Final answer:
The condition described involves both tensile and compressive stresses acting together due to axial and bending loads, known as combined stress. The correct option is D) Combined stress, which represents the algebraic sum of both types of stresses at any point in a structural member.
Step-by-step explanation:
The question is referring to a condition where both tensile and compressive stresses are acting simultaneously on a structural member due to the combination of axial loads and bending moments. This is a common scenario in structural engineering where elements such as beams and columns are subject to complex loading conditions that cause the material to experience both stretching and squeezing effects. The scenario described in the question is an example of combined stress, which is the algebraic sum of axial stress (due to axial load) and bending stress (due to moments).
When a long shelf sags under the weight of books, the top surface experiences compressive stress, while the bottom surface experiences tensile stress. This is because the bending causes one side of the shelf to shorten (compress), while the other side lengthens (tense). Therefore, the correct answer to the student's question is D) Combined stress.