Forces in Statically Determinate Beams and Frames

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


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