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In Hibbeler's book, Engineering Mechanics: Statics, on pages 264-265 it is mentioned that two assumptions are made in the design of a truss:

  1. All loadings are applied at the joints
  2. The members are joined together by smooth pins

Hibbeler goes on to say that because of these two assumptions, each truss member will act as a two-force member, and therefore the force acting at each end of the member will be directed along the axis of the member.

But why do these two assumptions lead to this conclusion?

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2 Answers2

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Because pin supports by definition, do not exert torques-moments and also if the loads are applied only upon the pins there are not also toques-moments along the members of the truss, thus they experience only compression or tension forces.

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If the force were not directed along the member there would be torque about the pin, and the member would rotate. In statics rotation is not allowed (i.e the truss would collapse, and that is bad!)

mike stone
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