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The Mechanical Properties of Junctions in Trees

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?. Debunking Stitching Fork geometry Inclusions.

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The Mechanical Properties of Junctions in Trees

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  1. The Mechanical Properties of Junctions in Trees Duncan Slater BSc BA(Hons) MSc PGCE MIfL MICFor AA Conference - Sept ‘11

  2. What’s the Forking Point? • Debunking • Stitching • Fork geometry • Inclusions Forks with included bark are prone to failure and can ruin the structure of a tree, and can cause damage to property or person

  3. Debunking

  4. Shigo’s diagram of branch anatomy

  5. Previous junction pulling tests

  6. “Medicalization of the fork”

  7. Uniform stress distributions FEA visualisation

  8. Trees still have lots to teach us! Strength of the fork = Geometry + Material + Probability of flexure

  9. Stitched up!

  10. Wood Density In general, wood is more dense at the join between two branches or limbs

  11. Wood Anatomy at Junctions Output of CT Scanning Images captured at the Henry Moseley X-Ray Imaging facility, University of Manchester

  12. Hazel Stem Fork Radial sections – images from HMXIF

  13. Oak Stem Fork Tangential sections – images from HMXIF

  14. Stitching… Dense, twisting grain stitches junctions together

  15. Tortuosity At the central apex of each junction, the grain is more twisted and tortuous, making it harder to split

  16. Quantifying the Effect The central region of a set of hazel forks was drilled or sawn out, to find its contribution to fork strength

  17. The shape of things to come

  18. Diameter Ratio Stronger Weaker A lower diameter ratio results in higher stresses at the apex of the fork when pulled apart

  19. Slimming down A rapid slimming down of the parent stem after the attachment results in higher stresses too Stronger Weaker

  20. The ‘Fillet’ The geometry of the top of the fork is very important in determining the stresses at the apex Stronger Weaker

  21. 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

  22. Thoughts included…

  23. 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!  )

  24. 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!  )

  25. Remodelling

  26. 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

  27. 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!

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