Lesson 6 – Truss analysis
Two-force members
A truss is a structure that consists entirely of two-force members. A two-force member is a
structure that has exactly two points where external forces can be applied. This means that
the net force acting at those two points must lie along the line that contains those two points
of application.
Suppose the only two places that forces can be applied is at points A and B. Suppose the
net force acting at A and the net force acting at B are equal in magnitude and opposite in
direction. Even though the net force is zero, the body is not in static equilibrium because
there is a net moment that causes it to rotate (clockwise in this case).
So, if a two-force member is in static equilibrium, the net forces acting at A and B must be
equal in magnitude and opposite in direction and they must point along the lines that
connect A and B. So, they must point directly towards each other as shown above, or they
can point directly away from each other as shown next.
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