| Overview |
| IL for Reactions |
| IL for Shear |
| IL for Moment |
| Summary |
| Homework Problems |
| Report Errors or Make Suggestions |
Section IF.1
Overview
Last Revised: 02/18/2026
An influence line describes the magnitude of a reaction, shear, or moment at one specific location on a structure for a load being placed elsewhere on the structure. An influence line is only good for determining the desired quantity at ONE specific location.
Influence lines can be computed qualitatively; however quantitative influence lines are typically sufficient when designing building structures and are used to determine where to place loads for maximum effect at the location for which the influence line is drawn. This paper will only look at qualitative influence lines.
Influence lines look like deflection diagrams, but they are not.
There are three types of influence lines, each with its own rule for developing the influence line:
- Reaction Influence Lines
- Found by imposing a unit displacement at the reaction in question. The resulting “deflected” shape is the influence line.
- Shear Influence Lines
- Found by cutting the member at the point of interest and imposing a unit shearing deformation. The resulting “deflected” shape is the influence line.
- Moment Influence Lines
- Found by introducing a hinge at the location of interest and imposing a unit rotation at the hinge. The resulting “deflected” shape is the influence line.
For these discussions, consider the two-span continuous beam in Figure 1.
Figure 1: Two-Span Continuous Beam