Conversion of Concentrated Loads on Wood Crossarms to 

 Loads Distributed at Each Pin Position 



By RICHARD C. EGGLESTON 



ONE 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 gen- 

 erally 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, 50- 

 pound 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. This objection was over- 

 come in recent tests made by the Bell Telephone Laboratories^, where the 

 loads were also distributed at each pin position. However, instead of sub- 

 jecting the 10-pin test arms to sudden 500-pound load increments (viz. 50 

 pounds at each of the 10 pin holes), the loads were applied gradually by a 

 hydraulic testing machine (Fig. 1, c). But, in spite of the advantages of 

 this machine method of distributed load application, it is probable that, be- 

 cause of the less elaborate apparatus involved in simple beam tests, there 

 will continue to be tests made by the concentrated load method. 



Where tests have been made by the concentrated load method, the ques- 

 tion arises how can the results be converted to a load-per-pin basis? A 

 conversion is needed before a fair comparison can be made of all test results, 

 and also to furnish the information generally most wanted, which is, as 



'Bell System Monograph No. B-1563, Strength Tests of Wood Crossarms. 



105 



