ES-130 Trusses - Method of Sections
The method of sections is most applicable when the forces in only some members are required. If, for the truss shown below only the forces in BF and BC are required, the truss may be section through BC, BF and EF as shown by the dotted line.

A free body diagram may then be drawn of the left or right portion of the truss. The left portion is preferable, since no reactions need to be found. If both portions contain reactions, they generally must be found before proceeding with the problem.

The FBD of the sectioned truss is shown here...

Note: All unknowns, not only those required, must be drawn on the FBD. Since only FBC and FBF are required, it is worthwhile to choose equations so that they may be found with minimal algebraic manipulation. A wise choice of points about which moments are taken frequently accelerates solution.

If moments are taken about point F, FBC, one of the required forces becomes the only unknown.

Summation of forces in the y-direction causes the second required force FBF to be the only unknown.

Note: The minus sign indicates that the assumption of member BF to be in tension was erroneous. The member is in compression.


Note:
  • Cuts do not have to be straight lines but may be curves.
  • Cuts may not be taken through joints.
  • Cuts through more than three members should be avoided, if possible.

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