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The method of sections analyzes trusses by making an imaginary cut through the structure and treating one part as a free body. The internal forces at the cut members can then be found using moment equations (often eliminating most unknowns) and force equilibrium. This method is faster than joint analysis when only a few member forces are needed.
From the method of joints, you know that every truss member carries either tension or compression along its axis, and that equilibrium at each pin produces two scalar equations. The method of joints is systematic but slow — to find the force in a member deep inside a large truss, you must work joint by joint from the supports inward. The method of sections takes a shortcut: instead of resolving the truss pin by pin, you slice the entire structure in half with an imaginary cut, expose the internal forces, and treat the resulting fragment as a rigid free body.
The cut must pass through exactly the members whose forces you want. By Newton's third law, the internal force in a cut member acts on your free body as an external force. If the cut passes through three members (the typical case for a simple truss), you have three unknowns and three equilibrium equations — the system is determinate. The key strategic insight is how to use moment equations. If you take moments about the point where two of the three cut members intersect, those two forces produce zero moment, and the equation isolates the third force directly, with no simultaneous equations to solve.
Consider a Pratt truss spanning a bridge. If you want only the force in the bottom chord midspan, joint-by-joint analysis requires many steps. Instead, cut a vertical slice through the midspan panel — through the diagonal, the top chord, and the bottom chord. Take moments about the intersection of the diagonal and top chord; only the bottom chord force contributes a moment arm, giving its magnitude in one equation. This is the power of the method: strategic moment centers eliminate two unknowns at once.
The method complements joint analysis rather than replacing it. Use it when you need forces in a small number of interior members without working through the whole truss. Use the method of joints when you need forces in all members, or when the truss is simple enough that joint-by-joint analysis terminates quickly. In practice, engineers often combine both: use support reactions and section cuts to find key interior members, then fill in the rest with joint equations. Your fluency with the moment of a force — knowing how to choose a convenient moment center to simplify the algebra — is what makes sections powerful in practice.