The mast of example 22 is fitted with cross-arms as shown in Fig. 44 con-
sisting of two channel irons (German standard section No. 10 = B. S. C. 3 approx.)
connected by bar iron 2| inches X -4 inch
section. The one-sided puU P is 1,240 lbs.
What stress are the channel irons sub-
jected to 1

No. 10 channel iron has a cross-section
of 2'1 square inches and a moment of
inertia J^, = -7 (inch)*. The distance of
the centre of figure from the neutral axis is
f inch and the resisting moment (or modulus
of section) W^ is 2-5 (inch)^.
The force P produces a tensile or compressive force in the two channel
irons of



-^S-

-^2'w- 10' — ;«--



I



Figf. 44a.
From Figs. 44 and 44a it can be seen that Z = 31 inches and e == llj inches.
k'



1,240 X 31

Hi ■



In addition to this the channel irons are also subjected to a twisting couple
due to the pull P of the wires on the insulators at a leverage of 10 inches.
This produces a bending moment in the channel irons with the leverage
Z = 31 inches and equal to :

ilfrf X I 1,240 X 10X31



M„



Hi



The total stress on one channel iron is therefore



P'l240lbs-mr







1,240 X 31 1,240 X 10 X 31



lU X 21



Hi X 2-5



= 15,200 lbs. per square inch.
Example 24.
Fiof. 45.
