Enter the magnitude of the load and the reaction at the specific point into the calculator to determine the influence line value, and this calculator can also evaluate any of the variables given the others are known.

## Influence Line Formula

The following formula is used to calculate the influence line value at a specific point on a structure:

IL = (W * R) / L

Variables:

- IL is the influence line value at a specific point
- W is the magnitude of the load
- R is the reaction at the specific point
- L is the length of the structure

To calculate the influence line value at a specific point, multiply the magnitude of the load by the reaction at that point, and divide the result by the length of the structure.

## What is a Influence Line?

An Influence Line is a graphical representation used in structural engineering to illustrate how a moving load on a structure, such as a bridge or a beam, influences the parameters of that structure, such as its reactions, internal forces, or deflections. The line is plotted on a graph where the x-axis represents the position of the load along the structure and the y-axis represents the value of the parameter being considered. The shape of the influence line depends on the structural system and the location of the parameter being evaluated. It helps engineers to determine the maximum effect of a moving load on a particular parameter and thus aids in the design and analysis of the structure.

## How to Calculate Influence Line?

The following steps outline how to calculate the Influence Line:

- First, determine the load position along the structure.
- Next, determine the reaction forces at the supports.
- Next, apply a unit load at the load position and determine the resulting deflection at various points along the structure.
- Repeat step 3 for multiple unit loads at different positions along the structure.
- Plot the deflection values obtained from step 3 and step 4 on a graph.
- Connect the plotted points to obtain the influence line.

**Example Problem:**

Use the following variables as an example problem to test your knowledge.

load position along the structure = 5m

reaction forces at the supports = 10kN and 15kN