Similar to the method of sections for trusses discussed in Art. 3.30, internal forces in statically determinate beams and frames also may be found by isolating a portion of these systems. A section should be chosen so that it will be possible to determine the unknown internal forces from only equations of equilibrium [Eq. (3.11) or 3.12)].
As an example, suppose that the forces and moments at point A in the roof purlin of the gable frame shown in Fig. 3.70a are to be calculated. Support B is a hinge. Support C is on rollers. Support reactions R1, H1, and R2 are determined from equations of equilibrium.
For example, summing moments about B yields