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Method of sections

The method of sections treats a whole part of the truss at once instead of one joint. You cut through at most three members, remove one part and put the member forces in as external forces. Then you can use all three equilibrium equations, including moment, on the remaining part.

∑Fx=0, ∑Fy=0, ∑M=0\sum F_x = 0,\ \sum F_y = 0,\ \sum M = 0equilibrium for the cut part

Symbols

SSforce in the cut memberN

Example

Cut through three members in a truss with a known load PP.

Take moments about the point where two of the unknown members meet – then only one unknown remains.

Choose the moment point where two of the three unknown members intersect, so they drop out of the equation.
Practise trusses and friction for free →

← Method of joints · Friction →

Part of Statics and Dynamics: Trusses and friction.