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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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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!)

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  • $\begingroup$ This is not correct. Pins don't exert torques , but even if we consider that the members were fixed in both ends, the truss wouldn't rotate at all! $\endgroup$
    – user98038
    Commented Dec 30, 2016 at 19:53
  • $\begingroup$ When a force by pin at one end of the member is not parallel to the member the force has a moment about about the pin at the other end. Since the force exerted by this second pin has to be equal and opposite to that exerted by the first, the result is a net torque on the member. $\endgroup$
    – mike stone
    Commented Jan 21, 2017 at 23:00

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