LOAD ON DERRICKS

273

Example.—Assuming the same conditions as in the previous example,
viz , derrick 48 feet long, span 24 feet long, led through a span block
on the mast shackled to an eyebolt 40 feet above the heel of the derrick.
A single fall is led down the derrick through a leading block to a winch
as shown in Figure 17. Find the thrust on the heel of the derrick and
the tension on the span.

TONS

AD i*

Fig. 17.—A Single Cargo Fall.
(i) In parallelogram ABDC, AB is the load (4
the resultant load on block A (7| tons).
(ii) In parallelogram ADEF, AD is 1\ tons, DE is the tension on
the span (2-| tons), AE is the thrust on the derrick (8| tons),
(iii) In parallelogram XGEH, XG^XH the load (4 tons), XK is
the stress on the heel block (5J tons).
(iv) In parallelogram YLNM, YL=YM the tension on the span
(2£ tons), YN is the stress on the span block (3| tons).
First, consider the stress at A in the figure due to the load and the
tension on the fall to hold the load.