The Mechanical Properties of Junctions
in Trees
Duncan Slater BSc BA(Hons) MSc PGCE MIfL MICFor
AA Conference - Sept ‘11
What’s the Forking Point?Forks with included bark are prone to failure and can ruin the structure of a tree, and can cause damage to property or person
• Debunking
• Stitching
• Fork
geometry
• Inclusions
Debunking
Shigo’s diagram of branch anatomy
Previous junction pulling tests
“Medicalization of the fork”
Uniform stress distributions
FEA visualisation
Trees still have lots to teach us!
Strength of the fork =
Geometry + Material + Probability of flexure
Stitched up!
Wood Density
In general, wood is more dense at the join between two branches or limbs
Wood density (kg/m3)
400 500 600 700 800 900
Forkwood Stemwood
Wood Anatomy at Junctions
Output of CT Scanning
Images captured at the Henry Moseley X-Ray Imaging facility, University of Manchester
Hazel
Stem Fork
Radial sections – images from HMXIF
Oak
Stem Fork
Tangential sections – images from HMXIF
Stitching…
Dense, twisting grain stitches junctions together
Tortuosity
At the central apex of each junction, the grain is more twisted and tortuous, making it
harder to split
Quantifying the Effect
The central region of a set of hazel forks was drilled or sawn out, to find its contribution to
fork strength
The shape of things to come
Diameter Ratio
A lower diameter ratio results in higher stresses at the apex of the fork when
pulled apart
Stronger Weaker
Slimming down
A rapid slimming down of the parent stem after the attachment results in higher
stresses too
Stronger Weaker
The ‘Fillet’
The geometry of the top of the fork is very important in determining the stresses at
the apex
Stronger Weaker
Fork Geometry
The fork is weakened if it has:• An open-topped bark inclusion
• A poor fillet shape
• A low (near 1:1) diameter ratio
• A slimming profile after the attachment
CAUTION!
THE TREE CAN REMODEL ITS JUNCTIONS
Thoughts included…
Bark Inclusions – Why?Better at conductance of sap
(Wolf Trees)
Probably caused by phototropism
(Being drawn to the light)
Never felt much tension
(Won’t reinforce junction)
Not wasteful of resources
(Clever old trees! )
Bark Inclusions – How?We can cause bark inclusions to become a problem!
Growing trees very close together in sheltered locations will give rise to many bark inclusions; even more so if we breed them to be fastigiate, upright cultivars.
Then we complain when the junction snaps a few years later, after we thin or move it
(Poor old trees! )
Remodelling
My Best Guidance on Forks• The fork is adapted for the location in which it has developed. If it has been sheltered for many years, avoid suddenly exposing it to far windier conditions
• Forks with bark inclusions, a poor fillet shape, a low diameter ratio and that slim down rapidly after their point of attachment are not safe, where found in exposed situations and/or connected to an elongate limb
Acknowledgments
• Dr. Roland Ennos• Dr. Robert Bradley at HMXIF• Phil Benn & Mike Carswell• David Elwell & Mike Heys• Myerscough College• BSc. (Hons) students Joe Barnes,
Claire Harbinson & Ian Williams
Thank You!