Why does an ideal truss member undergo only axial forces and not bending moments?
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Because that’s how the model works. They would just bend without any resistance under any bending moment.
That of course means all the nodes may not impart bending moments on the members.
In an *ideal* truss the joints are all pinned.
You can’t put a bending moment through a pin. It’s a hinge with zero rigidity, any theoretical rotation just rotates the joint without transferring any bending moment across it.
Since all forces are being applied to nodes (again in ideal situations) there is then no path for a bending force to get into a member. It forces all forces to be transferred solely by axial compression and tension.
In a non-ideal truss (ie a real one) there is *some* level of rigidity in the connections. But most of the time the stiffness of truss members against compression/tension is much much higher than their s stiffness in bending, so even if there is some rotation at a semi-rigid joint the rotation is very small and the bending stiffness is very low (compared to the axial stiffness) so negligible/almost negligible bending is transferred into the truss members.
/this is specifically for trusses made of triangles. There are other truss types (Eg vierendeel trusses) that do have bending however the design and use cases of these are often very different than most trusses.
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