others as repeatable. I wonder do they intend to publish this as a study at some point?