Conversion of Concentrated Loads on Wood Crossarms to Loads Distributed at Each Pin Position

NE of the most important requisites in all fields of engineering endeavor is knowledge of the strength of materials. The development of testing machines and techniques to study the basic properties of metals, plastics and wood products to withstand breaking forces has been a distinctive achievement during the last half century. All materials, whether they be part of a bridge, a building, a shipping crate, a telephone pole or a crossarm on a telephone pole, break under an excessive stress. To have accurate knowledge of the strength of the millions of crossarms used to carry the regular load of wires, which are frequently subjected to the extra loads of wind and ice, is most important in electrical communication. When strength tests of crossarms are made, the information most generally sought is how great a vertical load equally distributed at each insulator pin hole will the arms stand. In the past many crossarm tests have been made by the concentrated load method, where the arm is either supported at each end and loaded at the center, or supported at the center and loaded at the ends until failure occurs (Fig. 1, a and b). Some have been made by the distributed load method by placing, manually and simultaneously, 50pound weights in wire baskets suspended from each pin hole, and continuing such load applications until the arm fails. The method is objectionable chiefly because, in many of the tests, the loading is inadvertently carried past- the maximum loads the arms will support.