LOAD  Off DETIRTCKS

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(5) At the masthead make YL and yjf each equal to the tension
™   vSpaD, (°* t0nS) and comPlete tte Parallelogram YLNM    Join
XA, then YA represents the stress on the span block (144 tons)
The thrust on the derrick would be considerably reduced if the fall
were led from the derrick head to a lead block at the masthead and then
down to the winch as illustrated in Figure 19.
_ Assuming the same dimensions as before, viz, derrick 25 feet
height of span 20 feet up from the heel of derrick, length of span 18 feet'
Find the thntst on the derrick and the tension on the span (Fig 19) '
In the figure AB ,s equal to 10 unit-tons, AC to 5 urat-tons. Com-
plete the parallelogram ABDG. The length of AD gives the resultant
stress at A, 10J tons instead of 14 tons, as in previous example
Draw DE parallel to the span to meet the line of the derrict AE at
E Diaw EF parallel to AD In the parallelogram ADEF AE
gives the thrust on the derrick (12* tons) and AF the stress on the
span (4 tons) These measurements may be checked from the scale
on AB -with a pair of compasses
By leading the fall from the derrick end through a block at the
masthead instead of staught down the derrick as in Flgure 18, the
thrust on the heel of the derrick has been reduced from 17i to 12* tons
m tins example, and the stress on the span from 9J to 4 tons" thus
demonstrating the advantage of distributmg the stresses as widely as
may be convenient and practical.

Fig. 20.
Cargo Slings.—Example.—A beam 10 feet long and weigbino 20 cwts
• slung by means of a two-legged slug shackled to the ends of the beam'
Ihe legs are 6 feet long, find the tension on each leg