Load bearing capacity of alder, spruce and hemlock tail trees

Graduation date: 1985This paper presents an evaluation of the cable loading\ud support capacity of red alder, Alnus rubra Bong., Sitka\ud spruce, Picea sitchensis (Bong.) Carr, and western hemlock,\ud Tsuga heterophylla (Raf.) Sarg., tail trees. Capacity is\ud measured in terms of combined stress resulting from\ud compression and bending, rather than the traditional methods\ud of buckling or compressive stress alone.\ud Results from field tests to determine modLili of\ud elasticity, base stiffness values, and functions for moment\ud of inertia are presented to provide strength properties for\ud capacity analysis.\ud A two dimensional model with one guyline is used to\ud calculate the combined stress at points along the trees. In\ud addition to strength properties of each species, model\ud inputs include front and rear skyline angles, rigging\ud I\ud height, and the following guyline parameters: angle,\ud metallic area, unit weight, modulus of elasticity, and lower\ud end pretension.\ud The control calculations for each species are made with\ud the guyline angle equal to a rear skyline angle of 45\ud degrees. A\ud 3/411 guyline with 100 pounds of pretension is\ud used, and the skyline and guyline are placed at a height of\ud 30 feet, Given these conditions, it was found that a\ud skyline angle of about 15 degr.ees below horizontal maximized\ud combined stress per pound of skyline tension in alder and\ud spruce. An angle of about 10 degrees below horizontal was\ud found to maximize stress in hemlock per pound of skyline\ud tension.\ud Figures are presented which show that skyline tension\ud to a given level of stress may be a function of tree\ud diameter, if other variables are held constant.\ud Values for maximum allowable combined stress for each\ud species are set by adjusting published average values\ud downward. Calculations for 16 inch (diameter inside bark)\ud trees indicate that hemlock is able to withstand the\ud greatest skyline tension of the three species before\ud reaching its allowable stress, with alder and spruce\ud following in descending order.\ud A comparison is made between a 14 inch DIB Douglas-fir,\ud Pseudotsuga menziesii (Mirb. ) Franco, and a 16 inch DIB\ud alder, spruce and hemlock. Calculations indicate the\ud hemlock can withstand about 9% more skyline tension to its\ud allowable stress than the Douglas-fir. An alder slightly\ud over 17 inches DIB would be needed to support the same\ud tension, and a spruce with a DIB over 18 inches, which is\ud outside the range of field data, would be needed