On the mast = ^ ^ ^ X 24-5 X 175 = 560 lbs.
On the supports and insulators, say = 50 lbs.
The wind pressure on the lines and insulators acts at a height of 23 feet
above the earth, whilst the wind pressure on the poles acts at the centre of gravity
of these, or at a height of

15| + 2 X 10| ^ 24;5 _ ^^.^ f^^^
151+lOJ X 3 -^i^teet.
The effective moment acting at a height of 23 feet is therefore

{(625 + 50) X 23 + (560 X 11-4)} 12 = 263,000 Ib.-inches.
By equation 37 the diameter of pole at the ground level, assuming a stress
ij of 1,000 lbs. per square inch, should, therefore, be



r, „ ,^ 3/263,000 ,„ „ . ,
Z)„ = 215 sj-Y^ = 13-8 mches,

^^ £»„ = I X Z>„ = I X 13-8 = 9-2 inches.
OVERHEAD TRANSMISSION LINES 53

If the diameters found by the rule of the V. D. E. are used in place of these
the stress k^ will be less than 1,000 lbs. per square inch, thus :

,_, „ ,_ 3/263,000

15f = 215 ^ — j^ , or k^ = 680 lbs. per square inch.
Taking the maximum possible bending stress for pine to be 6,750 lbs. per
square inch, the factor of safety therefore works out

^^=10
680

The force P„ acting vertically along the axis of the pole is made up as follows :

lbs.
