two members, unloaded joint⇒both zero;third of three, two collinear, unloaded⇒zero
p. 98: if only two members meet and the joint has no external load and no support reaction, both are zero-force. Equilibrium forces that conclusion only when the two members are not collinear; a pair on one straight line can still carry axial force. The second printed case: three members, two of them collinear, same unloaded joint, and only the third member is zero-force.
F(kN)
Member force
Step-by-step solved example
Joint C is formed only by non-collinear members AC (horizontal) and BC (vertical). No external load and no support reaction acts at C. What are the two member forces?
Unloaded two-member joint: both forces are zero. If three members meet and two are collinear, only the third is zero — and only with no load and no reaction.
1. Conditions
Two members only, and the joint is unloaded. Both handbook conditions for this case are met.
2. Equilibrium
They are not collinear, so ΣFH = 0 kills the horizontal member and ΣFV = 0 kills the vertical member. Both are zero-force.
FAC=FBC=0
Answer: Both members are zero-force
8 practice questions
0/8 correct
1.Two non-collinear members meet at a joint with no load and no reaction. Their forces are
2.Three members, two collinear, no load and no reaction. The zero-force member is
3.A vertical load at a two-member joint means
4.A support reaction at the joint
5.In the three-member case the two collinear members
6.A joint solution that returns exactly zero means