FORCES IN WIRE ROPE SLINGS USED TO PREVENT LOG LOSS ON STEEP SLOPES

Abstract

Wire rope slings are sometimes used in steep forest terrain to prevent a felled tree from sliding or rolling down a steep slope. The slings are choked around the ‘stump’ and the ‘stem’ and shackled together, after which a standard technique is used to fell the tree. A study was carried out to determine the magnitude of the forces that occur in these slings when the tree is felled directly downhill to help accurately dimension the wire rope sling in this worst–case scenario. Field trials carried were out on 25 spruce trees (Picea sp.) with breast height diameters ranging from 36 to 59 cm, with heights of 28 to 43 m, weighing 13 to 47 kN, and on slopes of 57 to 84%. The trees were felled without any hindrance directly downhill and the forces measured ranged between 20 and 108 kN. Data analyses showed only a weak but not–significant correlation between the maximum force recorded and the tree dimensional data. The value of the force integrated over the time period up to the point when the tree became static, the overall impulse, gave a better correlation to the breast height diameter of the tree (r2 = 0.53). Using multiple regression analyses both the average crown radius and the ground slope improved the overall model only slightly. The two–tailed 95% confidence interval upper limit for the maximum force from this data set was 94 kN, which means as a minimum 18–mm wire rope slings should be used.