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

Step 1: Understanding influence line diagrams.
An influence line diagram represents how a response function (reaction, shear force, bending moment, etc.) at a fixed point of a structure varies as a unit load moves across the structure.
Step 2: Key characteristic of ILD.
In ILD analysis:
- The structure and the point of interest remain fixed.
- Only the position of the load changes along the span.
Step 3: Analysis of options.
(A) Correct, because the load moves while the point remains fixed.
(B) Incorrect, points do not change in ILD.
(C) Incorrect, both do not change simultaneously.
(D) Incorrect, load position does change.
Step 4: Conclusion.
Hence, the correct answer is (A).

Answer 2

Rather than stating the general definition again, we can verify this by walking through what actually happens when we construct an influence line diagram (ILD) for a specific response, such as the reaction at support A of a simply supported beam.

To construct this ILD, we place a unit load at a series of different locations along the span, one at a time, and for each position we calculate the resulting reaction at A. The location we are computing the response for (support A) never moves during this process — what changes from one calculation to the next is only where the unit load sits along the span. Plotting the computed reaction value against the load's position produces the influence line.

1. Points remain fixed, position of load changes: This matches the procedure exactly — the response is always evaluated at the same fixed point (support A here), while only the unit load's location is varied step by step.
2. Points change, position of loads remain fixed: This would mean recalculating the response at many different points for one fixed load position — that is instead how a bending moment or shear force diagram (for a fixed load) is constructed, not an influence line.
3. Both of them changes: If both the response point and the load position varied simultaneously, the resulting plot would mix together information from different scenarios and would not represent a meaningful influence line for a single fixed response quantity.
4. Neither of them changes: If nothing changed, only a single number would be produced, not a diagram at all, so this cannot describe how an ILD is built.

The worked example confirms that only the load's position is varied while the point of interest for the response stays fixed throughout.

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