How do engineers estimate the live loads that a bridge must carry?

Engineers estimate live loads—the weight of traffic such as cars, trucks, and trains—by considering the length of the bridge span.12

For short spans of less than 30 meters (100 feet), engineers design for the maximum conceivable load, as it is statistically likely that the bridge could be fully occupied by heavy vehicles simultaneously.1

However, for longer spans of 1,000 meters or more, designing for the absolute maximum load (such as bumper-to-bumper heavy trucks) is considered economically unreasonable because the probability of such an event is extremely low. In these cases, engineers use probable loads as the basis for their design calculations.1

This approach is essential because, while live loads dominate the design of shorter spans, the bridge's own weight (dead load) becomes the primary factor on longer spans, necessitating designs that prioritize structural efficiency and material optimization.1