NEW STAXDARD SPECIFICATIONS FOR WOOD POLES 517 



load assuming that the load is applied two feet from the top 

 and that the break would occur at the ground line. 

 (d) The classes from the lowest to the highest should be arranged in 

 approximate geometric progression, the increments in breaking 

 load between classes being about 25 per cent. 



Item "(/" is in accord with the preferred number principle, and the 

 increments chosen provide the lowest number of classes that are re- 

 quired in service. 



Tables of ten classes for each species, as shown in Table 2, have 

 been made a part of the standard specifications. Classes 8, 9, and 10, 

 defined simply by minimum top circumferences, have been provided 

 to cover poles purchased on a top size basis or for rural or other 

 lightly-loaded lines. Classes 1 to 7, defined primarily by their cir- 

 cumferences at six feet from the butt, have been designed to meet the 

 following breaking loads in pounds, assuming the conditions of item (c) : 



Class 1—4500 Class 5—1900 



Class 2—3700 Class 6—1500 



Class 3—3000 Class 7—1200 

 Class 4—2400 



The required circumferences at the ground line for the respective 

 species were calculated by means of the formula Mr — .000264f C^, 

 which is the well-known flexure formula applied to a cantilever beam 

 of circular cross section, and reduced to foot pound units. The ground 

 line circumferences thus obtained were converted into circumferences 

 at six feet from the butt by means of approximate average taper values 

 for the respective species. 



The breaking loads are ratings for the minimum size pole for the 

 given length and class based on the standard ultimate fiber stress for 

 the species. The average pole of a given class will usually be consid- 

 erably stronger than the class rating. The choice of sizes provided 

 in the tables is sufficiently extensive to enable the engineer to make 

 an economical selection of poles to meet specific requirements after the 

 load conditions of the line have been determined. 



Graphical charts have been prepared which show the relation be- 

 tween the dimension tables of some current specifications and the new 

 standards. These charts should be of material assistance to suppliers 

 and consumers who wish to compare the old with the new for inventory 

 or record purposes. Representative blocks from the charts appear in 

 Fig. 1. Comparisons for all lengths and classes may be found in the 

 complete charts that are obtainable from the American Standards 

 Association. 



