Printed from mechanics of materials stress m vable

Mechanics of Materials: Stress M. Vable
January, 2010
EXAMPLE 1.3
All members of the truss shown in Figure 1.12 have a cross-sectional area of 500 mm
2
and all pins have a diameter of 20 mm. Determine:
(a) The axial stresses in members BC and DE , (b) The shear stress in the pin at A , assuming the pin is in double shear.
PLAN
(a) The free-body diagram of joint D can be used to find the internal axial force in member DE. The free body diagram drawn after an
imaginary cut through BC, CF, and EF can be used to find the internal force in member BC. (b) The free-body diagram of the entire truss
can be used to find the support reaction at A, from which the shear stress in the pin at A can be found.
P 21 kN
A B
F E
D
G
C
2 m 2 m 2 m
2 m
Figure 1.12 Truss.
ANS.

1 9
Printed from:
Mechanics of Materials: Stress M. Vable
January, 2010
COMMENTS
1. We calculated the internal forces in each member before calculating the axial stresses, emphasizing the two steps Figure 1.2 of relat-
ing stresses to external forces.
2. In part (a) we could have solved for the force in BC by noting that EC is a zero force member and by drawing the free-body diagram
of joint C .
PROBLEM SET 1.1
Tensile stress