How it is that member FG is a zero force member?

[Ry122]

http://users.on.net/~rohanlal/truss.jpg
Can someone please explain to me how it is that member FG is a zero force member?
Isn't the only way for a member to be zero force if
1. Two non collinear members form a joint and no force is being exerted on this joint.
or 2.
When three members form a truss joint for which 1 member is joined at an angle with two collinear members and no force is exerted on this joint.

Member FG does not satisfy either of these.

 

[Mapes]

A pin-connected member is a zero-force member if no force is exerted along its axis (the pins can't transmit a moment). It should be clear that no horizontal force can be applied to FG at G. Therefore, it carries no horizontal force, or any force.

The same reasoning applies to BC, which is there to keep ACE from buckling.

 

[Ry122]

If the 50kN force had any horizontal component at all would this mean that FG would no longer be zero force?

 

[Mapes]

Correct. A free-body diagram around pin G would tell you that a counteracting force would be required to keep the pin from accelerating away. By the reasoning in my earlier post, only FG could supply this counteracting horizontal force.

 

[elbarto]

As has been already stated above, since the structure you have is made of truss members (ie pin connections) then each member is unable to carry/transmit any shear to adjacent members. Therefore the only capacity to resist the external vertical load at Node G must come from the tensile force in member GH.

Obviously if the 50 kN force was not coincident with member I GH (that is, not vertical) and hence no longer orthogonal to member FG, then we would expect member FG to carry some load.

If the 50 kN load was applied in the x-dirrection, then member GH would become a zero force member.

I can remember when I first began learning about zero force members in statics, I couldn't easily identify them. After drawing countless FBD's, it is now easier to see what is going on with the structure

hope this helps
Elbarto

 

[kenn87]

heys..would like to ask, does reactive forces due to a joint support cause the members attached to it to be non-zero force ? if say the member is vertically upward from the supporting pin joint.

 

[Mapes]

If the reaction force at a pin connection has a non-zero component along the axis of the member, then the member is not a zero-force member.