Influence lines are used in the design of a multi-span bridge under moving loads to identify what?

• A. Critical reactions, moments, and shears
• B. Temperature distribution
• C. Weather patterns
• D. Color mapping of steel

Answer: (A)

Influence lines express how the response at a specific location in a structure changes as a unit load moves across the structure. In a multi-span bridge under moving loads, they are used to locate the positions of the moving load that produce the largest values of reactions at supports, as well as bending moments and shear forces at sections. By drawing influence lines for each reaction and for the internal forces at a section, you can read off the maximum possible values as the unit load traverses the span. Those peak values identify the worst-case loading scenarios you must design for, ensuring members and connections can resist the strongest possible reactions and internal forces. The other options—temperature distribution, weather patterns, or color mapping—do not pertain to the structural response caused by moving loads, so they’re not what influence lines quantify.