Question

In influence line diagrams (ILD):

• A. Points remain fixed, position of load changes
• B. Points change, position of loads remain fixed
• C. Both of them changes
• D. Neither of them changes

Hint:

Influence line diagrams are mainly used for structures subjected to moving loads such as bridges.

Answer 1

The Correct Option is A

An ILD is built for one fixed response (say, reaction at a support or moment at a section) by moving a unit load to different positions and recording the response each time.

1. Points remain fixed, position of load changes: matches this process, since the response location never changes while the load's location is what varies.
2. Points change, position of loads remain fixed: this instead describes a bending moment or shear diagram for one fixed load, not an ILD.
3. Both change: would mix up different response points with different load positions, giving no single consistent influence line.
4. Neither changes: would give only one value, not a diagram.

So in an ILD, points remain fixed, position of load changes.

Answer 2

A third way to settle this is to look at what quantities are actually plotted on the two axes of an influence line diagram.

On the horizontal axis of an ILD, we plot the position of the unit load as it moves along the span. On the vertical axis, we plot the resulting value of one specific response (such as a particular reaction, shear, or moment) at one specific, predetermined location on the structure. Because the vertical axis is tied to a single, unchanging location by definition, that location cannot itself be a variable in the plot.

1. Points remain fixed, position of load changes: This matches the axis definitions exactly — the horizontal axis (load position) is the variable, while the response location behind the vertical axis is fixed by definition of the specific ILD being drawn.
2. Points change, position of loads remain fixed: This would require the vertical axis itself to represent a moving location, which contradicts the basic definition of an ILD as the response at one fixed location.
3. Both change: This would mean the diagram has two independent variables, which cannot be represented on a simple two-axis plot of load-position versus response value.
4. Neither changes: This would mean there is no independent variable at all, so no diagram (a line, showing variation) could be produced.

The very structure of the two axes used in an ILD confirms that only the load's position varies while the response point stays fixed.

Therefore, the correct answer is points remain fixed, position of load changes.