This paper presents an evaluation of the cable loading
support capacity of red alder, Alnus rubra Bong., Sitka
spruce, Picea sitchensis (Bong.) Carr, and western hemlock,
Tsuga heterophylla (Raf.) Sarg., tail trees. Capacity is
measured in terms of combined stress resulting from
compression and bending, rather than the traditional methods
of buckling or compressive stress alone.
Results from field tests to determine modLili of
elasticity, base stiffness values, and functions for moment
of inertia are presented to provide strength properties for
capacity analysis.
A two dimensional model with one guyline is used to
calculate the combined stress at points along the trees. In
addition to strength properties of each species, model
inputs include front and rear skyline angles, rigging
I
height, and the following guyline parameters: angle,
metallic area, unit weight, modulus of elasticity, and lower
end pretension.
The control calculations for each species are made with
the guyline angle equal to a rear skyline angle of 45
degrees. A
3/411 guyline with 100 pounds of pretension is
used, and the skyline and guyline are placed at a height of
30 feet, Given these conditions, it was found that a
skyline angle of about 15 degr.ees below horizontal maximized
combined stress per pound of skyline tension in alder and
spruce. An angle of about 10 degrees below horizontal was
found to maximize stress in hemlock per pound of skyline
tension.
Figures are presented which show that skyline tension
to a given level of stress may be a function of tree
diameter, if other variables are held constant.
Values for maximum allowable combined stress for each
species are set by adjusting published average values
downward. Calculations for 16 inch (diameter inside bark)
trees indicate that hemlock is able to withstand the
greatest skyline tension of the three species before
reaching its allowable stress, with alder and spruce
following in descending order.
A comparison is made between a 14 inch DIB Douglas-fir,
Pseudotsuga menziesii (Mirb. ) Franco, and a 16 inch DIB
alder, spruce and hemlock. Calculations indicate the
hemlock can withstand about 9% more skyline tension to its
allowable stress than the Douglas-fir. An alder slightly
over 17 inches DIB would be needed to support the same
tension, and a spruce with a DIB over 18 inches, which is
outside the range of field data, would be needed