The method of sections is developed based on the principle that if a truss structure is in equilibrium then part of the truss is also in equilibrium. Thus we can make any section(s) in a truss structure and parts of the truss structure should still be in equilibrium. We can select any convinient point as the reference point to apply the three equilibrium equations below.

**Procedure for method of sections**:

- Make a section through the members where internal forces need to be determined. Make sure the section resulting in maximum three unknown forces. If a section cannot produce the maximum of three unknown forces, then you must solve one or more unknown forces using either method of sections or method of joints. Also you may need to solve for external reaction forces before you can isolate the appropriate section.
- Draw the free-body diagram (FBD) for the part of sectioned truss that will give you more convinient way to solve the unknow forces.
- Assume the direction of the unknown forces. As recommendation, always assume the unknown forces to be in tension. Negative value in analysis result indicates that the actual force in the member is compression and the direction is the opposite.
- Apply the three equilibrium equations given above. Try to avoid forming equations that need be solved simultaneously. In using the moment equilibrium equation, try to take the moment about a point that lies at the intersection of lines of action of two unknown forces. In the case of having two parallel unknown forces, take equilibrium forces in the direction perpendicular of those two parallel unknown forces.

We should use this method if we are only interested in finding the internal forces of certain members, not the whole members of a truss structure.

{ 1 comment… read it below or add one }

mention some special cases with practical examples & their solutions