Which method uses free-body diagrams of sections of the truss to determine unknown forces?

Which method uses free-body diagrams of sections of the truss to determine unknown forces?

• A. Method of joints
• B. The method of sections
• C. Zero force member
• D. A redundant joint

Answer: (B)

This question tests the method that uses free-body diagrams of sections of the truss to determine unknown forces. In the method of sections, you cut through the truss to create a section that isolates part of the structure. Draw the free-body diagram of that section, including the external loads and the forces in the cut members. Since the truss is in static equilibrium, the sum of all forces and the sum of moments for that section must be zero. By choosing a cut that leaves only a small number of unknown member forces (typically three or fewer for a planar truss), you can solve those unknowns using the equilibrium equations. If a cut introduces too many unknowns, you pick a different cut. This approach contrasts with analyzing each joint directly (method of joints), which focuses on the forces at individual joints rather than a cut section. The idea of a zero-force member is a special case where a member carries no force under certain loading and geometry, not a method for finding all unknowns. A redundant joint refers to a statically indeterminate situation where extra members create more unknowns than equations, not a direct method for solving the forces.

This question tests the method that uses free-body diagrams of sections of the truss to determine unknown forces. In the method of sections, you cut through the truss to create a section that isolates part of the structure. Draw the free-body diagram of that section, including the external loads and the forces in the cut members. Since the truss is in static equilibrium, the sum of all forces and the sum of moments for that section must be zero. By choosing a cut that leaves only a small number of unknown member forces (typically three or fewer for a planar truss), you can solve those unknowns using the equilibrium equations. If a cut introduces too many unknowns, you pick a different cut.

This approach contrasts with analyzing each joint directly (method of joints), which focuses on the forces at individual joints rather than a cut section. The idea of a zero-force member is a special case where a member carries no force under certain loading and geometry, not a method for finding all unknowns. A redundant joint refers to a statically indeterminate situation where extra members create more unknowns than equations, not a direct method for solving the forces.