Structural Engineering

Influence Line

A graph of one response — a reaction, shear or moment at a point — as a unit load moves across a structure.

An influence line answers where a moving load should be placed to produce the largest value of one particular response. Its ordinate at any position is the response when a unit load stands there, so a concentrated load contributes its weight times the ordinate and a uniform load its intensity times the area under the line.

The Müller-Breslau principle gives the shape without calculation: remove the restraint that provides the response and impose a unit displacement there, and the deflected shape is the influence line. For moment at a point on a simple span it is a triangle peaking at ab/L over that point.

How much of the FE Civil exam is Structural Engineering?