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The term “two-force member” is really a misnomer, as there is actually only “one” unknown magnitude of a force involved. There are two forces acting on the member, however, they have the same magnitude but opposite directions.
What makes problems with two-force members present easier is that you only have to do equilibrium on such members once. Since two-force members must be loaded only at their ends, and their end connections must be pin connections, there are in general 4 unknown forces. However, you only have 3 equations of equilibrium for each member. Applying these 3 equations and doing a little algebra, reduces the 4 unknown forces down to only 1 unknown force. And stated earlier, this unknown force acts at each end, however in opposite directions.
Note that if you do not recognize that a member is a two-force member, all is not lost. Proceed as you would normally and at some point you will most likely realize one or more of the members are two-force members. And in the process, your understanding of the overall structure will be increased. – Dr. Tom
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