A Beginner's Guide to Structural Mechanics/Analysis

Qualitative Influence Lines

(c) 2026, T. Bartlett Quimby

Overview
IL for Reactions
IL for Shear
IL for Moment
Summary
Homework Problems
Report Errors or Make Suggestions

 

 

Section IL.5

Qualitative Influence Lines Summary

Last Revised: 02/17/2026

Influence lines are useful tools for determining WHERE to place moveable loads for maximum effect at some given location for reactions, shear, and moment. It is incumbent on the engineer to find the maximum effect when doing design to ensure that all possible load conditions are considered.

The principles presented here apply to all continuous, indeterminate, structures.

For the design of building structures that are commonly subjected to uniformly distributed loads, qualitative influence lines are typically sufficient. Quantitative influence lines are commonly used in bridge design where series of moving point loads are considered. The discussion here does not consider the computation of qualitative influence lines.