Influence lines are used in moving-load analysis to determine:

• A. How a unit load at a position affects reactions, shear, and moments.
• B. The color of the structure under load.
• C. The total weight of the beam.
• D. The dynamic response under earthquakes.

Answer: (A)

Influence lines show how moving a unit load along a structure changes the forces of interest at a point or section. They are built by placing a unit load at every possible position and recording the resulting reaction at a support, or the shear or bending moment at a particular section. The key idea is that the ordinate of the influence line for a given quantity (reaction, shear, or moment) at a location equals the value of that quantity produced by a unit load at that position. For a real moving load, you multiply the unit-load influence value by the actual load magnitude at each position and sum those contributions to get the actual response. This makes it possible to find maximum reactions or moments as the load travels along the beam. The other options don’t involve recomputing reactions, shear, or moments from a moving load, so they’re not what influence lines are used for.