Mechanical Grading of 6-Inch-Diameter Lodgepole Pine Logs for the Travelers' Rest and Rattlesnake Creek Bridges

Forest Products Laboratory (FPL) assistance was requested in mechanical grading of logs for two cable suspension bridges intended for pedestrian use in parks near Missoula, Montana. Two hundred ninety two lodgepole pine logs were obtained from a beetle-killed stand near Elk City, Idaho, by Porterbuilt, Inc., of Hamilton, Montana, and machined (dowelled) to a constant diameter of 6 in. The logs were visually graded by Timber Products Inspection, Inc., of Longview, Washington, and mechanically graded by FPL staff using procedures developed in previous research. Of the logs selected, 236 (80.8%) made at least No. 3 visual grade and were thus eligible for mechanical grading. Two hundred fifteen of the logs (74% of total) made a mechanical grade having an allowable bending strength–MOE assignment of 2150 Fb–1.4E, 21 of the logs (7%) that failed to make this mechanical grade could still maintain the assigned visual grade, and 56 logs (19%) would have to be used in non-structural applications. By comparison, of the 236 logs that made at least No. 3 visual grade, 188 (64%) graded as TP No.1, which is assigned allowable properties of 1250 Fb–1.1E. Thus mechanical grading is shown to provide a more efficient property assignment than does visual grading.