2/8 ELEMENTS OF ELECTRICAL ENGINEERING. 



in which S / is the maximum permissible fiber stress in pounds per square inch, W is 

 the resultant horizontal force in pounds acting on the insulator, and l f and d f are ex- 

 pressed in inches. 



The cross-arms on a corner pole are usually set so as to be parallel to the resultant 

 force due to wire tensions, and hence, except at the end of a line, this resultant force 

 does not produce a bending stress in the cross-arms. 



The unbalanced tensions of the wires produce a bending stress in the poles ; and 

 the diameter, d', of the pole at the ground and the height, I', of the pole, both in 

 inches, must satisfy equation (38), using for W f the resultant horizontal force due to 

 all of the wires. In most cases a corner pole is guyed or braced so that the bending 

 stress in the pole is to a great extent eliminated. 



(3) Stresses due to wind pressure vary with the direction as well as the velocity 

 of the wind. When the wind blows parallel to the line its effect is slight because the 

 wires are parallel to the wind. It is considered sufficient in practice to provide the 

 necessary strength to withstand a side wind giving a maximum pressure of from 20 to 

 30 pounds per square foot of surface, according to the degree of exposure of the line. 

 In calculating the force of a side wind on a cylinder like a pole or wire, the effective 

 exposed area is taken as two thirds of the product of the diameter of the cylinder 

 times its length. 



The effect of a side wind is to produce bending stresses in the insulator pins and 

 in the poles, and the dimensions of the pin in inches, as shown in Fig. 160, must 

 satisfy equation (38), where W f is the total force of the wind on the wire in 

 pounds, and S f is the maximum permissible fiber stress in pounds per square inch. 

 Also the height of I' of the pole and its diameter, </', at the ground, both in inches, 

 must satisfy equation (38), in which case W f is the force of the wind on all the 

 wires plus about half or two-thirds of the force of the wind upon the pole and cross- 

 arms. 



In estimating the stresses on pin, cross-arm and pole due to weight of wire and ice, 

 or the stresses due to wind pressure on the wires, a length of wire equal to the dis- 

 tance between adjacent poles must be assumed to be supported by each insulator. 



TENSILE STRENGTH OF TIMBER IN POUNDS PER SQUARE INCH. 



Cedar ( American ) n, ooo 



Chestnut 7,000 to 13,000 



Cypress 6,000 



Elm 6,000 to 10,000 



Oak 10,000 



Pitch pine 7,600 



Yellow pine..." 5,000 to 12,000 



White pine 8,oco 



Red wood (California) ii,oco 



Spruce 5,ooo to 10,000 



The usual factor of safety being 4 to 6, the permissible fiber stress in pounds per 

 square inch is from one-sixth to one-fourth of the values given in this table. 



