Think Safety Factor, not Strength Loss

To help stop unnecessary tree felling or damaging crown reduction tree work to your most important trees. We've updated and much improved our explanation of why t/R ratios don't work.

Why t/R ratios don't work

Think Safety Factor, not Strength Loss

Explains why you can't make a credible decision about Likelihood of Failure, or the risk, based on a residual wall thickness from, sonic tomography, micro-drills, or estimation with a sounding hammer.

Isn't t/R = 0.3 a guide? Or a starting point?
We've seen occasional comments like this on our social media feeds.

If Why t/R ratios don't work hasn't persuaded you, t/R = 0.3 doesn't work as a guide, or starting point. Here's some additional explanation about why.

t/R = 0.3 could be too much hollowness for a particular species of tree (Material properties). With particular crown dimensions and location (Load). And a particular stem diameter and geometry (Form).

Improve the Material properties, and t/R = 0.3 would be fine.

Or, lower the height, or have a more sheltered location (Load), and t/R = 0.3 would be fine.

Or, increase the section modulus by widening the diameter, and t/R = 0.3 would be fine.

t/R = 0.4 could be too much hollowness for a particular species of tree (Material properties). With particular crown dimensions and location (Load). And a particular stem diameter and geometry (Form)

...etc

And so forth, through every t/R ratio.

t/R = 0.3
A tree that's 50% hollow might be a tree with a good Residual Safety Factor.

t/R = 0.7
A tree that's 9% hollow might be a tree with a Residual Safety Factor that's too low.

Your critical t/R could be any ratio. It's just one part of the 'Form' in the tree Safety Factor puzzle.

When to use 'Why t/R ratios don't work'
You're welcome to use Why t/R ratios don't work to question tree felling or crown reductions when sonic tomography or micro-drill readouts are used to justify them.

Similarly, this will help you question media posts showing photos of stumps from felled trees, which are decayed or hollow, to justify the work.

Hollow stump porn

Sonic tomography, micro-drills, and sounding hammers can be a really useful kit. But, even when they're used well, they're only telling you a small part of the Likelihood of Failure story.