Drawing Free-Body Diagrams for Pin-Jointed Truss Structures

[high93]

I have attached the problem as an image. I am unsure of how to draw the free body diagram of the truss structure. Would there be reaction forces in the Y-direction at the supports A and E? if there are, then the member AB would not be a two-force member. If there isnt, then Fy of the whole system will not be zero. I'm really confused! Any help would be greatly appreciated..thank you.

 

[high93]

maybe there is only a Y reaction force at point E as member EC is a three force member?

 

[Chopin]

You know there must be a vertical reaction force somewhere in the diagram to counter the 10kN force--if there weren't, the whole thing would slide up the wall. So A and/or E must be providing a downward force.

As for A, you know the torque is zero on the member (with the fulcrum defined as the wall point). Since the only possible source of torque is the pin at B, that means B must be providing 0 vertical force to AB. That means there's no vertical force to be countered by a reaction force at A, so therefore it must be 0. That means all the vertical reaction force must be coming from E.

That's basically just a more long-winded way of saying what you already said, which is that AB is a two-force member, so the reaction force at A must be directed along its length.

 

[high93]

thank you very much. However, I'm still unable to proceed... can anyone give me a hint for the next step to solve the problem please?

 

[high93]

bump

 

[Chopin]

Given the discussion above, you should be able to draw a free-body diagram for each member now. Have you been able to do that?