LOADS ON DERRICKS

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Construction—Make ZTT equal to 80 feet, from a convenient scale,
and erect perpendiculars WX=70 feet and Z 7=100 feet. With
centre X and radius 50 feet on a pair of compasses describe an arc,
and with centre Y and radius 60 feet describe another arc cutting the
first one at A. Join AX and A Y. The 10-ton weight will be suspended
from A. AB is now drawn vertically upwards and made equal to 10
equal parts to represent unit-tons and not necessarily from the same
scale as before as the parallelogram would probably be too big. BC is
then drawn parallel to YA and BD parallel to XA The length of
AD measured in the same units as AB gives the tension on the pendant
A Y (9 J tons), and the length of AC the tension on XA (6 tons).

Fig 15—Derrick Apparatus.
Derricks.—A piece of school apparatus to demonstrate the thrust
on the heel of a derrick and the tension on the span is shown in Figure 15.
A given weight is seen hanging from the end of the derrick, its heel
being fitted into a circular weighing balance which registers the thrust
at the heel, the tension on the span being registered on a fiat balance.
The thrust on the heel of a derrick and the tension on the span
leading to the mast due to a weight hanging from the derrick end
may be determined by means of a parallelogram.
Example.—A derrick 48 feet long is kept upended by means of a
span 24: 'feet long attached to a point on the mast 40 feet vertically
above the heel of the derrick. Find the tension on the span and the