In Chapter 6 of Hibbeler's Structural Analysis (9th Edition) , the focus shifts to Influence Lines for Statically Determinate Structures

Chapters 1-5 introduce basic concepts: types of structures, loads, equilibrium, trusses, and beams. By Chapter 6, the training wheels come off. The chapter focuses on determining (normal force, shear force, and bending moment) at specific points within a beam or frame.

An is a graph that represents the variation of a reaction, shear force, or bending moment at a specific point in a structure as a concentrated unit load (usually 1 kN or 1 kip) moves across the structure. Influence Lines vs. Shear/Moment Diagrams