Chapter 8 Internal Forces
One of the fundamental assumptions we make in statics is that bodies are rigid, that is, they do not deform, bend, or change shape. While we know that this assumption is not true for real materials, we are building the analytical tools necessary to analyze deformation. In this chapter you will learn to compute the forces and moments inside a object which hold it together as it supports its own weight and any applied loads.
The chapter begins with a discussion of internal forces and moments and defines a new sign conventions especially for them. Next we will determine internal forces at a specific point within a rigid body. Finally, we develop three techniques to find internal forces at every point throughout a beam. Note that we use the words internal forces when we are referring to both “internal forces and internal bending moments.”
Determination of the internal forces is the first step in the engineering design of a structure. A properly designed structure must safely support all expected external loads, including live loads, dead loads, wind and earthquake loads. External loads produce internal forces, which in turn creates stresses, strains, and deformations in the structure. In a successful design, the shape, size, and material must all be carefully chosen to limit them to safe values. You are advised to pay attention, and master this topic.