Final answer:
The additional moment developed in a structural member when its longitudinal axis deviates from the line of action of a compressive force is called a bending moment. This is due to eccentric loading, causing one side of the member to experience compressive stress and the other tensile stress.
Step-by-step explanation:
The additional moment developed in a structural member when its longitudinal axis deviates from the line of action of a compressive force is known as a bending moment. This occurs because the line of action of the applied force does not pass through the centroid or center of mass of the member, which causes the member to bend or deflect. In such cases, one side of the member experiences compressive stress while the other side experiences tensile stress, similar to the deformation described in Figure 12.20 (a) of an object bending downward. Whenever such eccentric loading occurs, it's crucial to account for the bending moment produced to ensure the structural member can withstand the applied loads without failure.