BMP - Tree Risk Assessment - 55

Appendix 2
Loads on Trees
When evaluating tree risk, the tree risk assessor needs to assess the
effects of expected loads on the likelihood of failure. " Load " is a generic
term describing the result of various forces acting on a structure. The two
natural forces that exert loads on trees are gravity and wind. Gravity acts
as a constant pull on the mass of the tree, generating static load from selfweight and the weight of water (condensation, rain, snow, or ice) on leaves
and branches.
Energy from wind exerts a dynamic (changing) force on leaves and branches
from friction and pressure. Wind forces within the canopy vary because wind
energy is transferred to the
leaves and branches as the
wind moves through the
crown, causing branches
to bend and twist. In addition, there may be loads
from climbers, rigging
operations, epiphytes,
or other sources. These
forces all result in stresses
and strains in the tree
structure.
Bending moments
are generated by lateral
forces acting on a lever
arm. Their magnitude
depends on the amount
of force and the length of
the lever arm. The center
of wind load can only be
estimated because of the
varying nature of wind
and the tree crown. The
Figure A2-1. Wind affects the entire tree crown
but is often calculated as the force (F) at a single
lever arm length is often
point acting at the center of pressure. The bending
approximated as the
moment (M) of the tree is calculated by multiplying
height of the center of
the resultant force by the distance from the ground
(L) to the center of pressure.
the crown.

F

L

M

55



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