Explanation:

As shown in Figure (a), for the beam BC, the clockwise moment 15 kNm at left hand end B produces a sagging (positive) bending moment. At end C, the clockwise moment of 30 kNm produces a hogging (negative) bending moment.

Figure

For the column AB, the anticlockwise moment of 15 kNm at end A produces tension on left hand or outer face (as plotted). The anticlockwise moment of 15 kN at end B produces tension on right hand (inner) face. They will be plotted on the opposite sides of the AB.