BMP - Tree Risk Assessment - 57

Loads on a tree lead to internal stresses. Stress is a force exerted over an
area, mathematically defined using the formula:
Stress = Force/Area
There are four basic stresses within a tree:
Compression is squeezing a material.
Tension is stretching or pulling a material, the opposite of compression.
Shear stress occurs when components of a material attempt to slide
relative to one another.
Torsion is a type of shear stress caused by a twisting force.
Stresses can occur alone or in combination. When gravity acting on a branch
pulls or bends it downward, the bending moment creates tension in the top
of the branch (fibers are stretched) while the bottom of the branch is under
compression (fibers are pushed together). In the middle of the branch, the
" neutral plane " experiences shear stress, where the fibers in tension and
those in compression meet and try to slide in opposite directions. Bending,
therefore, involves at least three stresses: compression, tension, and shear, and
may also have a torsional
component.
Torsional stress occurs when a branch, trunk,
or root twists, leading
to maximum stress near
Shear
the perimeter. Trees with
asymmetrical crowns
may experience an unTension
even wind load, resulting
in higher levels of torsion.
Compression
The stress formula
shows that stress increases either when there is
an increase in load or
when there is a decrease
Torsion
in cross-sectional area.
Cross-sectional area can
be decreased by cuts or
injuries to the trunk or by Figure A2-2. The four basic stresses within a tree are
internal loss of wood or compression, tension, shear, and torsion.
57



BMP - Tree Risk Assessment

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Table of Contents
BMP - Tree Risk Assessment - Cover1
BMP - Tree Risk Assessment - Cover2
BMP - Tree Risk Assessment - i
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BMP - Tree Risk Assessment - Table of Contents
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