BMP - Tree Risk Assessment - 56

Bending Moment (M) = Force (F) × Lever Arm Length (L)
The most common wind-related lateral force on a tree is called " drag. "
Because wind force varies with the square of the velocity of the wind, a small
increase in wind speed can result in a large increase in drag force. Wind speed
changes with height above ground, so taller trees and trees at higher altitudes
are subject to higher winds.
Drag is also dependent on the wind-intercepting surfaces (frontal area
and volume of crown facing the wind); wind resistance (drag coefficient)
of the tree; and the density of the air. Wind-intercepting surfaces and drag
coefficient of the tree change with wind velocity due to the streamlining and
reconfiguration of leaves, twigs, and branches. The degree of streamlining
differs among tree species and with individual trees. Within a range of wind
speeds, the higher the wind speed, the more streamlining.
Wind energy needs to be either dissipated within the tree or transferred
into the roots and soil. At low wind velocities, leaf and twig movement
dissipates most of the wind energy. As wind speed increases, progressively larger branches move. When wind forces are large enough to bend
the trunk, a greater amount of energy is transferred to the soil via the
root system.
Dynamic movement also affects the loads. Gusts and turbulences generate loads on the tree at different frequencies and intensities, causing a
complex dynamic reaction. Wind moves branches in different directions in
a seemingly uncoordinated fashion. This movement acts to dissipate wind
energy and slow the movement of larger branches in a process called " mass
damping, " which results in reductions in trunk loading and oscillation.
Branches and twigs with different diameters and lengths add to damping
by moving at different speeds with different loads. If a tree lacks interior
or lower branches, there is less damping within the crown, so more force
is transferred to the trunk. This can lead to a higher stress in the branches,
branch attachments, trunk, or roots.
When trees are close to other trees or structures, or have a normal branch
development within the crown, energy can be transferred or dissipated
when branches collide. Trees also may lose leaves, twigs, and branches in
high winds, which further dissipates energy and reduces wind resistance by
providing a smaller frontal area. Branch loss also may adversely reduce the
amount of damping and branch collisions that occur.

BMP - Tree Risk Assessment

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