Influence Line Diagrams MCQ with answer and solution

Concept

An Influence Line Diagram (ILD) represents the variation of a structural response (like shear force or bending moment) at a specific fixed section as a unit load moves across the structure.

Step-by-step solution

1. Understand the definition of an influence line diagram (ILD). 2. An ILD is drawn for a specific fixed section of a structure. 3. The vertical ordinate of the ILD gives the value of a reaction, shear force, or bending moment at that specific section when a moving unit load is placed at any position along the span. 4. Therefore, an influence line for shear force shows the variation of shear force at one section caused by a moving load over the entire structure. Answer: (C) variation of the shear force at one section caused by a moving load over the entire structure

Explanation

An influence line diagram is specific to one fixed section of a beam or frame. It shows how a structural action (like shear force or moment) at that section changes as a unit load moves across the entire structure.

Why the other options are wrong

A: This describes a standard Shear Force Diagram (SFD) for fixed loads, not an ILD. B: An ILD considers a moving load, not a single fixed static load. D: This is incorrect because an ILD focuses on a single section at a time, not the entire structure simultaneously.

Shortcut

Remember: Shear Force Diagram (SFD) = Fixed load, variation over all sections. Influence Line Diagram (ILD) = Fixed section, moving load over all positions.

Common mistake

Students often confuse a Shear Force Diagram (SFD) with an Influence Line Diagram (ILD). SFD shows variation along the length for fixed loads, while ILD shows variation at one point for moving loads.