Civil Engineering Reference
In-Depth Information
m
G
A
B
H
1m
E
D
C
F
m
W
W
W
2
2
6
1m
(a)
Shear force
W
2
ve
ve
W
2
(b)
ve
Bending moment
F
IGURE
4.10
Shear
forces and bending
moments in a truss
1.5
W
(c)
m
A
45
°
W
Internal shear force
2
F
AC
D
F
IGURE
4.11
Internal shear force
in a truss
m
The same result applies to all the internal diagonals whether to the right or left of the
mid-span point since the shear force is constant, although reversed in sign, either side
of the load. The two outer diagonals are in compression since their vertical components
must be in equilibrium with the vertically upward support reactions. Alternatively, we
arrive at the same result by considering the internal shear force at a section just to the
right of the left-hand support and just to the left of the right-hand support.
If the diagonal AC was repositioned to span between D and B it would be subjected
to an axial compressive load. This situation would be undesirable since the longer a
compressionmember, the smaller the load required to cause buckling (seeChapter 21).
Therefore, the aim of truss design is to ensure that the forces in the longest members,
