About determine zero force member

[mchei]

i am confused on the determination of zero force member

from the above pic, the coloured beam is zero member, i know that when u view from joint B, 2 beams are zero member, but when u view from A, beam_AB is not zero, are there any contradiction?

also, i don't know why when F_AB is zero, then F_BC is also zero? becoz the beam BC don't have any supporting foce on it, am i correct?

finally, i have found 2 rule on wiki but i am confused on it.
do zero force member must not exist when u view the joint that is applied a load or it is a supporting point?

furthermore, is zero force member have zero force acting on it?

2. Homework Equations

1. If only two members meet in an unloaded joint, both are zero-force members.
2. If three members meet in an unloaded joint of which two are in a direct line with one another, then the third (perpendicular) member is a zero-force member.
3. If two members meet in a loaded joint and the line of action of the load coincides with one of the members, the other member is a zero-force member.

3. The Attempt at a Solution

coloured beam is zero force member

 

[PhanthomJay]

i am confused on the determination of zero force member

from the above pic, the coloured beam is zero member, i know that when u view from joint B, 2 beams are zero member, but when u view from A, beam_AB is not zero, are there any contradiction?

There is no contradiction, because AB force is still zero when looking at joint A. The support reaction force from A goes into member AC as a vertical component of the force in AC, and the horizontal component of the force in AC goes into AG.

also, i don't know why when F_AB is zero, then F_BC is also zero? becoz the beam BC don't have any supporting foce on it, am i correct?

Since F_AB is 0, then, yes, there is no force applied to BC. There can't be any force in AB , because if there were, there would be no place to balance its vertical component.

finally, i have found 2 rule on wiki but i am confused on it.

Rule 2 is wrong...remove the word 'perpendicular'. DG is a zero force member, it does not have to be perpendicular.

do zero force member must not exist when u view the joint that is applied a load or it is a supporting point?

No, untrue. See rule 3, only one of the members may have zero force, or none of the members, but not all the members.

furthermore, is zero force member have zero force acting on it?

yes, no internal force acts in a zero force member, and hence, it can have no force acting on it.

2. Homework Equations

1. If only two members meet in an unloaded joint, both are zero-force members.
2. If three members meet in an unloaded joint of which two are in a direct line with one another, then the third (perpendicular) member is a zero-force member.
3. If two members meet in a loaded joint and the line of action of the load coincides with one of the members, the other member is a zero-force member.

3. The Attempt at a Solution

coloured beam is zero force member