How are influence lines used in the design of a multi-span bridge under moving trucks?

• A. Influence lines identify critical positions where moving loads maximize reactions, moments, and shears, guiding member sizing and reinforcement placement.
• B. Influence lines predict weather effects on bridge deformation.
• C. Influence lines show color-coded stress maps in real time.
• D. Influence lines are used only for aesthetic design.

Answer: (A)

Influence lines show how a unit load moving across a bridge changes the response at a point of interest, such as a support, a reaction, or a bending moment in a member. For a multi-span bridge carrying moving trucks, they are used to locate the positions of the truck where reactions, moments, and shear reach their extreme values. Those extreme values are the controlling cases for design, so influence lines guide where to size members and where to place reinforcement or stiffeners. In practice, you construct influence lines for the reactions at supports and for the bending moments in each span, then place the actual truck (or trucks) along the bridge to find where the product of the load with the influence line is largest, giving the worst-case forces. This accounts for the variation as the truck moves from span to span and over internal supports, including the different sign of moments over supports in continuous spans. The other ideas—predicting weather effects, providing real-time color maps, or being used only for aesthetics—do not describe how influence lines drive structural design for moving loads.