________________________________________________________________________
PROBLEM (11.9) The jib crane shown in Fig. P11.9 is of capacity W = 20 kN. For α =
30o and a factor of safety of ns = 3, calculate the required minimum cross-sectional area for
circular steel bar AB (E = 200 GPa).
SOLUTION
BC
F
20
0: 40
sin30
yBCo
FF===kN
kN
∑
0: 40cos30 34.64
o
xAB
FF== =
∑
W=20 kN
30o
B
AB
F
232
64
229
34.64 10 (3)(2.5)
, 0.329 10
(200 10 )
cr AB s
EI
PF I m
fL
π
π
−
×
== = = ×
×
So, 46
0.329 10 , 25.44
4
c
I
cmm
π
−
× =
2
==
22
(25.44) 2033Ac mm
ππ
== =
Justification of the formula used:
12.7rI A mm== 2500 12.7 197Lr
=
= O.K.
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