Cut through at most three unknown members so the free-body of one side is solvable with three equilibrium equations. Take moments about a joint where two unknowns meet to isolate the third.
Fi(kN)
Member force
Step-by-step solved example
Simple triangular truss span 8 m, height 3 m, 12 kN down at the ridge. Find the bottom chord by a section through the three members.
Cut through three members; ΣM about a joint kills two unknowns.
1. Reactions
Each support 6 kN up.
2. Section
ΣM about the ridge: both rafters pass through it, so their arms are zero. Bottom-chord arm is the 3 m height: T×3 = 6×4 → T = 8 kN tension.
T=8kN
Answer: Bottom chord T = 8 kN (tension)
10 practice questions
0/10 correct
1.Cut at most how many unknowns?
2.Method of joints uses
3.A zero-force member at an unloaded joint with two collinear members is the
4.Tension pulls
5.If a section cuts four members
6.ΣM about a joint is useful to
7.Truss members are assumed
8.Compression pushes
9.A section through the midspan of a Pratt truss typically finds
10.If joints are actually rigid, extra moments mean