Module 17
Influence lines and moving loads
What happens when the load will not stay still: influence lines, the Müller-Breslau principle, and finding the worst position of a load train.
What this module covers
- Explain what an influence line is and how it differs from a bending moment diagram
- Construct influence lines for reaction, shear and bending moment on a simple span
- Use the Müller-Breslau principle to sketch an influence line without calculation
- Find the effect of a distributed load from the area under an influence line
- Locate the worst position of a train of moving loads
Lessons
The one diagram people consistently misread — and why.
Start lesson →A way to draw an influence line by releasing the structure instead of calculating.
Start lesson →Where do you park the lorry to do the most damage?
Start lesson →When the load reaches the girder through cross beams, and how truss members respond to it.
Start lesson →
Module checkpoint
Check what you have taken in
4 questions
Question 1
What does the horizontal axis of an influence line represent?
Question 2
A simply supported beam spans 8.00 m. What is the peak ordinate of the influence line for bending moment at mid-span, in metres?
Question 3
For a simply supported span of 8.00 m, what is the magnitude of the jump in the shear influence line at a section 3.00 m from the left support?
Question 4
To maximise an effect using a distributed live load, which parts of the structure should be loaded?