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Method of joints

In a truss each member is a two-force member: the force acts along the member, either in tension or compression. The method of joints isolates one joint at a time and applies equilibrium there. With only two equilibrium equations in the plane, you can solve at most two unknown member forces per joint.

∑Fx=0, ∑Fy=0\sum F_x = 0,\ \sum F_y = 0equilibrium at the joint, at most 2 unknowns

Symbols

SSmember force (positive = tension)N

Example

Assume tension in every member. If you get S1=−8S_1 = -8 kN,

the member is in compression with 8 kN, not a mistake.

Start at a joint with at most two unknown members, and let the sign tell you when it becomes compression.
Practise trusses and friction for free →

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Part of Statics and Dynamics: Trusses and friction.