7
METHOD OF SECTIONS
Learning Objectives
1). To employ the method of sections to evaluate the axial force
carried by selected members in a truss.
2). To do an engineering estimate of the load in select members
of a truss.
Procedure
1). Draw a FBD of the entire truss showing the reaction forces
at the supports and the external loads. Write the
equilibrium equations and solve for as many unknowns as
possible.
2). Locate the force members to be evaluated. Identify whether
any of these forces can be determined by observation (e.g.,
zero-force members).
3). Identify section to be used and draw a FBD of the section
including any support reactions, external loads and internal
forces of sectioned members. Remember, the cutting plane
must cut through the members of interest. Also the cutting
plane need not be straight, it may be curved.
4). Write the equilibrium equations for one of the two sections.
The equations for either half of the section will yield the
same member forces.
5). Three equilibrium equations are available, so up to three
unknowns can be solved with a single section.
6). At times more than one section may be necessary.
