Free-Body Diagram  
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The figure shows the free body diagram around the flange at B. The free body diagram cuts across the rod above and the post below. The axial forces on the rod and the post are F subscript 1 directed vertically upwards and F subscript 2 directed vertically downwards respectively.

First, consider equilibrium. Draw a free-body diagram (FBD) around the junction of the two axial members. In this case, draw a free-body diagram around flange B.

The FBD cuts through members (1) and (2), respectively. Assume that there could be an internal axial force in each member cut by the FBD. Further, assume that the force in each member is tension.

Note that we always assume tension even though we intuitively recognize that the internal forces for this particular problem are likely to be compression.