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zero-force members are used to increase the stability of the truss
during construction and to provide added support if the loading is
changed.
The zero-force members of a truss can generally be found by
inspection of each of the joints. For example, consider the truss shown
in Fig. 1-75,a. If a free-body diagram of the pin at joint A is drawn,
Fig. 1-75,b, it is seen that members AB and AF are zero-force
members. (We could not have come to this conclusion if we had
considered the free-body diagrams of joints F or B simply because
there are five unknowns at each of these joints.) In a similar manner,
consider the free-body diagram of joint D, Fig. 1-75,d. Here again it is
seen that DC and DE are zero-force members. From these
observations, we can conclude that if only two members form a truss

Σ F = 0; F AB = 0
x
Σ F = 0; F AF = 0
y

a b

0 0
Σ F = 0; F DE +=
x
Σ F = 0; F DC sinθ = 0;
y
F DC = 0 since sinθ ≠ 0
c (d) d (c)
Fig. 1-75.

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